Chemistry Questions and Answers

Answer: B

Homonuclear molecules contain a covalent bond.

Both atoms are of the same element, so their electronegativities are identical and neither can draw the shared pair towards itself. The bond is therefore purely covalent and non-polar, as in \(\mathrm{H_2}\), \(\mathrm{N_2}\) and \(\mathrm{Cl_2}\).

Answer: C

Alloys of mercury are called amalgams.

Mercury dissolves most metals at room temperature, and dental amalgam - mercury with silver, tin and copper - is the familiar example. Iron is the notable exception, which is why mercury can be stored and shipped in iron flasks.

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

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

This is nuclear fission.

\[\mathrm{^{235}_{92}U + ^1_0n \rightarrow ^{141}_{56}Ba + ^{92}_{36}Kr + 3\,^1_0n + energy}\] The two fragments are of comparable mass, and the extra neutrons released can trigger further fissions, giving a chain reaction. Hahn and Strassmann discovered it in 1938.

Answer: A

The experiment established the size of the nucleus.

From the closest approach of alpha particles scattered straight back, Rutherford put the nucleus at about \(10^{-15}\ \mathrm{m}\) across - some ten thousand times smaller than the atom, so that the atom is almost entirely empty space.

Answer: B

HF has the greatest ionic character.

By Fajans' rules and Pauling's relation, ionic character rises with the electronegativity difference between the atoms. That difference is \(4.0-2.1=1.9\) for H–F, against 0.9 for H–Cl, 0.7 for H–Br and 0.4 for H–I. HF is about 43 per cent ionic, which also explains its hydrogen bonding and unusually high boiling point.

Answer: B

There are four isomers of \(\mathrm{C_4H_8}\) that are alkenes.

They are but-1-ene, cis-but-2-ene, trans-but-2-ene and 2-methylpropene. The first and last differ in the carbon skeleton (chain isomerism), but-1-ene and but-2-ene in the position of the double bond, and the two but-2-enes are geometrical isomers. Cyclobutane and methylcyclopropane share the formula but are not alkenes.

Answer: D

Positive ions are formed by the loss of electrons.

Only the electrons are held loosely enough to be removed by ordinary chemical or electrical means. Removing a proton would change the element itself, and that is a nuclear process, not a chemical one.

Answer: C

The mass number equals the number of nucleons.

Nucleons are the protons and neutrons taken together, so \[A = Z + N\] where \(Z\) is the number of protons and \(N\) the number of neutrons. Electrons weigh too little to matter.