Living things on Earth are very many and very different, so scientists sort them into groups and subgroups. This chapter explains the Five Kingdom system given by R. H. Whittaker and then classifies microorganisms (microbes) into bacteria, protozoa, fungi, algae and viruses, with their size and features.
Key points
Dividing living organisms into groups and subgroups on the basis of similarities and differences is called biological classification (वर्गीकरण).
Whittaker used 5 criteria: complexity of cell structure, complexity of the organism's body, mode of nutrition, life style (producer/consumer/decomposer) and phylogenetic relationship.
The five kingdoms are Monera, Protista, Fungi, Plantae and Animalia.
Monera: all bacteria and blue-green algae; unicellular, prokaryotic (no distinct nucleus or cell organelles); autotrophic or heterotrophic.
Protista: unicellular, eukaryotic; move with pseudopodia, cilia or flagella. Euglena and Volvox are autotrophs; Amoeba and Paramoecium are heterotrophs.
Fungi: non-green, eukaryotic, heterotrophic; most are saprotrophs; cell wall is made of chitin. Examples: yeast, Aspergillus, Penicillium, mushroom.
Bacteria (1–10 µm) reproduce by binary fission and can double their number in about 20 minutes in good conditions. Shapes: coccus, bacillus, spirillum, vibrio.
Algae are aquatic and photosynthetic, e.g. Chlorella, Chlamydomonas.
Viruses (10–100 nm) are seen only with an electron microscope. They are DNA or RNA covered by a protein coat and multiply only inside living cells. They are called "organisms at the edge of living and non-living".
National Institute of Virology, Pune (founded 1952, under ICMR) does research on viruses.
Important terms
Prokaryotic cell – cell without a membrane-bound nucleus and cell organelles.
Eukaryotic cell – cell with a true nucleus enclosed in a nuclear membrane.
Saprotroph – organism that absorbs food from dead and decaying matter.
Bacteriophage – a virus that attacks bacteria.
Chitin – tough complex sugar that forms the cell wall of fungi.
Formulas
1 metre = 10⁶ micrometres (µm)
1 metre = 10⁹ nanometres (nm)
Size order (small to big): Virus < Bacteria < Fungi/Algae < Protozoa
5 important questions
Q1. What criteria did Whittaker use for classification?
Complexity of cell structure (prokaryotic/eukaryotic), complexity of body (unicellular/multicellular), mode of nutrition, life style (producer, consumer, decomposer) and phylogenetic relationship.
Q2. Write the characteristics of viruses.
They are 10–100 nm in size and seen only with an electron microscope. They are a DNA or RNA molecule inside a protein coat. They multiply only inside a living host cell, then destroy it. They cause diseases like polio, influenza and AIDS.
Q3. Explain nutrition in fungi.
Fungi have no chlorophyll, so they cannot make food. Most are saprotrophs: they absorb nutrients from decaying organic matter and dead bodies of plants and animals.
Q4. Which organisms are included in Kingdom Monera?
All bacteria and blue-green algae. They are unicellular and prokaryotic, without a distinct nucleus or cell organelles.
Q5. True or false: Lactobacilli are harmful bacteria.
False. Lactobacilli are useful bacteria; they turn milk into curd.
Exam tip
Learn the five kingdoms with one example each, and the ascending order of size: virus, bacteria, fungi/algae, protozoa.
Health is not only the absence of disease. This chapter explains what disease is, how diseases are classified, some important infectious diseases (with pathogen, spread and prevention), non-infectious diseases like cancer, diabetes and heart disease, misuse of medicines, generic medicines and a healthy lifestyle.
Key points
Health is a state of complete physical, mental and social well-being, not merely the absence of disease.
Disease (रोग) is a disturbance in the physiological or psychological processes of the body; each disease has specific symptoms.
By duration: chronic (long-lasting) and acute (short) diseases. By reason: hereditary (e.g. Down's syndrome) and acquired. Acquired diseases are infectious (common cold, flu, dengue) or non-infectious (diabetes, heart disease).
Infectious diseases spread through contaminated air, water, food or vectors (insects and animals).
TB is caused by Mycobacterium tuberculosis (spread through air, spitting); prevented by BCG vaccine and DOT treatment.
Dengue is caused by a flavivirus (DEN 1–4) and spread by Aedes aegypti; platelet count falls. Malaria – female Anopheles; elephantiasis – female Culex.
Swine flu is caused by influenza A (H1N1) virus; first case in Mexico in 2009. AIDS is caused by HIV and diagnosed by the ELISA test. Rabies spreads through bites of infected animals; hydrophobia is its main symptom.
Cancer is uncontrolled, abnormal growth of cells; a lump of cancer cells is a malignant tumour. Tobacco, alcohol, junk food and low-fibre diet are possible causes.
Diabetes occurs when the pancreas secretes too little insulin, so blood glucose is not controlled.
Heart attack: call 108 and give compression-only life support (press the centre of the chest at 100–120 per minute).
Generic medicines are sold without a patent, are equal in quality and cheaper. Pradhan Mantri Jan Aushadhi Yojana began on 1 July 2015.
Important terms
Pathogen – microbe that causes disease.
Vector – insect or animal that carries a pathogen to humans.
Chronic disease – disease that lasts a long time.
Hydrophobia – extreme fear of water seen in rabies.
Generic medicine – medicine made without a patent, cheaper but of equal quality.
Formulas
Health days to remember:
7 April – World Health Day
14 June – World Blood Donation Day
29 September – World Heart Day
14 November – World Diabetes Day
5 important questions
Q1. Distinguish between infectious and non-infectious diseases.
Infectious diseases spread from one person to another through air, water, food or vectors (e.g. TB, cholera). Non-infectious diseases do not spread by contact; they arise in the body due to lifestyle, heredity etc. (e.g. diabetes, cancer).
Q2. Identify the odd term: malaria, hepatitis, elephantiasis, dengue.
Hepatitis – the others spread through mosquitoes; hepatitis spreads through contaminated water, needles and blood.
Q3. What are the main reasons of diabetes and heart diseases?
Heredity, obesity, lack of exercise and mental stress; for heart disease also smoking, alcohol, diabetes and high blood pressure.
Q4. What can be prevented by drinking boiled and filtered water?
Water-borne diseases like cholera, typhoid, dysentery and hepatitis.
Q5. Why are generic medicines cheap?
Their formula is already available, so there is little spending on research and no patent cost, but quality is the same as branded medicines.
Exam tip
Make a table: disease – pathogen – mode of spread – prevention. Many 2-mark questions come from it.
A force is a push or pull that comes from interaction between two objects. This chapter covers contact and non-contact forces, balanced and unbalanced forces, inertia, pressure and its units, atmospheric pressure, buoyant force, Archimedes' principle, density and relative density.
Key points
Contact force acts through direct contact (muscular force, friction). Non-contact force acts without contact (magnetic, gravitational, electrostatic force).
Friction always acts opposite to the direction of motion. It helps us walk; we slip on a banana peel because friction is reduced.
Balanced forces: equal and opposite, net force zero, no motion. Unbalanced forces: net force is not zero, so the object moves towards the bigger force (e.g. tug of war).
Forces in the same direction add up; forces in opposite directions subtract. Force has magnitude and direction, so it is a vector quantity.
A force can start motion, stop motion, change speed or direction, and change shape.
Inertia (जडत्व) is the tendency of an object to stay in its existing state. Types: inertia of rest, inertia of motion and directional inertia.
Pressure is the force acting perpendicularly on unit area. Smaller area means more pressure – that is why pins and nails are pointed.
Atmospheric pressure is the pressure due to the air around the Earth. Air pressure at sea level is 1 atmosphere.
When an object is put in a fluid, an upward buoyant force acts on it.
Archimedes' principle: when an object is partly or fully immersed in a fluid, an upward buoyant force acts on it, equal to the weight of the fluid displaced.
Ships, submarines, lactometers and hydrometers work on Archimedes' principle.
Important terms
Inertia – tendency of a body to remain in its state of rest or motion.
Pressure – force per unit area acting perpendicular to a surface.
Buoyant force – upward force exerted by a fluid on an object in it.
Density – mass per unit volume.
Relative density (specific gravity) – density of a substance divided by density of water.
Formulas
Pressure = Force / Area; SI unit N/m² = pascal (Pa)
1 bar = 10⁵ Pa; 1 atmosphere ≈ 101 × 10³ Pa ≈ 1 bar = 10³ mbar; 1 mbar = 10² Pa = 1 hPa
Density = Mass / Volume; SI unit kg/m³
Relative density = Density of substance / Density of water (no unit)
Weight = mg (g = 9.8 m/s²)
5 important questions
Q1. A box of weight 50 N rests on an area of 0.25 m². Find the pressure.
Pressure = Force / Area = 50 / 0.25 = 200 N/m² (200 Pa).
Q2. Density of iron is 7.85 × 10³ kg/m³ and of water 10³ kg/m³. Find relative density of iron.
Relative density = 7.85 × 10³ / 10³ = 7.85 (no unit).
Q3. State Archimedes' principle and give two uses.
When an object is partly or fully immersed in a fluid, an upward buoyant force acts on it equal to the weight of the fluid it displaces. Uses: building ships and submarines; lactometer and hydrometer.
Q4. Why does the coin fall into the glass when the card below it is flicked quickly?
Because of inertia of rest. The card moves away fast, but the coin tends to stay at rest, so it falls straight into the glass.
Q5. Why do nails have pointed ends?
A pointed end has a very small area, so the same force produces a very large pressure and the nail goes in easily.
Exam tip
Always convert cm² to m² (divide by 10,000) before finding pressure in pascal.
When electrons in a conductor are made to flow, we get current electricity. This chapter explains electrostatic potential, potential difference, electric current and its unit, different electric cells (dry cell, lead-acid cell, Ni-Cd and lithium-ion cells), electric circuits, connecting cells in series and parallel, and the magnetic effect of electric current (electromagnet and electric bell).
Key points
Positive charge tends to flow from a point of higher electric level to a point of lower electric level. This level is called electrostatic potential (विद्युत विभव).
Current flows only when there is a potential difference between two points; its SI unit is the volt (V).
Electrons flow from the negative terminal to the positive terminal; conventional current is shown in the opposite direction.
Electric current is the charge flowing through a conductor in one second. Its SI unit is the ampere (A). Current is a scalar quantity.
An electric cell keeps a constant potential difference between its two terminals.
Dry cell: zinc case is the negative terminal; graphite rod in the centre is the positive terminal; surrounded by MnO₂ paste; electrolyte is a wet paste of ZnCl₂ and NH₄Cl. It gives small current but has long shelf life and works in any position.
Lead-acid cell: Pb and PbO₂ electrodes in dilute sulphuric acid; about 2 V; rechargeable; gives large current – used in cars, trucks and UPS.
Ni-Cd and lithium-ion cells are rechargeable; lithium-ion cells store more energy and are used in mobile phones and laptops.
The closed path through which current flows is an electrical circuit. Symbols are used to draw circuit diagrams.
In series connection, the positive terminal of one cell joins the negative terminal of the next; potential differences add up. Several cells joined together form a battery.
When current flows in a wire, a magnetic field is produced around it (magnetic needle deflects).
A coil of wire around an iron screw becomes a magnet only while current flows – an electromagnet. An electric bell works using an electromagnet.
Important terms
Potential difference – difference in electric potential between two points; unit volt.
Electric current – rate of flow of electric charge; unit ampere.
Electric cell – device that maintains a constant potential difference using chemical reactions.
Battery – two or more cells connected together.
Electromagnet – temporary magnet made by passing current through a coil around soft iron.
Formulas
Current I = Charge Q / time t; 1 ampere = 1 coulomb / 1 second (1 A = 1 C/s)
Cells in series: total potential difference = sum of potential differences of cells (e.g. 3 cells of 1.5 V = 4.5 V)
5 important questions
Q1. What is potential difference? What is its unit?
The difference in electric potential between two points in a circuit. It makes charges flow. SI unit: volt (V).
Q2. Describe the construction of a dry cell.
A zinc container acts as the negative terminal. A graphite rod at the centre is the positive terminal, surrounded by MnO₂ paste. Between them is a wet paste of ZnCl₂ and NH₄Cl as the electrolyte.
Q3. Why are lead-acid cells used in vehicles?
They can deliver a large current and can be recharged after discharging.
Q4. Three cells of 1.5 V are joined in series. What is the total potential difference?
Total = 1.5 + 1.5 + 1.5 = 4.5 V.
Q5. How does an electric bell work?
When the switch is pressed, current flows through the coil, which becomes an electromagnet and pulls the iron strip, so the striker hits the gong. This breaks contact with the screw, magnetism vanishes, the strip springs back, contact is made again and the cycle repeats quickly.
Exam tip
Practise drawing the dry cell and lead-acid cell with correct labels and the circuit symbols for cell, bulb and switch.
An atom is the smallest particle of an element that keeps its chemical identity. This chapter traces how ideas about the atom developed (Kanad, Democritus, Dalton, Thomson, Rutherford, Bohr), describes the subatomic particles, atomic number and mass number, electronic configuration, valency and isotopes with their uses.
Key points
Kanad (6th century BC) called the smallest particle "Paramanu"; Democritus called it "atom" (Greek atomos = cannot be cut).
Dalton (1803): matter is made of indivisible, indestructible atoms; atoms of an element are alike.
Thomson (1904) – plum pudding model: positive charge spread through the atom, electrons embedded in it. He showed the electron is about 1800 times lighter than a hydrogen atom.
Rutherford (1911) – nuclear model: a small positively charged nucleus at the centre holds almost all the mass; electrons revolve around it.
Bohr (1913) – stable orbit model: electrons revolve in fixed shells (K, L, M, N) with definite energy; they do not lose energy while in a shell.
Nucleus has protons (positive, mass about 1 u) and neutrons (no charge, mass about 1 u). Electrons (negative) have negligible mass.
An atom is neutral because the number of protons = number of electrons.
Maximum electrons in a shell = 2n²: K = 2, L = 8, M = 18, N = 32.
The outermost shell is the valence shell; its electrons are valence electrons. Valency (संयुजा) is related to them. A complete octet (or duplet for helium) means valency zero – inert gases.
Isotopes are atoms of the same element with the same atomic number but different mass numbers, e.g. C-12, C-13, C-14.
Uses of isotopes: U-235 in nuclear power, Co-60 in cancer treatment, I-131 in treating goitre, radioactive isotopes to find leaks in underground pipes.
Important terms
Atomic number (Z) – number of protons in the nucleus.
Mass number (A) – total number of protons and neutrons.
Electronic configuration – distribution of electrons in the shells of an atom.
Valence electrons – electrons in the outermost shell.
Isotopes – atoms of the same element with different mass numbers.
Formulas
Number of neutrons N = A − Z
Maximum electrons in nth shell = 2n²
1 u = 1.66 × 10⁻²⁷ kg
Valency = x (number of valence electrons) when x ≤ 4; valency = 8 − x when x ≥ 4
5 important questions
Q1. State the postulates of Bohr's atomic model.
Electrons revolve around the nucleus in stable shells of definite energy. While in a shell they do not lose energy. An electron can jump to a higher shell by absorbing energy and return by giving out energy.
Q2. The atomic number of calcium is 20 and mass number is 40. Find its protons, electrons and neutrons.
Matter can be classified in two ways: by physical state (solid, liquid, gas) and by chemical composition (element, compound, mixture). This chapter explains the particle picture of the three states, types of mixtures (solution, suspension, colloid), compounds, molecular formulae and valency.
Key points
An intermolecular force of attraction acts between particles of matter. It is strong in solids, moderate in liquids and very weak in gases.
Solids have definite shape and volume and are almost incompressible. Liquids have definite volume but no definite shape (they flow). Gases have neither definite shape nor volume and are highly compressible.
Element: only one type of atom (e.g. O₂, N₂). Compound: molecules made of two or more types of atoms joined chemically (e.g. H₂O, CO₂). Mixture: two or more elements/compounds mixed physically (e.g. air, brass).
Elements are metals, non-metals and metalloids.
Brass = copper + zinc; bronze = copper + tin (alloys are mixtures).
A part of matter having uniform composition is called a phase.
Homogeneous mixture: one phase (salt in water). Heterogeneous mixture: two or more phases (sand in water).
Solution (द्रावण): homogeneous mixture; transparent; passes through filter paper. The component in largest amount is the solvent; others are solutes; mixing them is dissolution.
Suspension: heterogeneous mixture of a liquid and solid; particles larger than 10⁻⁴ m; separated by filtration (e.g. muddy water).
Colloid: heterogeneous mixture with particles about 10⁻⁵ m, spread evenly; not separated by filtration (e.g. milk, smoke, fog).
In a compound the properties of the constituents are lost (iron + sulphur heated gives iron sulphide, which is not attracted to a magnet).
A molecular formula shows the number of atoms of each element in one molecule. It can be written from valencies by cross-multiplication.
Important terms
Phase – part of matter with uniform composition.
Solvent – the component of a solution present in largest proportion.
Solute – component(s) dissolved in the solvent.
Colloid – heterogeneous mixture in which tiny particles are evenly dispersed.
Valency – combining capacity of an atom, the number of bonds it forms.
Formulas
Cross-multiplication: element A (valency a) + element B (valency b) → formula AbBa, then simplify.
Example: Al (3) and O (2) → Al₂O₃; Ca (2) and Cl (1) → CaCl₂.
Particle size: solution < colloid (about 10⁻⁵ m) < suspension (more than 10⁻⁴ m)
5 important questions
Q1. Distinguish between homogeneous and heterogeneous mixtures.
A homogeneous mixture has one phase and uniform composition (salt solution). A heterogeneous mixture has two or more phases and non-uniform composition (sand in water).
Q2. What is a colloid? Give examples.
A heterogeneous mixture in which very small particles (about 10⁻⁵ m) are evenly dispersed and do not settle or get filtered. Examples: milk, smoke, fog.
Q3. Write the molecular formula of the compound formed by aluminium (valency 3) and chlorine (valency 1).
By cross-multiplication: AlCl₃.
Q4. Why do gases have no definite shape or volume?
The intermolecular force in gases is very weak, so particles move freely and fill all the available space.
Elements are classified into metals, non-metals and metalloids. This chapter compares the physical and chemical properties of metals and non-metals, explains noble metals and purity of gold (carat), corrosion and its prevention, and alloys.
Key points
Metals (धातू) are lustrous, hard, malleable, ductile, sonorous and good conductors of heat and electricity. They have high density and high melting points.
Exceptions among metals: mercury and gallium are liquids at room temperature; sodium and potassium are soft and can be cut with a knife; Na, K, Li are lighter than water (lithium 0.53 g/cc); lead is a poor conductor.
Silver, copper and aluminium are the best conductors of heat. Gold, silver and aluminium are highly malleable.
Non-metals are brittle, non-lustrous, bad conductors, and have low density and low melting points. They exist as solids (C, S, P), a liquid (Br₂) or gases (H₂, N₂, O₂).
Exceptions among non-metals: diamond is the hardest natural substance; graphite conducts electricity; diamond and iodine have lustre.
Metalloids (As, Si, Ge, Sb) have properties between metals and non-metals.
Metals have 1–3 valence electrons and lose them to form cations (Na → Na⁺ + e⁻). Non-metals have 4–7 valence electrons and gain electrons to form anions (Cl + e⁻ → Cl⁻).
Metal oxides are basic; non-metal oxides are acidic (CO₂ + H₂O → H₂CO₃).
Metals react with dilute acids to give salt and hydrogen gas. Na and K react with cold water; magnesium needs steam. Non-metals do not react with dilute acids.
Noble metals (gold, silver, platinum, palladium, rhodium) occur free in nature and do not react easily. 24 carat gold is 100% pure; ornaments use 22 carat (91.66%).
Corrosion: metals react with gases of the air in presence of moisture – iron gets reddish rust, copper gets a green coating, silver turns black. Prevented by oil, grease, paint, varnish or galvanising (zinc coating).
An alloy is a homogeneous mixture of metals (or metal with a non-metal). Stainless steel = iron + carbon + chromium + nickel; bronze = copper + tin. The iron pillar near Qutub Minar has not rusted in about 1500 years.
Important terms
Malleability – property of being beaten into thin sheets.
Ductility – property of being drawn into wires.
Sonority – property of producing a ringing sound when struck.
Corrosion – slow damage of a metal by reaction with air and moisture.
Alloy – homogeneous mixture of two or more metals, or a metal and a non-metal.
Formulas
Metal + oxygen → metal oxide (basic)
Metal + dilute acid → salt + hydrogen (Zn + 2HCl → ZnCl₂ + H₂)
Q1. Why are ornaments not made from 24 carat gold?
Pure gold is very soft, so ornaments would bend or break. Copper or silver is added and 22 carat gold is used.
Q2. Identify the odd term: gold, silver, iron, diamond.
Diamond – it is a non-metal (form of carbon); the others are metals.
Q3. Why are non-metal oxides called acidic?
They dissolve in water to form acids (e.g. SO₂ + H₂O → H₂SO₃) and react with bases to form salt and water.
Q4. How can corrosion be prevented?
By applying oil, grease, paint or varnish, or by coating with a non-corroding metal like zinc (galvanising). This stops air and moisture reaching the metal.
Q5. Write the uses of noble metals.
Ornaments and medals (gold, silver, platinum), medicines (silver is antibacterial), electronic devices, and catalysts (platinum, palladium).
Exam tip
Remember the exceptions (Hg, Ga, Na, K, Li, Pb, diamond, graphite) – they are asked as "odd one out" and "give reasons".
Human activities like industrialisation, growing population, mining, transport and overuse of fertilisers and pesticides are polluting the Earth. This chapter explains air, water and soil pollution – their causes, effects and prevention – along with ozone depletion, greenhouse effect, acid rain and India's pollution-control laws.
Key points
Pollution (प्रदूषण) is contamination of the natural environment that harms ecosystems. Substances that cause it are pollutants; they may be natural or man-made, degradable or non-degradable.
Air pollution: natural causes – volcanoes, earthquakes, dust storms, forest fires, microbes; man-made causes – burning fuels and waste, factories, atomic plants and blasts.
Main air pollutants: SO₂ (irritates eyes and respiratory tract), CO (reduces oxygen-carrying capacity of blood), oxides of nitrogen (lung irritation), particulate matter and dust (respiratory diseases, silicosis).
London smog (1952) and the Bhopal gas tragedy (night of 2 December 1984) are famous air-pollution disasters.
CFCs and other chemicals deplete the ozone layer, which protects us from ultraviolet rays.
CO₂, methane, nitrous oxide and CFCs trap heat; they are greenhouse gases. More of them causes global warming.
Oxides of sulphur and nitrogen mix with rain water to form acids – acid rain. It increases acidity of soil and water and damages buildings and living things.
Water pollution: sewage, industrial effluents, fertilisers, pesticides, oil spills, washing clothes, immersing offerings. Effects: diseases (hepatitis, typhoid), low dissolved oxygen, death of aquatic animals, excess algal growth.
Soil pollution: change in physical, chemical and biological properties of soil that lowers fertility. Caused by industrial waste, chemical fertilisers, insecticides, weedicides and domestic waste.
Air, water and soil pollution are linked – e.g. wet waste dumped in the open pollutes soil, then water and air.
Laws: Water (Prevention and Control of Pollution) Act 1974, Air Act 1981, Environment Protection Act 1986. Maharashtra Pollution Control Board and Central Pollution Control Board supervise them.
Important terms
Pollutant – a substance that harms the natural functioning of an ecosystem.
Acid rain – rain containing sulphuric and nitric acids formed from oxides of sulphur and nitrogen.
Greenhouse effect – trapping of the Sun's heat near the Earth by certain gases.
Global warming – rise in average temperature of the Earth.
Smog – smoke mixed with fog.
Formulas
Important dates and laws:
London smog – 5 to 9 December 1952
Bhopal gas tragedy – 2 December 1984
Water Act 1974, Air Act 1981, Environment Protection Act 1986
5 important questions
Q1. What is acid rain? Write its effects.
Oxides of sulphur and nitrogen from burning coal and fuels dissolve in rain water forming acids; this falling rain is acid rain. It makes soil and lakes acidic, kills aquatic life, damages plants and buildings, and puts heavy metals into the food chain.
Q2. Explain the relation between greenhouse effect and global warming.
Greenhouse gases trap the heat radiated by the Earth. When their amount rises, more heat is trapped and the Earth's average temperature increases – this is global warming.
Q3. Suggest four measures to prevent air pollution.
Use public transport and CNG/electric vehicles; fit filters on factory chimneys; do not burn garbage or crop waste; plant more trees; ban CFCs.
Q4. Vomiting and dysentery after eating pani puri – which pollution is this?
Water pollution (contaminated water carrying pathogens).
Q5. What are the effects of soil pollution?
Soil fertility decreases, toxic substances enter crops and the food chain, and harmful substances leach into water, increasing water pollution.
Exam tip
For "which pollution?" questions, look for the clue word: smoke/breathing = air, stomach illness = water, poor crops = soil.
This chapter deals with three disasters – earthquake, fire and landslide. It explains their causes, effects, the precautions to take, earthquake-resistant buildings, types of fire and methods of putting out fire, and how a school can prepare a disaster management plan.
Key points
An earthquake (भूकंप) is the sudden shaking of the Earth's surface. Shocks start at the focus inside the Earth; the point on the surface just above it is the epicentre, where damage is greatest.
Earthquakes are recorded by a seismograph (seismometer); their intensity is measured on the Richter scale. Roughly 12,400–14,000 earthquakes occur on Earth every year.
Causes: plate movement, volcanic eruptions, stress of big dams, mining, underground atomic tests, steam formed from water entering hot cracks.
Effects: loss of life, collapse of buildings, roads and pipelines, fires in cities, change in river courses and water table, tsunamis if under the sea.
During an earthquake: stay calm, take cover under a table or bed, or sit in a corner covering the head; outside, stay away from tall buildings, trees and poles; use stairs, not the lift; switch off main electric supply; use a torch, not candles.
Earthquake-resistant buildings are built according to Indian Standard codes like IS 1893 and IS 13920.
Classes of fire: A – solids (wood, paper); B – flammable liquids (petrol, oil); C – gases (LPG, acetylene); D – reactive metals (sodium, potassium); E – electrical.
Methods of extinguishing fire: cooling with water, smothering (cutting off air using sand, soil or foam), and removing flammable material. Electrical fires: cut power and use CO₂ extinguisher, never water.
A landslide happens when rocks and soil, weakened by water in cracks and weathering, slide down a slope. Causes: heavy rain, earthquakes, cutting trees, digging hills for roads. Malin village in Pune district was buried by a landslide.
Every school should prepare a disaster management plan with committees, a map and emergency contacts.
Important terms
Focus – the point inside the Earth where an earthquake starts.
Epicentre – the point on the Earth's surface directly above the focus.
Seismograph – instrument that records earthquake waves.
Earthquake-resistant building – building that is not damaged by ground movement up to a certain limit.
Landslide – sudden sliding of rocks and soil down a slope.
Formulas
Key facts:
Intensity of earthquake – Richter scale
Fire triangle: fuel + oxygen (air) + heat; remove any one and the fire stops.
Codes for earthquake-resistant design: IS 1893, IS 13920.
5 important questions
Q1. What precautions should be taken during an earthquake?
Do not panic or run. Take shelter under a table or bed or sit in a corner covering your head. Outside, keep away from buildings, trees and electric poles. Use stairs, not lifts. Switch off the main power and avoid candles after the quake.
Q2. Why should water not be used on a fire caused by electricity or oil?
Water conducts electricity and may cause shock. Oil is lighter than water, so it floats and the fire spreads. Use sand, foam or a CO₂ extinguisher.
Q3. Write the causes of landslides.
Heavy rain, earthquakes, floods, cutting of trees causing soil erosion, and digging hills while building roads.
Q4. Write any four effects of an earthquake.
Loss of life and property, damage to roads, bridges and pipelines, fires in cities, tsunami in coastal areas, change in water table and river courses.
Q5. What are the three methods of extinguishing fire?
Cooling with water, smothering by cutting off air (sand, soil, foam), and keeping flammable materials away from the fire.
Exam tip
Learn the five classes of fire with one example each and the correct extinguishing method.
The cell is the structural and functional unit of life. This chapter describes the parts of plant and animal cells – cell wall, plasma membrane, cytoplasm – how substances move in and out (diffusion, osmosis, endocytosis), and the structure and function of each cell organelle such as nucleus, ER, Golgi complex, lysosome, mitochondria, plastids and vacuoles.
Key points
Plant cells have a cell wall, plastids and a large central vacuole; animal cells lack a cell wall and plastids and have small vacuoles.
Cell wall: strong, elastic coat made mainly of cellulose and pectin; supports and protects the cell. Found in plants, algae and fungi.
Plasma membrane: thin, elastic covering made of two layers of phospholipids with proteins. It is selectively permeable and helps maintain homeostasis.
Endocytosis (taking in) and exocytosis (throwing out) use energy; diffusion and osmosis do not.
Osmosis (परासरण) is movement of water from a region of more water to less water through a selectively permeable membrane.
In a hypotonic solution water enters the cell (endosmosis – raisins swell). In a hypertonic solution water leaves (exosmosis); the cytoplasm shrinks – plasmolysis. In an isotonic solution there is no net movement.
Cytoplasm is the sticky fluid between the membrane and nucleus; it is the site of many chemical reactions.
Nucleus controls all activities of the cell and carries chromosomes (DNA).
Endoplasmic reticulum transports substances; rough ER has ribosomes, smooth ER does not. Ribosomes make proteins.
Golgi complex (discovered by Camillo Golgi, Nobel Prize 1906 with Santiago Cajal) modifies, packs and dispatches proteins and other substances.
Lysosomes contain digestive enzymes; they burst and digest old or damaged cells, so they are called suicide bags.
Mitochondria produce energy (ATP) – the powerhouse of the cell. Mitochondria and plastids have their own DNA and ribosomes.
Plastids are only in plant cells: chloroplasts (green, photosynthesis), chromoplasts (colours of flowers and fruits), leucoplasts (store food).
Vacuoles store water, food and waste; in plants they keep the cell turgid.
Important terms
Organelle – specialised part of a cell with a specific function.
Selectively permeable membrane – membrane that lets some substances pass but not others.
Plasmolysis – shrinking of cytoplasm when a cell loses water in a hypertonic solution.
Homeostasis – keeping the inner environment of the cell steady.
Cytosol – liquid part of cytoplasm excluding organelles.
Formulas
Quick table:
Organelle
Function
Nucleus
Controls the cell
Mitochondria
Energy production
Ribosome
Protein synthesis
Golgi complex
Packing and dispatch
Lysosome
Digestion, suicide bag
Chloroplast
Photosynthesis
5 important questions
Q1. Why are lysosomes called suicide bags?
When a cell becomes old or damaged, lysosomes burst and their enzymes digest the cell itself.
Q2. What happens when raisins are kept in water and then in sugar solution?
In water (hypotonic) water enters by endosmosis and raisins swell. In thick sugar solution (hypertonic) water leaves by exosmosis and they shrink.
Q3. Why are mitochondria called the powerhouse of the cell?
They break down food (cellular respiration) and release energy in the form of ATP used by the cell.
Q4. Write three differences between plant cell and animal cell.
Plant cells have a cell wall, plastids and a large central vacuole; animal cells have no cell wall, no plastids and small vacuoles.
Q5. Why do wooden doors fit tightly in the rainy season?
The wood absorbs water by imbibition/osmosis from moist air and swells.
Exam tip
Practise labelled diagrams of the plant cell, animal cell and mitochondrion – they carry easy marks.
A group of organs working together to do a specific job is an organ system. This chapter studies two vital systems of the human body – the respiratory system (breathing and cellular respiration) and the circulatory system (heart, blood vessels, blood, blood groups and blood pressure).
Key points
Respiration has three steps: external respiration (breathing in and out through the lungs), internal respiration (gas exchange between blood and tissue fluid) and cellular respiration (oxidation of glucose in cells to release energy as ATP).
Respiratory organs: nose (filters air with hair and mucus) → pharynx → wind pipe (trachea) with sound box (larynx) → bronchi → lungs.
A lid (epiglottis) closes the wind pipe while swallowing, so we should not talk while eating.
Lungs are covered by a double layer called pleura and are made of tiny air sacs called alveoli, which give a huge surface for gas exchange.
Haemoglobin, an iron-containing protein in RBCs, carries oxygen.
The diaphragm is a muscular partition between chest and abdomen. When ribs rise and the diaphragm lowers, pressure on lungs falls and air rushes in (inhalation).
The heart (हृदय) is a muscular organ with four chambers; it beats about 72 times per minute, pumping about 75 ml of blood per beat. William Harvey described blood circulation in 1628.
Arteries carry blood away from the heart; veins carry blood to the heart; capillaries connect them.
Blood plasma is 90–92% water. One cubic millimetre of blood has 50–60 lakh RBCs.
ABO blood groups were discovered by Karl Landsteiner in 1900. With the Rh factor there are eight blood groups. O is the universal donor, AB the universal recipient.
Blood pressure is the pressure of blood on the walls of arteries; it is measured with a sphygmomanometer. Systolic (during contraction) and diastolic (during relaxation); normal about 120/80 mm of Hg.
A healthy person above 18 years can donate about 350 ml blood; the fluid part is restored within 24 hours.
Important terms
Organ system – group of organs working together for a specific function.
Alveoli – tiny air sacs in lungs where gas exchange happens.
Cellular respiration – slow oxidation of food in cells to release energy.
Blood pressure – pressure exerted by blood on the walls of arteries.
Normal BP: systolic 90–119 mm Hg, diastolic 60–79 mm Hg; hypertension stage I: 140–159 / 90–99
5 important questions
Q1. Explain the three steps of respiration.
External respiration: inhaling and exhaling through lungs. Internal respiration: O₂ moves from blood to tissue fluid and CO₂ from tissue fluid to blood. Cellular respiration: glucose is oxidised in cells with oxygen, giving CO₂, water and energy (ATP).
Q2. Why is a person with blood group O called a universal donor?
Blood group O has no A or B antigens on RBCs, so it can be given to people of any blood group.
Q3. What is systolic and diastolic pressure?
Pressure recorded during contraction of the heart is systolic; during relaxation it is diastolic. Normal is about 120/80 mm of Hg.
Q4. How is the structure of lungs suited for gas exchange?
Lungs have millions of alveoli with very thin walls surrounded by capillaries, giving a large surface area for quick exchange of O₂ and CO₂.
Q5. Why should we not talk while eating?
While talking the wind pipe is open, so food particles may enter it and cause choking.
Exam tip
Draw and label the respiratory system and the internal structure of the heart neatly; know the direction of blood flow.
Sour substances contain acids and bitter, slippery substances contain bases. This chapter explains acids and bases, how indicators (laboratory and natural) identify them, natural acids in food, properties and uses of acids and bases, and neutralisation – the reaction of an acid with a base to form salt and water.
Key points
Acids (आम्ल) taste sour, are soluble in water and are corrosive. Acids in food are natural (organic) acids and are weak.
Strong mineral acids: sulphuric acid (H₂SO₄), hydrochloric acid (HCl), nitric acid (HNO₃). They burn the skin.
To dilute a concentrated acid, add the acid slowly to water – never add water to concentrated acid.
Bases taste bitter/astringent and feel slippery, e.g. lime water Ca(OH)₂, baking soda NaHCO₃, caustic soda NaOH, soap. Bases soluble in water are alkalis.
Water is neutral – neither acidic nor basic.
Indicators are substances that change colour in acid or base. Litmus paper is made from lichen extract.
Natural indicators: turmeric, hibiscus, rose, red cabbage. Turmeric turns red with soap (alkali).
In a chemical change the composition of a substance changes and new substances with new properties are formed. This chapter explains how to recognise chemical changes, how to write word and chemical equations, natural and man-made chemical changes, and how atoms join by ionic and covalent bonds.
Key points
Signs of a chemical change: change in colour or taste, gas bubbles, sound, heat or light, a solid disappearing or forming.
Adding baking soda to lemon juice gives bubbles of CO₂ and the sour taste reduces – a chemical change (neutralisation).
In an equation, reactants are written on the left, products on the right, with an arrow pointing to the products.
Natural chemical changes: respiration, photosynthesis, rusting, ripening of fruit. Man-made: cooking, making lime, burning fuels.
Respiration: glucose + oxygen → carbon dioxide + water + energy. CO₂ turns lime water milky.
Photosynthesis: carbon dioxide + water → glucose + oxygen (in sunlight, in green plants).
During a chemical reaction old bonds break and new bonds form.
Noble gases have a complete octet/duplet, so they do not form bonds. Other atoms form bonds to complete their octet.
Ionic (electrovalent) bond (आयनिक बंध): formed by transfer of electrons; the oppositely charged ions attract each other. Example: Na (2,8,1) gives one electron to Cl (2,8,7) → Na⁺Cl⁻. MgCl₂: Mg gives 2 electrons, one to each Cl.
Covalent bond: formed by sharing a pair of valence electrons between two atoms. Example: H₂ (H–H), Cl₂, H₂O, CH₄; O₂ has a double bond, N₂ a triple bond.
Number of bonds an atom forms = its valency.
Important terms
Reactants – substances that take part in a reaction.
Products – new substances formed in a reaction.
Chemical equation – reaction written using chemical formulae.
Ionic bond – bond formed by electrostatic attraction between oppositely charged ions after electron transfer.
Covalent bond – bond formed by sharing of valence electrons.
Q1. Classify as physical or chemical change: ripening of mango, melting of ice, darkening of cut potato, bursting of balloon.
Chemical – ripening of mango, darkening of cut potato. Physical – melting of ice, bursting of balloon.
Q2. Explain the formation of sodium chloride.
Sodium (2,8,1) loses one electron to become Na⁺ (2,8). Chlorine (2,8,7) gains it to become Cl⁻ (2,8,8). The opposite ions attract and form an ionic bond.
Q3. What is a covalent bond? Explain with H₂.
A bond formed by sharing valence electrons. Each hydrogen atom has one electron; both share them as a pair, so each gets a complete duplet: H–H.
Q4. Why do noble gases not form chemical bonds?
Their valence shell already has a complete octet (or duplet for helium), so they have no tendency to lose, gain or share electrons.
Q5. What happens when you blow into lime water? Write the equation.
It turns milky because CO₂ forms insoluble calcium carbonate: Ca(OH)₂ + CO₂ → CaCO₃ + H₂O.
Exam tip
Practise electron-dot or shell diagrams for NaCl, MgCl₂, H₂, Cl₂ and H₂O – they are frequently asked.
Heat is a form of energy that flows from a hotter object to a colder one. This chapter covers sources of heat, the difference between heat and temperature, temperature scales and their conversion, thermometers, specific heat and the calorimeter, and the expansion of solids, liquids and gases on heating.
Key points
SI unit of heat is the joule (J); CGS unit is the calorie. 1 calorie = 4.18 J. One calorie is the heat needed to raise the temperature of 1 g of water by 1 °C.
Sources of heat: Sun (nuclear fusion of hydrogen into helium), Earth (geothermal energy), chemical energy (burning fuels), electricity, atomic energy (fission of uranium, thorium) and air.
Our sense of hot and cold is relative, so we use a thermometer to measure temperature.
Heat is the total kinetic energy of the atoms of a substance; temperature (तापमान) is related to their average kinetic energy.
Temperature scales: Celsius (°C), Fahrenheit (°F) and Kelvin (K). Water freezes at 0 °C = 32 °F = 273.15 K and boils at 100 °C = 212 °F = 373.15 K.
Celsius and Fahrenheit readings are equal at −40°.
Clinical thermometer measures body temperature (normal 37 °C); maximum–minimum thermometer records the highest and lowest temperature of the day.
Specific heat is the heat needed to raise the temperature of unit mass of a substance by 1 °C. Water has a high specific heat (1 cal/g °C).
A calorimeter is used to measure heat exchanged; heat lost by the hot object = heat gained by water and calorimeter.
Most substances expand on heating and contract on cooling. Solids show linear, areal and volume expansion; different materials have different expansion coefficients.
Liquids expand more than solids, and gases expand the most. Gaps are left in railway tracks and bridges for expansion.
Important terms
Temperature – measure of how hot or cold an object is.
Specific heat – heat required to raise the temperature of 1 kg (or 1 g) of a substance by 1 °C.
Calorimeter – device used to measure heat.
Coefficient of linear expansion – change in length per unit length for 1 °C rise in temperature.
Geothermal energy – heat from inside the Earth.
Formulas
(F − 32)/9 = C/5
K = C + 273.15
Heat Q = m × c × ΔT (m = mass, c = specific heat, ΔT = change in temperature)
For water: heat (cal) = mass (g) × change in temperature (°C)
Linear expansion: l₂ − l₁ = λ × l₁ × ΔT, where λ = coefficient of linear expansion
5 important questions
Q1. How much heat is needed to raise the temperature of 1.5 kg water from 15 °C to 45 °C?
Mass = 1500 g, ΔT = 30 °C. Heat = 1500 × 30 = 45,000 cal = 45,000 × 4.18 = 1,88,100 J.
Q2. Convert 68 °F into Celsius and Kelvin.
(68 − 32)/9 = C/5 → 36/9 = C/5 → C = 20 °C. K = 20 + 273.15 = 293.15 K.
Q3. At what temperature are Celsius and Fahrenheit readings equal?
Put F = C: (C − 32)/9 = C/5 → 5C − 160 = 9C → C = −40. So at −40°.
Q4. Distinguish between heat and temperature.
Heat is a form of energy, the total kinetic energy of atoms, measured in joule. Temperature shows hotness, related to the average kinetic energy, measured in °C, °F or K.
Q5. Why are gaps left between rails of a railway track?
In summer the rails expand. Gaps give room for expansion; otherwise the rails would bend.
Exam tip
Remember both conversion formulas and always change kg to g when using calories.
Sound is produced by vibrating objects and needs a medium to travel. This chapter explains how a tuning fork produces compressions and rarefactions in air, why sound cannot travel in vacuum, the frequency of sound, how the human voice box and musical instruments produce sound, and how a loudspeaker works.
Key points
Every sound comes from a vibrating object. A tuning fork has two prongs and a stem; when struck, the prongs vibrate.
When the prongs move outward, air near them is pushed together – a region of high pressure and density called compression. When they move inward, a region of low pressure and density is formed – rarefaction.
A series of compressions and rarefactions moving away from the source is a sound wave. The air itself does not travel; the state of compression and rarefaction (energy) travels.
When sound waves reach the ear, the eardrum vibrates and the brain senses sound.
Sound needs a material medium (solid, liquid or gas). In the bell-jar experiment, the ringing becomes fainter as air is pumped out. Astronauts on the Moon cannot hear each other directly because there is no air.
Frequency (वारंवारता) is the number of vibrations (cycles) per second; its unit is hertz (Hz). A tuning fork may be marked 512 Hz. The musical note "sa" is about 256 Hz.
Humans hear sounds roughly between 20 Hz and 20,000 Hz.
In the human larynx, two vocal cords vibrate when air from the lungs passes through the slit between them. Muscles make the cords tight or loose to change the voice.
In string instruments, frequency is changed by changing tension or vibrating length of the string. Wind instruments vibrate an air column; percussion instruments vibrate a stretched membrane or metal.
A loudspeaker has a coil and magnet; varying current makes the coil and its cone vibrate to produce sound.
Loudness is measured in decibels (dB). Sound around 100 dB or more can be harmful – this is noise pollution.
Important terms
Compression – region of a sound wave with high pressure and density.
Rarefaction – region of a sound wave with low pressure and density.
Frequency – number of vibrations per second; unit hertz (Hz).
Medium – material through which sound travels.
Decibel (dB) – unit for measuring loudness.
Formulas
Frequency = number of vibrations / time (in seconds); 1 Hz = 1 vibration per second
Audible range of humans: 20 Hz to 20,000 Hz
5 important questions
Q1. Explain how a tuning fork produces sound.
When struck, its prongs vibrate. Moving outward they compress the air (compression); moving inward they create rarefaction. These alternate regions travel through air as a sound wave.
Q2. Describe an experiment to show that sound needs a medium.
Put a ringing electric bell in an airtight bell jar connected to a vacuum pump. As air is pumped out, the sound becomes fainter and almost stops, though the bell is still seen vibrating. So sound needs a medium.
Q3. Why can't astronauts on the Moon hear each other directly?
The Moon has no atmosphere, so there is no medium for sound to travel.
Q4. What is frequency? A body makes 1024 vibrations in 2 seconds. Find its frequency.
Frequency is vibrations per second. Frequency = 1024 / 2 = 512 Hz.
Q5. How is human voice produced?
Air from the lungs passes through the narrow slit between the vocal cords in the larynx, making them vibrate. Changing the tension of the cords changes the voice.
Exam tip
Draw the tuning fork diagram showing compression (C) and rarefaction (R) labels – it is often asked.
We see objects when light from them enters our eyes. This chapter explains reflection of light, the terms used (incident ray, normal, angles), the laws of reflection and their verification, regular and irregular reflection, and devices like the kaleidoscope and periscope that use reflection.
Key points
When light falls on a surface and turns back, it is called reflection of light (प्रकाशाचे परावर्तन).
The ray falling on the surface is the incident ray; the point where it falls is the point of incidence; the ray going back is the reflected ray.
The normal is a line perpendicular to the mirror at the point of incidence.
Angle of incidence (i) is between the incident ray and the normal; angle of reflection (r) is between the reflected ray and the normal.
Laws of reflection: (1) angle of reflection = angle of incidence; (2) incident ray, reflected ray and normal lie in the same plane; (3) incident ray and reflected ray are on opposite sides of the normal.
The laws can be verified with a mirror, pins and protractor at angles like 30°, 45° and 60°.
Regular reflection happens from a smooth, polished surface; parallel rays stay parallel after reflection – a clear image forms.
Irregular (diffused) reflection happens from a rough surface; parallel rays scatter in different directions. Laws of reflection are still obeyed at every point.
We see the Moon's image in water because moonlight (reflected sunlight) is again reflected by water.
A kaleidoscope uses three mirrors arranged as a triangle to form many images and beautiful designs that rarely repeat; it is used by wallpaper and cloth designers.
A periscope uses two parallel plane mirrors at 45°; it helps see objects above obstacles, e.g. from a submarine.
Important terms
Incident ray – ray of light falling on a surface.
Reflected ray – ray that returns from the surface.
Normal – perpendicular drawn to the surface at the point of incidence.
Regular reflection – reflection from a smooth surface.
Irregular reflection – reflection from a rough surface.
Formulas
∠i = ∠r
If the angle between the incident ray and the mirror is x, then ∠i = 90° − x.
Angle between incident and reflected rays = ∠i + ∠r = 2∠i
5 important questions
Q1. State the laws of reflection of light.
The angle of incidence equals the angle of reflection; the incident ray, reflected ray and normal lie in the same plane; and the incident and reflected rays lie on opposite sides of the normal.
Q2. The reflected ray makes 60° with the normal. What angle does the incident ray make with the normal?
By the first law, ∠i = ∠r = 60°.
Q3. An incident ray makes 35° with the mirror surface. Find the angle of reflection.
∠i = 90° − 35° = 55°. So ∠r = 55°.
Q4. Distinguish between regular and irregular reflection.
Regular reflection is from a smooth surface; parallel rays reflect parallel and a clear image forms. Irregular reflection is from a rough surface; reflected rays scatter, so no clear image forms.
Q5. What is a periscope? Where is it used?
A device with two parallel plane mirrors at 45° in a tube that lets us see objects above or around an obstacle. It is used in submarines and in trenches.
Exam tip
Always measure angles from the normal, not from the mirror – this is the most common mistake.
Materials made in factories by processing natural materials are called man-made materials, e.g. plastic, thermocol, glass and artificial fibres. This chapter explains types of plastic and their uses, the effect of plastic and thermocol on the environment, the 4 R's, and types of glass and their uses.
Key points
Plastic is a man-made material made of organic polymers that shows plasticity (can be moulded).
Thermoplastic softens on heating and can be moulded again and again, e.g. polythene, PVC, polystyrene, polypropylene.
Thermosetting plastic, once set in a mould, cannot be reshaped by heating, e.g. bakelite, melamine, polyurethane, polyester. Used for electric switches and cooker handles.
Properties of plastic: does not corrode or rust, light in weight, poor conductor of heat and electricity, can be made in any colour or shape, not affected by water or humidity.
Plastic is non-biodegradable; it takes thousands of years to decompose, while wood takes 10–15 years. Plastic waste blocks drains, harms animals and pollutes soil and water.
Follow the 4 R's: Reduce, Reuse, Recycle, Recover.
Thermocol is a form of polystyrene. It is non-biodegradable; burning it releases poisonous gases; styrene in it may cause cancer, and reheating food in thermocol is harmful.
Glass (काच) is made by heating sand (silica) with soda and limestone at about 1500 °C (soda-lime glass).
Types of glass: silica glass (little expansion – lab glassware), borosilicate glass (heat-resistant – medicine bottles, lab ware), alkali silicate glass ("water glass", soluble in water), lead glass (decorative, shiny), optical glass (lenses), coloured glass (metal oxides added).
Glass is non-degradable but can be recycled; making glass releases gases like CO₂ and SO₂.
Important terms
Man-made material – material produced in factories from natural raw materials.
Polymer – large molecule made by joining many small units.
Thermoplastic – plastic that can be softened and reshaped by heating.
Thermosetting plastic – plastic that cannot be reshaped by heating once set.
Biodegradable – can be broken down by microbes in nature.
Formulas
Key facts:
Raw materials of glass: sand (SiO₂) + soda (Na₂CO₃) + limestone (CaCO₃)
4 R's: Reduce – Reuse – Recycle – Recover
5 important questions
Q1. Distinguish between thermoplastic and thermosetting plastic.
Thermoplastic softens on heating and can be moulded again (polythene, PVC). Thermosetting plastic cannot be reshaped by heating once it is set (bakelite, melamine).
Q2. Why are electric switches and cooker handles made of bakelite?
Bakelite is a thermosetting plastic; it is a bad conductor of heat and electricity and does not soften on heating.
Q3. Why are plastic tanks used to store chemicals?
Plastic does not corrode or react with most chemicals and is not affected by moisture.
Q4. What are the harmful effects of thermocol?
It is non-biodegradable; burning it releases poisonous gases; styrene from it may cause cancer and can mix with hot food; factory workers may get eye, skin and breathing problems.
Q5. Which type of glass is used for laboratory apparatus and why?
Borosilicate (or silica) glass, because it expands very little on heating and does not crack easily.
Exam tip
Make a two-column list of plastics and their uses – match-the-pairs questions come straight from it.
Living (biotic) and non-living (abiotic) factors constantly interact; the structure formed by these relationships is an ecosystem. This chapter explains components of an ecosystem, producers, consumers and decomposers, niche, interactions and flow of energy, land and aquatic biomes, and how human activities damage ecosystems.
Key points
Biotic factors are living (plants, animals, microbes); abiotic factors are non-living (air, water, soil, sunlight, temperature, minerals).
An ecosystem (परिसंस्था) is formed by biotic and abiotic factors and their interactions. It is also called a biotic community.
Abiotic factors are physical (air, water, sunlight, soil) and chemical – inorganic (H, O, N, Ca, Fe, K, Na) and organic (carbohydrates, proteins, lipids).
Producers (autotrophs) – green plants make food by photosynthesis. Consumers (heterotrophs) – primary (herbivores), secondary and tertiary consumers. Decomposers – microbes that convert dead bodies into inorganic substances, recycling nutrients.
Abiotic factors decide which organisms can survive in a place and in what numbers; organisms in turn change the abiotic factors.
The position and role of an organism in relation to others is its niche, e.g. a sunflower gives oxygen and provides food and shelter to insects.
Energy from the Sun enters through producers and passes to consumers; some is lost as heat at each step, while minerals are recycled by decomposers.
Large areas with similar climate and abiotic factors are biomes – land biomes (grassland, forest, desert) and aquatic biomes (fresh water and marine). The Earth itself is one vast ecosystem.
Decomposers act at every level and return minerals to the soil.
5 important questions
Q1. What is an ecosystem? Name its components.
The structure formed by interactions between living organisms and their non-living surroundings. Components: abiotic (physical and chemical factors) and biotic (producers, consumers, decomposers).
Q2. Why are microbes called decomposers?
They convert organic substances in dead plants and animals into inorganic substances, which go back into the soil and are reused by plants.
Q3. What is a niche? Give an example.
The position and role of an organism in its ecosystem. A sunflower plant releases oxygen and gives food and shelter to ants and honey-bees.
Q4. Which biotic factors are affected by building a dam?
Land plants and animals in the flooded area are destroyed or displaced; fish movement in the river is blocked; organisms of running water are affected downstream.
Q5. Give an example of a food chain in a grassland.
Stars are gigantic spheres of hot gas that are born, live and die. This chapter describes galaxies, properties of the Sun as an ordinary star, how stars are born from clouds of gas and dust, how they produce energy, and their end stages – white dwarf, neutron star or black hole – depending on their mass.
Key points
A galaxy is a collection of billions of stars, their planetary systems and interstellar clouds. Galaxies are spiral, elliptical or irregular. Our galaxy, the Milky Way (Mandakini), is spiral and has about 10¹¹ stars.
Telescopes on Earth and on satellites above the atmosphere give us information about the universe.
About 4000 stars are visible to the naked eye. The Sun is an ordinary star; it looks biggest only because it is nearest.
The Sun: 72% hydrogen, 26% helium, 2% heavier elements; mass 2 × 10³⁰ kg (about 3.3 lakh times Earth); radius 6,95,700 km (about 100 times Earth); surface temperature 5800 K; centre 1.5 × 10⁷ K; age 4.5 × 10⁹ years.
Stars' masses are measured in units of the Sun's mass (M☉); they range from about 1/10 to 100 times the Sun's mass.
Birth of stars: a cloud of gas and dust contracts due to gravity; its centre becomes hot and dense, nuclear fusion of hydrogen into helium begins and a star (तारा) is born.
Stars are classified into three groups by mass, and each group has the same path of evolution.
Stars of mass less than 8 M☉: become a red giant, then a white dwarf. A white dwarf is about the size of the Earth and extremely dense – one spoonful weighs a few tonnes. When the Sun becomes a red giant it will swallow Mercury and Venus.
Stars of mass 8 to 25 M☉: red giant → supergiant (size up to 1000 times) → supernova explosion → neutron star, made entirely of neutrons.
Stars of mass more than 25 M☉: after supernova they become a black hole, whose gravity is so strong that even light cannot escape.
A supernova seen in 1054 AD left behind the Crab Nebula.
Important terms
Galaxy – huge group of billions of stars, gas and dust.
Nuclear fusion – joining of light nuclei (hydrogen) to form a heavier nucleus (helium), releasing energy.
Red giant – stage in which a star swells and its surface cools and looks red.
White dwarf – small, very dense, white end stage of a low-mass star.
Supernova – huge explosion at the end of a massive star's life.
Black hole – end stage of the most massive stars; nothing, not even light, escapes it.
Formulas
Key facts:
Less than 8 M☉ → red giant → white dwarf
8 to 25 M☉ → red giant → supergiant → supernova → neutron star
More than 25 M☉ → supergiant → supernova → black hole
5 important questions
Q1. Why is the Sun called an ordinary star?
Billions of stars have greater or smaller mass, size and temperature than the Sun. It only looks big and bright because it is the nearest star to us.
Q2. How is a star born?
A huge cloud of gas and dust in a galaxy contracts under its own gravity. Its centre becomes very hot and dense, hydrogen fusion begins, and energy is produced – a star is born.
Q3. What will happen to the Sun at the end of its life?
It will become a red giant, swelling enough to swallow Mercury and Venus, and finally become a white dwarf.
Q4. What is a neutron star?
The end stage of a star of 8 to 25 solar masses after a supernova; it is made completely of neutrons whose pressure balances gravity for ever.
Q5. Name the three types of galaxies. To which type does our galaxy belong?
Spiral, elliptical and irregular. The Milky Way is a spiral galaxy.
Exam tip
Draw the flow chart of a star's evolution by mass – it answers several questions at once.