Chemistry Questions and Answers

Answer: D

Dolomite contains both magnesium and calcium.

Its formula is \(\mathrm{CaMg(CO_3)_2}\). Magnesite is \(\mathrm{MgCO_3}\), calcite \(\mathrm{CaCO_3}\), and carnallite \(\mathrm{KCl\cdot MgCl_2\cdot 6H_2O}\), which has magnesium but no calcium.

Answer: D

Atoms are composed of electrons and nuclei.

The nucleus holds the protons and neutrons and almost all the mass; the electrons occupy the surrounding orbitals and determine chemical behaviour. Saying 'electrons and protons' leaves out the neutrons, which is why the fuller answer is the right one.

Answer: D

LiCl shows the greatest covalent character.

By Fajans' rules, covalent character increases when the cation is small and highly charged and the anion large and easily polarised. \(\mathrm{Li^+}\) is the smallest of these cations, so it distorts the chloride ion most strongly. This is why LiCl dissolves in organic solvents such as ethanol and acetone, while CsCl does not.

Answer: D

Cathode rays have both mass and charge.

They are electrons. They are deflected by electric and magnetic fields, which shows they carry charge, and they turn a light paddle-wheel placed in their path, which shows they carry momentum and therefore mass.

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.