General Science Previous Year Questions (PYQ) with Answers – Page 91

The number of protons in the nucleus defines an element, and no two elements share it. The atomic number (Z) is the number of protons in an atom's nucleus, and in a neutral atom it also equals the number of electrons, which decide chemical behaviour.

A transformer heats up because of energy lost inside it: heat from current in the windings and hysteresis loss in the iron core.

Sleeping sickness, or African trypanosomiasis, is caused by the protozoan Trypanosoma brucei. Trypanosoma is a flagellated protozoan that lives in the blood.

Melanin absorbs the ultraviolet rays of sunlight and shields the DNA of skin cells from damage. Melanin is made by cells called melanocytes in the lower layer of the epidermis.

Bronze is an alloy of copper and tin, usually about 88 parts copper to 12 parts tin. Adding tin makes copper harder, stronger and easier to cast, and bronze also resists corrosion well.

A hygrometer measures humidity, that is, the amount of water vapour or moisture in the air. The name comes from the Greek "hygros", meaning wet.

The green pigment of leaves is chlorophyll, and a magnesium atom sits at the centre of every chlorophyll molecule.

Pure water is neutral, and a neutral solution has a pH of 7 at 25 °C. pH is the negative logarithm of the hydrogen ion concentration.

Ultraviolet photons carry more energy than visible light, not less, so this statement is wrong. The energy of a photon is E = hc/λ, so a shorter wavelength means higher energy.

Fungal cell walls are built mainly of chitin, whereas plant cell walls are built of cellulose. Chitin is a tough polysaccharide made of units of N-acetylglucosamine, a sugar that carries nitrogen.

This filarial roundworm causes elephantiasis, also called lymphatic filariasis. The adult worms live in the lymphatic vessels and block the flow of lymph, so the legs, arms or genitals swell and the skin thickens, giving the disease its name.

Wool is an animal fibre made mainly of the protein keratin. Polymers are giant molecules built from many small repeating units called monomers.

Sucralose is made from ordinary table sugar, sucrose, by changing its molecule. In making sucralose, three hydroxyl (OH) groups of the sucrose molecule are replaced by chlorine atoms.

Platinum only gets hot and may glow; it does not react, and on cooling the crucible is exactly as it was, so the change is physical.

For reflection at a plane mirror, the deviation is 180° − 2i, and with i = 40° this gives 180° − 80° = 100°.

Rutherford's gold foil experiment showed that an atom's positive charge and almost all its mass sit in a tiny central nucleus.

Tendons are made of tightly packed parallel bundles of collagen fibres, which give them great pulling strength. Collagen is the most abundant protein in the human body.

A light year is the distance that light travels in vacuum in one year at about 3 into 10 to the power 8 metres per second, which works out to roughly 9.46 into 10 to the power 12 kilometres; astronomers use it because distances between stars are awkward to write…

Quartz is crystalline silicon dioxide (SiO₂), an electrical insulator, not a semiconductor. A semiconductor has a conductivity between that of a conductor and an insulator, and its conductivity rises with temperature or when impurities are added (doping).

With equal length and area, the wire of lower conductivity, aluminium, has the higher resistance. Resistance is R = ρl/A, where ρ is resistivity, and resistivity is the reciprocal of conductivity (ρ = 1/σ).