Chemistry: chemical reactions, acids, bases and salts
Why is respiration considered an exothermic reaction?
Because glucose is oxidised in our cells and the process releases energy, which the body uses for its activities. Any reaction that gives out heat or energy is exothermic - respiration is simply a slow, controlled oxidation.
Balance the equation: Fe + H₂O → Fe₃O₄ + H₂
3Fe + 4H₂O → Fe₃O₄ + 4H₂. Balance the metal first (3 Fe), then oxygen from water (4 O needs 4 H₂O), then check hydrogen (8 H on the left, so 4 H₂ on the right).
Why does the colour of copper sulphate solution change when an iron nail is dipped into it?
Iron is more reactive than copper, so it displaces copper from the solution. The blue copper sulphate turns pale green as iron sulphate forms, and a reddish-brown copper deposit appears on the nail. This is a displacement reaction and also a redox reaction.
Why does tooth decay begin when the pH of the mouth falls below 5.5?
Below that pH, the acid produced by bacteria acting on sugar starts dissolving the calcium phosphate of tooth enamel. Using a mildly basic toothpaste neutralises the excess acid and stops the process.
Give the chemical formula and one use each of baking soda, washing soda and plaster of Paris.
Baking soda is NaHCO₃, used in baking and as an antacid. Washing soda is Na₂CO₃·10H₂O, used for removing hardness from water. Plaster of Paris is CaSO₄·½H₂O, used for setting fractured bones.
Why is plaster of Paris stored in a moisture-proof container?
Because it reacts with water and sets into a hard solid mass of gypsum (CaSO₄·2H₂O). Once it has absorbed moisture it can no longer be used for casting.
What are amphoteric oxides? Give two examples.
Oxides that react with both acids and bases to form salts and water. Aluminium oxide (Al₂O₃) and zinc oxide (ZnO) are the standard examples.
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Chemistry: metals, non-metals and carbon compounds
Why does carbon form a very large number of compounds?
Because of tetravalency and catenation. Carbon has four valence electrons, so it forms four strong covalent bonds, and it can bond to other carbon atoms in chains, branches and rings - giving an enormous number of possible structures.
Why does carbon form covalent rather than ionic bonds?
Losing four electrons would need very high energy, and gaining four would leave a small nucleus holding eight electrons - both are unfavourable. Sharing electrons achieves a stable octet at a much lower energy cost.
Distinguish between saturated and unsaturated hydrocarbons, including how each burns.
Saturated hydrocarbons (alkanes) have only single carbon-carbon bonds and burn with a clean blue flame. Unsaturated hydrocarbons (alkenes and alkynes) contain double or triple bonds and burn with a yellow sooty flame because of incomplete combustion.
Why do soaps not work effectively in hard water?
Hard water contains calcium and magnesium ions, which react with soap to form an insoluble precipitate called scum. Soap is consumed forming scum instead of micelles, so cleaning fails. Detergents work because their calcium and magnesium salts remain soluble.
Explain the cleansing action of soap.
A soap molecule has a hydrophobic hydrocarbon tail and a hydrophilic ionic head. Tails dissolve into an oil droplet while heads stay in the water, forming a micelle that keeps the dirt suspended so it can be rinsed away.
Why is a layer of copper carbonate seen on copper vessels, and how is it removed?
Moist air corrodes copper to form green basic copper carbonate. Acidic substances such as lemon or tamarind dissolve it, which is why they restore the shine on copper utensils.
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Biology: life processes, control and coordination
Why is double circulation necessary in humans?
Because blood passes through the heart twice in one complete cycle, which keeps oxygenated and deoxygenated blood separate. Warm-blooded animals need this efficiency to meet their high energy demand for maintaining body temperature.
What are the two products of anaerobic respiration in yeast and in human muscles?
In yeast, glucose breaks down to ethanol and carbon dioxide. In our muscle cells during vigorous exercise, it forms lactic acid, whose accumulation causes cramps.
How is the small intestine adapted for absorption?
It is long and internally folded into villi, which enormously increase the surface area. Villi are richly supplied with blood vessels that carry absorbed food to every cell.
What is the role of the stomata, and how do they open and close?
Stomata allow gaseous exchange and transpiration. When guard cells take in water they swell and become curved, opening the pore; when they lose water they shrink and the pore closes.
Draw and describe the path of a reflex action.
Receptor to sensory neuron to spinal cord (where the relay neuron connects) to motor neuron to effector muscle. The response is produced in the spinal cord without waiting for the brain, which is why reflexes are fast.
Name one plant hormone each for growth, shoot elongation, cell division and growth inhibition.
Auxin promotes growth and directs phototropism; gibberellin causes stem elongation; cytokinin promotes cell division; abscisic acid inhibits growth and causes wilting and stomatal closure.
Why is iodine necessary in our diet?
The thyroid gland needs iodine to make thyroxine, which regulates carbohydrate, protein and fat metabolism. A deficiency causes goitre - a visible swelling of the thyroid gland.
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Biology: reproduction, heredity and environment
Name the mode of asexual reproduction in Amoeba, Hydra, Spirogyra, Planaria and Rhizopus.
Amoeba - binary fission; Hydra - budding; Spirogyra - fragmentation; Planaria - regeneration; Rhizopus - spore formation. These five are asked almost every year.
How is sex determined in human beings?
All eggs carry an X chromosome. A sperm carries either X or Y, so a fertilising X-sperm produces a girl (XX) and a Y-sperm produces a boy (XY). The father's contribution therefore determines the sex of the child.
In Mendel's experiments, what are the F₂ ratios for a monohybrid and a dihybrid cross?
A monohybrid cross gives a 3:1 phenotypic ratio (1:2:1 genotypic). A dihybrid cross gives 9:3:3:1, which demonstrates that the two traits are inherited independently.
Distinguish between acquired and inherited traits with one example each.
Acquired traits arise during an individual's lifetime and are not passed on, because they do not change the DNA of germ cells - for example, learning to swim. Inherited traits are encoded in genes and passed to offspring, for example eye colour.
Why is the number of trophic levels in a food chain limited to three or four?
Because only about 10% of the energy at one trophic level transfers to the next. After three or four levels, too little energy remains to support another population.
What is biomagnification?
The progressive accumulation of non-biodegradable substances such as pesticides at successively higher trophic levels, since they cannot be broken down or excreted. Top consumers, including humans, therefore carry the highest concentrations.
Why is the depletion of the ozone layer a cause for concern, and what caused it?
Ozone absorbs harmful ultraviolet radiation; its depletion increases skin cancer and eye damage. Chlorofluorocarbons (CFCs) used in refrigerants and sprays were the main cause, which is why their production has been restricted internationally.
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Physics: light and the human eye
This unit supplies the paper's most reliable numericals. Learn the sign convention first - distances measured against the direction of incident light are negative - because almost every lost mark here is a sign error rather than an arithmetic one.
- ✓Mirror formula: 1/v + 1/u = 1/f, with magnification m = −v/u.
- ✓Lens formula: 1/v − 1/u = 1/f, with magnification m = v/u.
- ✓Power of a lens: P = 1/f, with f in metres, measured in dioptres. Convex lenses have positive power, concave negative.
- ✓Refractive index: n = c/v = sin i / sin r.
An object is placed 15 cm from a concave mirror of focal length 10 cm. Find the image position and magnification.
Using u = −15 cm and f = −10 cm: 1/v = 1/f − 1/u = −1/10 + 1/15 = −1/30, so v = −30 cm - a real image 30 cm in front of the mirror. m = −v/u = −(−30)/(−15) = −2, so the image is inverted and twice the size.
A convex lens of focal length 20 cm forms an image of an object placed 30 cm away. Find the image distance and nature.
With u = −30 cm and f = +20 cm: 1/v = 1/f + 1/u = 1/20 − 1/30 = 1/60, so v = +60 cm. m = v/u = 60/(−30) = −2, so the image is real, inverted and magnified twice.
Find the power of a lens of focal length 25 cm.
P = 1/f with f in metres = 1/0.25 = +4 D. Always convert centimetres to metres first - this is the most common slip in the chapter.
Why does the sky appear blue?
Because air molecules scatter shorter wavelengths much more strongly than longer ones, and blue light is scattered most among the visible wavelengths reaching us. This scattered blue light reaches our eyes from all directions.
Why does the sun appear reddish at sunrise and sunset?
Near the horizon, sunlight travels a much longer path through the atmosphere, so most of the blue light is scattered away before reaching us and mainly the longer red wavelengths get through.
Why do stars twinkle while planets do not?
Stars are effectively point sources, so atmospheric refraction keeps changing the apparent brightness of the light reaching us. Planets are extended sources - a collection of many point sources whose variations average out, so the light appears steady.
Name the defects of vision corrected by concave and convex lenses.
Myopia (short-sightedness) is corrected by a concave lens of suitable power; hypermetropia (long-sightedness) by a convex lens. Presbyopia, in which both near and distant vision weaken with age, needs bifocal lenses.
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Physics: electricity and magnetic effects
- ✓Ohm's law: V = IR. Resistance of a conductor: R = ρl/A.
- ✓Series: R_s = R₁ + R₂ + R₃. Parallel: 1/R_p = 1/R₁ + 1/R₂ + 1/R₃.
- ✓Power: P = VI = I²R = V²/R. Energy in commercial units: 1 kWh = 1 unit.
- ✓Heating effect (Joule's law): H = I²Rt.
A potential difference of 12 V is applied across a 4 Ω resistor. Find the current and the power dissipated.
I = V/R = 12/4 = 3 A, and P = VI = 12 × 3 = 36 W. You could also use P = V²/R = 144/4 = 36 W as a check.
Find the equivalent resistance of 2 Ω and 3 Ω in series, and in parallel.
In series, 2 + 3 = 5 Ω. In parallel, 1/R = 1/2 + 1/3 = 5/6, so R = 1.2 Ω. Note that a parallel combination is always smaller than the smallest individual resistance - a quick sanity check.
How many units of energy does a 100 W bulb consume in 10 hours?
Energy = 100 W × 10 h = 1000 Wh = 1 kWh, which is 1 unit.
Why is tungsten used for the filament of an electric bulb?
It has a very high melting point, so it can glow white-hot without melting, and high resistivity, so it heats strongly for the current passing through.
Why is a fuse connected in the live wire, and why must the earth wire be present?
The fuse is placed in the live wire so that when it melts, the supply to the appliance is genuinely cut off. The earth wire provides a low-resistance path to the ground so that any leakage current flows away safely instead of through a person touching the metal casing.
State Fleming's left-hand and right-hand rules and where each applies.
The left-hand rule gives the direction of force on a current-carrying conductor in a magnetic field, so it applies to motors. The right-hand rule gives the direction of induced current when a conductor moves in a field, so it applies to generators.
What is the frequency of AC supply in India, and how often does its direction change?
50 Hz, which means the current reverses direction 100 times a second - twice in each cycle. This is a frequently asked one-mark question.
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How to use these questions
- ✓Answer before reading the solution. Reading a model answer feels like studying and produces almost no retention.
- ✓For every 'why' question, make sure your answer names the cause. 'Because blue is scattered more' scores; 'because of scattering' usually does not.
- ✓Write out numericals fully - formula, substitution with units, answer with unit. Marking schemes give separate credit for each step.
- ✓Keep one page per unit of the questions you got wrong, and revise only those pages in the final week.
- ✓Practise diagrams separately: ray diagrams for mirrors and lenses, the human eye, a neuron, the nephron, the digestive system and a soap micelle.
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