Methyl ethyl ketone is also known as:

The correct answer is B, Solution, salt water, brass. A homogeneous mixture has the same composition and properties throughout and no visible boundary between its parts, so a true solution such as salt water qualifies, and so does brass, an alloy of copper and zinc, since alloys are solid solutions. In salt water the particles are smaller than one nanometre, which is why the mixture looks clear and does not settle or scatter light. A is wrong because it adds colloid and suspension, both of which are heterogeneous. C is wrong because colloids such as milk and toothpaste have dispersed particles between one and one thousand nanometres that scatter a beam of light, the Tyndall effect, so they are heterogeneous. D is wrong for the same reason, as it lists only colloids and a suspension. Exam tip: a solution cannot be filtered and shows no Tyndall effect, a colloid shows the Tyndall effect but does not settle, and a suspension settles on standing.

The correct answer is A, Gamma rays. Gamma rays are electromagnetic waves of the highest frequency and shortest wavelength, given out by the nucleus of a radioactive atom and in nuclear reactions, and their energy lets them kill cancer cells in radiotherapy. Cobalt-60 is the source used in the gamma knife and in the teletherapy units of Indian hospitals; gamma rays also sterilise surgical instruments and are used to preserve onions and potatoes. They carry no charge and no mass, so a magnetic field does not bend them, and only thick lead or concrete stops them. B is wrong because ultraviolet waves come from the Sun and from lamps, and are much weaker. C is wrong because visible light is the narrow band our eyes can see. D is wrong because X-rays come from electrons striking a metal target outside the nucleus, not from the nucleus itself. Exam tip: the order of increasing frequency is radio, microwave, infrared, visible, ultraviolet, X-ray, gamma.

The correct answer is C, Archimedes principle. The principle says a body immersed wholly or partly in a fluid is pushed up by a force equal to the weight of the fluid it pushes aside, and that upward force is called buoyancy or upthrust. It explains why a body feels lighter in water, why a ship of steel floats while a nail of steel sinks, and why a hydrometer and a lactometer work. A body floats when its density is less than that of the fluid and sinks when it is more, which is why an iron ball sinks in water but floats in mercury. A is wrong because Boyle's law links the pressure and volume of a gas at constant temperature. B is wrong because Charles law links the volume of a gas to its temperature at constant pressure. D is wrong because Pascal's law says pressure applied to an enclosed fluid is passed on equally in all directions, the idea behind the hydraulic brake and lift. Exam tip: Archimedes gives upthrust, Pascal gives the hydraulic press.

The correct answer is C, ethylene, the only plant hormone that is a gas. It hastens the ripening of fruit, encourages leaves and flowers to drop and is used commercially in the form of ethephon to ripen mangoes, bananas and tomatoes evenly; a ripe fruit kept with unripe ones ripens them because it gives off ethylene. A is wrong because auxin is the hormone of cell elongation, bending towards light and apical dominance, and in high doses it works as a weedkiller. B is wrong because gibberellin lengthens stems, breaks seed dormancy and causes bolting in rosette plants; it is associated with growth in height rather than ripening. D is wrong because cytokinin promotes cell division, delays the ageing of leaves and helps shoots form in tissue culture. The hormone to pair with stress is abscisic acid, which closes the stomata and enforces dormancy.

The correct answer is B, a stem. The potato is an underground stem tuber, and the proof is on its skin: the hollows called eyes are axillary buds with tiny scale leaves, and buds of this kind are never found on a true root. That is why a cut piece of potato with an eye grows into a whole new plant. A is wrong because the familiar modified roots are the sweet potato, carrot, radish, turnip and beetroot, none of which carries buds. C is wrong because modified leaves give us the fleshy scales of an onion bulb, the tendrils of a pea and the spines of a cactus, but not the potato. D is wrong because the potato is not a reproductive structure in the floral sense; the potato plant does bear flowers separately. Group the modified stems for revision: potato tuber, ginger and turmeric rhizomes, onion and garlic bulbs, colocasia corm.