Chemistry Questions and Answers

Answer: A

Citric acid is the organic acid.

It is \(\mathrm{C_6H_8O_7}\), a carbon compound with three carboxyl groups, found in citrus fruit. Nitric, sulphuric and phosphoric acids contain no carbon and are mineral acids.

Answer: C

Rusting is corrosion, and it is a chemical change.

Iron combines with oxygen and water to form hydrated iron(III) oxide, a new substance with different properties from the metal. The change cannot be undone by physical means, which is what marks it as chemical rather than physical.

Answer: A

Quartz glass is amorphous.

It has short-range order but no long-range periodicity, so it has no sharp melting point and its properties are the same in all directions. Chrome alum, silicon carbide and graphite are all crystalline, with regularly repeating lattices.

Answer: C

One mole of sodium atoms is the heaviest, at 23 g.

A mole of anything is \(6.022\times10^{23}\) particles, so the mass is simply the molar mass: water 18 g, ammonia 17 g, sodium 23 g, hydrogen \((\mathrm{H_2})\) 2 g.

Answer: C

The nucleus consists of protons and neutrons.

Together they are called nucleons and account for almost the whole mass of the atom. The electrons occupy the surrounding orbitals; the nucleus itself is about \(10^{-15}\) m across, some hundred thousand times smaller than the atom.

Answer: A

The \(\mathrm{H^+}\) ion contains no electrons.

A hydrogen atom has one proton and one electron; removing that electron leaves a bare proton. This is why \(\mathrm{H^+}\) never exists free in solution - it attaches at once to a water molecule to give the hydronium ion \(\mathrm{H_3O^+}\).

Answer: D

Potassium reacts most violently with cold water.

Reactivity increases down Group 1 as the outer electron becomes easier to lose. Sodium melts into a ball and fizzes; potassium reacts fast enough that the hydrogen ignites with a lilac flame. Calcium reacts steadily and magnesium hardly at all with cold water.

Answer: B

The sawdust catches fire because concentrated nitric acid is a powerful oxidising agent.

It oxidises the cellulose rapidly, and the heat evolved is enough to raise the wood above its ignition temperature. Concentrated sulphuric acid, by contrast, chars sugar and wood by dehydration, leaving black carbon rather than setting it alight.

Answer: C

Zirconium is essential in the construction of nuclear reactors.

Zircaloy is used to clad the fuel rods because zirconium has an exceptionally small neutron-capture cross-section - it lets neutrons pass rather than absorbing them - while resisting corrosion by hot water at high pressure. Cobalt and nickel absorb neutrons far more strongly.

Answer: B

This illustrates the law of definite proportions.

Proust's law says a given compound always contains the same elements combined in the same fixed ratio by mass, whatever its source. Water from any origin is 1 part hydrogen to 8 parts oxygen by weight.