Chemistry Questions and Answers

Answer: B

The wrong statement is that among isoelectronic species, the smaller the positive charge on the cation, the smaller the ionic radius.

The relation is the opposite: for a fixed number of electrons, a larger nuclear charge pulls them in harder, so radius decreases as the positive charge grows. \(\mathrm{Na^+}\) is larger than \(\mathrm{Mg^{2+}}\), which is larger than \(\mathrm{Al^{3+}}\). The other three statements are all correct.

Answer: C

Homonuclear molecules contain a covalent bond.

Both atoms are of the same element and so have identical electronegativity; neither can pull the shared pair towards itself, and the bond is purely covalent and non-polar - as in \(\mathrm{H_2}\), \(\mathrm{O_2}\) and \(\mathrm{Cl_2}\).

Answer: C

The mixture is gunpowder.

The classic proportions are about 75 per cent potassium nitrate, 15 per cent charcoal and 10 per cent sulphur. The nitrate supplies oxygen, the charcoal is the fuel and the sulphur lowers the ignition temperature: \[\mathrm{2KNO_3 + 3C + S \rightarrow K_2S + N_2 + 3CO_2}\]

Answer: A

Nail polish remover contains acetone.

Propanone dissolves the nitrocellulose and resins of the polish quickly and evaporates at once, being volatile. It also strips the natural oils from the nail, which is why acetone-free removers use ethyl acetate instead.

Answer: C

Cadmium does not liberate hydrogen from water.

Sodium, lithium and potassium are all alkali metals, high in the reactivity series, and react with even cold water. Cadmium lies below hydrogen in the series and is unaffected by water at ordinary temperatures.

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: A

Washing soda is sodium carbonate, \(\mathrm{Na_2CO_3\cdot 10H_2O}\).

It removes hardness by precipitating calcium and magnesium as their insoluble carbonates, and its solution is alkaline by hydrolysis, which helps it cut grease. Sodium bicarbonate is baking soda.

Answer: D

Aspirin is acetylsalicylic acid.

It is made by acetylating salicylic acid with acetic anhydride, and works by irreversibly inhibiting the cyclo-oxygenase enzymes, so cutting the production of prostaglandins that cause pain, fever and inflammation. Methyl salicylate is oil of wintergreen.

Answer: A

Ozone is produced in photochemical smog.

Sunlight splits nitrogen dioxide from vehicle exhaust, \[\mathrm{NO_2 + h\nu \rightarrow NO + O},\qquad \mathrm{O + O_2 \rightarrow O_3}\] and the ozone then reacts with unburnt hydrocarbons to give PAN and aldehydes. Hydrocarbons and nitrogen oxides are the ingredients rather than the products.

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.