The small pores on the lower surface of the leaf are called ________

The correct answer is D, pinna. The pinna is the outer, visible flap of the ear, and its funnel shape collects sound from the surroundings.

Sound gathered by the pinna travels down the ear canal to the tympanic membrane, or eardrum, which starts to vibrate. These vibrations pass through the three small bones of the middle ear, the malleus, incus and stapes, to the cochlea, the coiled fluid filled part of the inner ear. There the vibrations are turned into nerve signals, and the auditory nerve carries them to the brain, where they are recognised as sound.

A is wrong: the auditory nerve carries signals to the brain but does not collect sound. B is wrong: the tympanic membrane vibrates when sound reaches it and does not gather sound itself. C is wrong: the cochlea lies deep inside the ear and changes vibrations into nerve impulses.

Exam tip: the path of sound is pinna, ear canal, eardrum, then malleus, incus and stapes, then cochlea, auditory nerve and brain.

The correct answer is A, 2-butanone. Methyl ethyl ketone has a methyl group on one side of the carbonyl and an ethyl group on the other, which makes four carbons in all with the carbonyl at the second position, so its IUPAC name is butan-2-one or 2-butanone. Ketones end in the suffix -one, and the number tells which carbon carries the carbonyl group. It is a colourless liquid used as a solvent in paints, varnishes, adhesives and printing inks, and is found in small amounts in some fruits. B is wrong because propanone, better known as acetone, has three carbons and is dimethyl ketone. C is wrong because methyl pentanone would need six carbons. D is wrong because 2-butanol is an alcohol with a hydroxyl group, not a ketone, though it can be oxidised to 2-butanone. Exam tip: count the carbons on both sides of the carbonyl and add one for the carbonyl carbon itself.

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.