Chemistry Questions and Answers

Answer: A

Rusting increases the weight of iron.

Rust is hydrated iron(III) oxide, \(\mathrm{Fe_2O_3\cdot xH_2O}\), so oxygen and water from the air are added to the metal: \[\mathrm{4Fe + 3O_2 + 2xH_2O \rightarrow 2Fe_2O_3\cdot xH_2O}\] The rusted article is heavier, though weaker, because it now contains atoms that came from outside.

Answer: D

Water drops are spherical because of surface tension.

Molecules at the surface are pulled inwards by their neighbours, since there are none above them, and the liquid therefore contracts to the shape with the least surface area for a given volume. For a fixed volume that shape is a sphere.

Answer: A

Tungsten is used for bulb filaments.

It has the highest melting point of any metal, 3,410 °C, so it can be run white-hot without melting, and its low vapour pressure means it evaporates slowly. The bulb is filled with argon or nitrogen to slow that evaporation further.

Answer: C

Calcium sulphate dihydrate, \(\mathrm{CaSO_4\cdot 2H_2O}\), is gypsum.

Heating it to about 120 °C drives off most of the water and gives plaster of Paris, \(\mathrm{CaSO_4\cdot\frac{1}{2}H_2O}\). Blue vitriol is copper sulphate and Epsom salt magnesium sulphate.

Answer: C

Sodium is the pure element.

An element consists of atoms of one kind only. Glass and cement are both mixtures of several compounds - glass mainly of silicates of sodium and calcium, cement of calcium silicates and aluminates.

Answer: D

Phosphorus is kept under water to stop it catching fire in air.

White phosphorus ignites spontaneously at about 30 °C, close to room temperature in a warm climate. It is insoluble in water and denser than it, so a covering layer keeps oxygen away without dissolving or reacting with the element.

Answer: C

Air is a mixture.

Its components are not chemically combined, keep their own properties, and are present in proportions that vary with place and altitude. They can be separated by the purely physical process of fractional distillation of liquid air.

Answer: D

Nitrogen forms a maximum of four covalent bonds.

It has five valence electrons and no d-orbitals, so it can form three ordinary covalent bonds and one further coordinate bond by donating its lone pair - as in the ammonium ion \(\mathrm{NH_4^+}\). The absence of d-orbitals is why nitrogen cannot expand its octet as phosphorus does in \(\mathrm{PCl_5}\).

Answer: A

Argon is the most abundant noble gas in the atmosphere, at 0.93 per cent by volume.

Nearly all of it is argon-40, produced by the radioactive decay of potassium-40 in the crust. It is the third commonest gas in air, after nitrogen and oxygen.

Answer: B

Monazite sand is found on the Travancore coast of Kerala.

The beach sands of Chavara and Manavalakurichi are among the richest monazite deposits in the world. Monazite is a phosphate of cerium and other rare earths containing about 8–10 per cent thorium, and India's thorium reserves are the basis of the third stage of its nuclear power programme.