General Science Previous Year Questions (PYQ) with Answers – Page 55

A desiccant is a substance that absorbs moisture from its surroundings and keeps things dry, and silica gel is the most widely used one.

Silver reacts with traces of hydrogen sulphide in the air and forms a black coating of silver sulphide. This slow attack on a metal by its surroundings is called corrosion, and on silver it is usually called tarnishing.

Zinc oxide is amphoteric: it reacts with acids as a base and with strong alkalis as an acid, giving a salt and water in both cases.

Bleaching powder is calcium oxychloride, CaOCl₂, and DDT is dichlorodiphenyltrichloroethane; chlorine is the one element common to both. Bleaching powder is made by passing chlorine gas over dry slaked lime.

Turmeric is a natural indicator: soap is basic and turns the yellow stain reddish brown, and rinsing with plenty of water removes the soap so the stain turns yellow again.

These are the only two elements that are liquid at standard room temperature and pressure: mercury is a metal and bromine is a non-metal.

The Swedish chemist Berzelius proposed around 1813–1814 that each element be written with one or two letters taken from its name, the system still used today.

On heating, sodium bicarbonate releases carbon dioxide gas, whose bubbles make dough rise and turn cakes and bread soft and spongy.

A positive power means a converging (convex) lens, and f = 1/P = 1/2 = 0.5 m. The power of a lens is the reciprocal of its focal length in metres: P = 1/f. Its SI unit is the dioptre (D), and 1 D is the power of a lens…

Ammonium chloride sublimes: on heating, it changes directly from a solid into vapour without melting, and the vapour turns back into solid on a cool surface.

Helicobacter pylori is a spiral-shaped bacterium that lives in the lining of the stomach. It survives the strong acid by producing the enzyme urease, which makes ammonia around it, and it inflames and weakens the protective mucus layer, so acid can damage the wall and cause a peptic ulcer.

At sunrise and sunset sunlight passes through a much thicker layer of air, which scatters away most of the blue light and leaves mainly red and orange to reach our eyes.

Biogas is mostly methane, usually about 50 to 75 per cent, and methane is the part that burns. Biogas is made when bacteria break down cattle dung, crop waste, kitchen waste or sewage in the absence of oxygen, a process called anaerobic digestion, inside a closed digester.

Analogous structures do the same job but evolved separately, and the wings of birds and bats are a standard example.

Pearls are harvested from pearl oysters, which are bivalve molluscs. When a foreign particle, such as a grain of sand or a parasite, gets inside the shell, the oyster's mantle covers it layer after layer with nacre, the shiny material also called mother of pearl.

A galvanometer detects whether a small electric current is flowing in a circuit and shows its direction. A moving-coil galvanometer has a coil placed between the poles of a magnet.

Of the four, X-rays lie farthest towards the short-wavelength, high-energy end of the electromagnetic spectrum. All electromagnetic waves travel at the speed of light in vacuum but differ in wavelength.

All rose plants of one kind belong to a single species, the basic unit of classification. A species is a group of individuals that are alike in their main features and can interbreed in nature to produce fertile offspring.

Algae are the predominantly aquatic members of the Kingdom Plantae. They are simple, chlorophyll-bearing organisms with a thallus body that is not divided into true roots, stems and leaves.

Ultraviolet light is an electromagnetic wave, and all electromagnetic waves are transverse, not longitudinal. In an electromagnetic wave, the electric and magnetic fields vibrate at right angles to each other and to the direction in which the wave travels, which is the mark of a transverse wave.