Chemistry Questions and Answers

Answer: D

The cyclotron is used for accelerating particles.

Lawrence's design of 1930 sends charged particles spiralling between two D-shaped electrodes in a magnetic field, with an alternating voltage across the gap giving them a push on each half-turn. The period of revolution \[T = \frac{2\pi m}{qB}\] is independent of speed, which is what lets a single fixed frequency keep accelerating them.

Answer: D

Contact lenses are made from Lucite, that is polymethyl methacrylate.

PMMA is glass-clear, hard and easily shaped to an exact curvature, which made it the material of the first hard contact lenses. Modern soft lenses use hydrogels such as poly-HEMA and silicone hydrogels, which let far more oxygen reach the cornea.

Answer: C

Conduction electrons are more mobile because they collide less frequently.

An electron moves through the conduction band, which is largely empty, so its mean free path between scattering events is long. A hole moves by successive electrons hopping into it within the crowded valence band, so its effective mass is larger and its progress slower - in silicon, electron mobility is about three times hole mobility.

Answer: D

Such elements are metalloids.

They lie along the staircase dividing metals from non-metals: boron, silicon, germanium, arsenic, antimony and tellurium. They have a metallic lustre but are brittle, and conduct electricity poorly - a property that becomes useful as semiconduction.

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: C

Fluorine's reactivity follows from its very high electronegativity.

At 4.0 it is the most electronegative element, so it strips electrons from almost anything and forms strong bonds with a large release of energy. Its small size and the weakness of the F–F bond, about \(155\ \mathrm{kJ\,mol^{-1}}\), reinforce this. Fluorine has no d-orbitals available at all.

Answer: C

Batteries hold sulphuric acid.

A lead-acid cell uses about 35 per cent \(\mathrm{H_2SO_4}\) as electrolyte: \[\mathrm{Pb + PbO_2 + 2H_2SO_4 \rightarrow 2PbSO_4 + 2H_2O}\] The acid is consumed on discharge, so the electrolyte's specific gravity falls - which is how a hydrometer reads a battery's state of charge.

Answer: D

Metallic lustre is due to the free electrons.

The delocalised electrons of the metallic bond absorb visible light of all wavelengths and re-emit it immediately, so the surface reflects nearly everything that reaches it. The same electron sea gives metals their electrical and thermal conductivity.

Answer: D

Sodium thiosulphate is the photographic chemical, known as hypo.

It fixes the image by dissolving the unexposed silver bromide, which would otherwise darken in daylight: \[\mathrm{AgBr + 2Na_2S_2O_3 \rightarrow Na_3[Ag(S_2O_3)_2] + NaBr}\]

Answer: C

Petroleum is found in sedimentary rocks.

It forms from marine organic matter buried in fine-grained sediments and cooked over millions of years, then migrates upward through porous rock until an impermeable cap traps it. Igneous and metamorphic rocks form at temperatures that would destroy the hydrocarbons.