Chemistry Questions and Answers

Answer: D

The mixture is separated by fractional distillation.

Ethanol boils at 78 °C and water at 100 °C, a difference too small for simple distillation, so a fractionating column is needed. Even then only 95.6 per cent ethanol can be obtained, because the mixture forms an azeotrope at that composition.

Answer: D

Acid rain is caused by oxides of nitrogen and sulphur dioxide.

\(\mathrm{SO_2}\) from burning coal and \(\mathrm{NO_x}\) from vehicle engines are oxidised in the air and dissolve in rain: \[\mathrm{2SO_2 + O_2 + 2H_2O \rightarrow 2H_2SO_4},\qquad \mathrm{4NO_2 + O_2 + 2H_2O \rightarrow 4HNO_3}\] Rain below about pH 5.6 counts as acid rain.

Answer: B

Laughing gas is nitrous oxide, \(\mathrm{N_2O}\).

Humphry Davy named it for the euphoria it induces, and it is still used as a mild anaesthetic and analgesic mixed with oxygen. Nitric oxide, NO, and nitrogen dioxide, \(\mathrm{NO_2}\), are quite different and are toxic.

Answer: B

Hydrogen is used to make vanaspati.

Passed through the warm oil over a nickel catalyst, it adds across the carbon–carbon double bonds, converting the unsaturated liquid oil into a saturated semi-solid fat. The process is called hydrogenation, or hardening of oils.

Answer: D

Radioactivity is measured with a Geiger–Müller counter.

Radiation entering the tube ionises the gas inside; the ions are accelerated by a high potential difference and produce an avalanche that registers as a pulse. Each pulse is one detected particle, so the count rate measures the activity.

Answer: C

The nucleus contains protons and neutrons.

These are the nucleons, and they hold virtually the whole mass of the atom in a volume about \(10^{-15}\) m across. The electrons occupy the space outside the nucleus, which is why the atom as a whole is some hundred thousand times larger.

Answer: D

The group displacement law was given by Soddy and Fajans.

It states that alpha emission moves an element two groups to the left in the periodic table, while beta emission moves it one group to the right. That follows from the change in atomic number: \(-2\) for alpha, \(+1\) for beta.

Answer: C

A water molecule can form up to four hydrogen bonds.

It has two hydrogen atoms, each able to donate a bond, and two lone pairs on the oxygen, each able to accept one. This tetrahedral capacity is why ice has an open hexagonal lattice and is less dense than liquid water.

Answer: B

Butane is used in cigarette lighters.

It liquefies under modest pressure at room temperature - a few atmospheres are enough - so a large amount of fuel is stored in a small transparent reservoir, and it vaporises instantly on release. Propane would need a much stronger container.

Answer: B

The correct order is \(\mathrm{Al_2O_3 < MgO < Na_2O < K_2O}\).

Basicity of an oxide increases as the element becomes more metallic - that is, down a group and to the left across a period. \(\mathrm{Al_2O_3}\) is amphoteric, MgO weakly basic, and \(\mathrm{Na_2O}\) and \(\mathrm{K_2O}\) strongly basic, with potassium the most electropositive of the four.