Chemistry Questions and Answers

Answer: C

Formaldehyde is used to preserve specimens.

Formalin, a 40 per cent aqueous solution, cross-links the amino groups of proteins and so hardens the tissue and kills any bacteria or fungi that would decompose it. Its own pungency and toxicity are the drawback.

Answer: B

Isotopes have the same number of protons.

That is what makes them the same element and gives them the same chemistry, since chemistry depends on the electrons, which match the protons. They differ in neutron number and therefore in atomic mass.

Answer: C

Long chains of repeating carbon-based units are polymers.

They are built from small molecules called monomers - ethylene giving polythene, and amino acids giving proteins. Hydrocarbons are simply compounds of carbon and hydrogen, of any length, and need not be chains of repeating units.

Answer: B

Alum is used in the purification of water.

Potash alum, \(\mathrm{K_2SO_4\cdot Al_2(SO_4)_3\cdot 24H_2O}\), releases \(\mathrm{Al^{3+}}\), whose high charge neutralises the negative charge on suspended clay particles so that they coagulate and settle. It is also used to stop bleeding from shaving cuts, for the same coagulating reason.

Answer: B

The mixture is separated by sublimation.

Naphthalene sublimes - it passes straight from solid to vapour on warming - and recondenses on a cold surface, while the sand, which is non-volatile, stays behind. Camphor, iodine and ammonium chloride behave the same way.

Answer: C

Gamma rays are high-energy electromagnetic waves.

They are photons of no rest mass and no charge, with wavelengths below about \(10^{-11}\ \mathrm{m}\), emitted when a nucleus falls from an excited state to a lower one. Being uncharged, they are undeflected by electric and magnetic fields and are the most penetrating of the three radiations.

Answer: B

Dust is removed by electrophoresis, in the Cottrell precipitator.

Smoke is a colloidal sol of solid particles carrying an electric charge. Passing the flue gas between electrodes at a high potential difference discharges the particles, so they lose their mutual repulsion, coagulate and fall - and only the clean gas leaves the chimney.

Answer: B

An atom bomb works by nuclear fission.

A supercritical mass of uranium-235 or plutonium-239 is assembled so that each fission releases neutrons which cause more fissions, uncontrolled: \[\mathrm{^{235}_{92}U + ^1_0n \rightarrow ^{141}_{56}Ba + ^{92}_{36}Kr + 3\,^1_0n}\] Fusion powers the hydrogen bomb, which uses a fission device as its trigger.

Answer: D

Nitrogen has the highest first ionisation potential of the four.

Ionisation energy rises across a period, so oxygen should exceed nitrogen - but nitrogen's \(2p^3\) sub-shell is exactly half filled and therefore unusually stable, while oxygen's \(2p^4\) has one paired orbital whose electron–electron repulsion makes removal easier. So the order is N > O > C > B.

Answer: A

The breath of an acutely diabetic patient smells of acetone.

Without enough insulin the body cannot use glucose and burns fat instead, producing ketone bodies - acetoacetate, \(\beta\)-hydroxybutyrate and acetone. Acetone is volatile and is exhaled, giving the sweet, nail-polish smell of diabetic ketoacidosis.