Chemistry Questions and Answers

Answer: D

All the above are chemical weathering.

Oxidation turns iron minerals to rust and crumbles the rock; carbonation dissolves limestone as \[\mathrm{CaCO_3 + H_2O + CO_2 \rightarrow Ca(HCO_3)_2}\] which is how caves form; and acid rain accelerates both. Each alters the rock's chemical composition, unlike physical weathering by frost or heat.

Answer: A

Photosynthesis is \(\mathrm{6CO_2 + 6H_2O \rightarrow C_6H_{12}O_6 + 6O_2}\).

Light captured by chlorophyll drives the reaction, which stores solar energy as glucose. The reverse equation is respiration, which releases that energy again. Note that the oxygen given off comes from the water, not from the carbon dioxide.

Answer: B

When a bond forms, the atoms share or transfer electrons.

Transfer gives an ionic bond and sharing a covalent one; either way the atoms reach a more stable electron configuration and the system falls to lower energy. Protons and nuclei are not involved - moving those would be a nuclear process, not a chemical one.

Answer: B

Secondary structure is the alpha helix and the beta pleated sheet.

These are local, regular foldings of the polypeptide backbone, held by hydrogen bonds between the carbonyl oxygen of one residue and the amide hydrogen of another. The primary structure is the amino-acid sequence, the tertiary the overall fold of one chain, and the quaternary the assembly of several chains.

Answer: A

Proteins are built from amino acids.

Twenty of them occur in proteins, each with an amino group, a carboxyl group and a distinctive side chain on the same carbon. They join by peptide bonds formed with the loss of water, and the sequence is dictated by the gene.

Answer: A

Iron recovers copper from copper sulphate solution.

\[\mathrm{Fe + CuSO_4 \rightarrow FeSO_4 + Cu}\] Iron is above copper in the reactivity series, so it reduces \(\mathrm{Cu^{2+}}\) to the metal. Silver and mercury lie below copper and cannot; sodium would attack the water first.

Answer: D

Ammonia is by far the most soluble of these gases in water.

About 700 litres of ammonia dissolve in one litre of water at room temperature, because \(\mathrm{NH_3}\) hydrogen bonds to water and partly reacts with it. Carbon dioxide dissolves to less than one litre per litre, nitrogen barely at all.

Answer: A

Silver iodide is used to produce artificial rain.

Its crystal lattice is close in structure to that of ice, so supercooled droplets in a cloud freeze on it and grow heavy enough to fall. Sprayed from aircraft or fired from the ground, it is the commonest cloud-seeding agent.

Answer: D

The fixer is sodium thiosulphate, known in photography as hypo.

After development, the unexposed silver bromide must be removed or the print would darken in light. Thiosulphate dissolves it as a soluble complex: \[\mathrm{AgBr + 2Na_2S_2O_3 \rightarrow Na_3[Ag(S_2O_3)_2] + NaBr}\]

Answer: A

Mesons are held to bind the nucleons.

Yukawa predicted in 1935 that a particle of intermediate mass carries the strong nuclear force, and the pion was found in 1947. The exchange of pions between protons and neutrons is the residual effect of the colour force acting between their quarks.