Chemistry Questions and Answers

Answer: D

Ferric chloride works because it coagulates the blood and so seals the vessels.

Blood is a negatively charged colloidal sol. The trivalent \(\mathrm{Fe^{3+}}\) ion neutralises that charge very efficiently - by the Hardy–Schulze rule, coagulating power rises sharply with the charge on the oppositely charged ion - so the sol clots at once.

Answer: B

The atomic number gives the number of electrons in a neutral atom, because it equals the number of protons.

The proton count defines the element and is what the atomic number really counts; the electron count matches it only while the atom is uncharged. The number of nucleons is the mass number instead.

Answer: C

Carbon tetrachloride was used as a fire extinguisher, under the name pyrene.

Its vapour is much denser than air, so it blankets the fire and cuts off oxygen, and it does not conduct electricity, which made it useful on electrical fires. It is no longer used, because hot \(\mathrm{CCl_4}\) can form phosgene.

Answer: C

Such species are isoelectronic.

\(\mathrm{N^{3-}}\), \(\mathrm{O^{2-}}\), \(\mathrm{F^-}\), \(\mathrm{Ne}\), \(\mathrm{Na^+}\) and \(\mathrm{Mg^{2+}}\) all have 10 electrons. Isotopes share the proton number, isobars the mass number, and isotones the neutron number.

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.