General Science Previous Year Questions (PYQ) with Answers – Page 47

In the light reaction of photosynthesis, water is broken up; its hydrogen is used to make food and its oxygen is given out.

A shoot grows upward, away from the pull of gravity, so it is negatively geotropic. A tropism is a directional growth movement of a plant part in response to a stimulus: towards the stimulus is positive, away from it is negative.

Plaster of Paris is made from gypsum, not from common salt. When gypsum, calcium sulphate dihydrate (CaSO4·2H2O), is heated to about 373 K, it loses part of its water and becomes calcium sulphate hemihydrate (CaSO4·½H2O), which is Plaster of Paris; it sets hard again when mixed with water and is…

Anthracene sublimes on heating, while common salt does not, so heating the mixture separates them. Sublimation is the change of a solid directly into vapour without passing through the liquid state.

On a hot day the air just above a road or desert sand is much hotter, and so optically rarer, than the air higher up.

At room temperature and normal pressure only two elements are liquids: mercury, a metal, and bromine, a non-metal. Mercury melts at about −39 °C and is used in thermometers and barometers because it stays liquid over a wide range and does not wet glass.

An angle of incidence of zero means the ray falls along the normal, that is, perpendicular to the surface. By Snell's law, n₁ sin i = n₂ sin r; if i is zero, sin i is zero, so sin r is also zero and the angle of refraction is zero.

Lime water is a clear solution of calcium hydroxide, Ca(OH)₂. When carbon dioxide is passed through it, insoluble calcium carbonate, CaCO₃, forms as a fine white precipitate, which makes the lime water turn milky.

Vegetables and fruits supply most of the vitamins and minerals in a vegetarian diet, which is why they are called protective foods.

Silver bromide is the light-sensitive compound coated on black and white photographic film and paper, because light breaks it down into tiny grains of silver metal.

A pollen grain lands on the stigma, grows a pollen tube that travels down through the style, and enters the ovary to reach an ovule, where fertilisation takes place.

The radicle is the embryonic root, and it is the first part to come out of a germinating seed, growing downward to form the primary root.

This is the defining condition of simple harmonic motion, written F = −kx, where k is the force constant and the minus sign shows that the force always points back towards the mean position.

The frequency of a wave is set by its source, so it does not change when light passes from air into glass; only the speed and wavelength change.

Beats are heard when two sound waves of slightly different frequencies overlap, so their combined loudness rises and falls regularly.

Starlight bends as it passes through layers of air of changing temperature and density, and because these layers keep moving, the star's apparent position and brightness flicker.

. A light-year works as a unit of distance only because light in a vacuum always travels at the same speed, so the distance it covers in a year is fixed.

Triclosan is an antibacterial and antifungal agent added mainly to personal-care products. It has been used in antibacterial soaps, hand washes, toothpastes, deodorants, shaving creams, mouthwashes and cosmetics, and sometimes in kitchen items and textiles to stop the growth of germs.

Bisphenol A is the main building block from which polycarbonate plastic is made. Polycarbonate is a hard, clear and shatter-resistant plastic once widely used for reusable water bottles, baby feeding bottles, food containers, CDs and spectacle lenses.

. When quicklime reacts with water a large amount of heat is released, so the reaction is exothermic.