Chemistry Questions and Answers

Answer: D

Diamond is the hardest form of carbon, rating 10 on the Mohs scale.

Every atom is \(sp^3\) hybridised and bonded to four neighbours in a rigid three-dimensional network, so deforming it means breaking covalent bonds. Graphite's atoms lie in sheets held together only by weak forces, which is why it is soft.

Answer: C

Polonium has the greatest number of isotopes, some thirty-three of them.

All are radioactive; polonium has no stable isotope at all. Heavy elements have many possible neutron numbers, none of which gives a stable nucleus, so the count of known isotopes is large.

Answer: D

Acid rain is caused by oxides of nitrogen and sulphur dioxide.

\(\mathrm{SO_2}\) from burning coal and \(\mathrm{NO_x}\) from vehicle engines are oxidised in the air and dissolve in rain: \[\mathrm{2SO_2 + O_2 + 2H_2O \rightarrow 2H_2SO_4},\qquad \mathrm{4NO_2 + O_2 + 2H_2O \rightarrow 4HNO_3}\] Rain below about pH 5.6 counts as acid rain.

Answer: A

Silica is the major component of glass.

Ordinary soda-lime glass is about 75 per cent \(\mathrm{SiO_2}\), with sodium carbonate to lower the melting point and limestone to make the product insoluble. Adding boron oxide instead gives borosilicate glass, which expands little on heating.

Answer: B

Nitrogen forms six well-characterised oxides.

They are \(\mathrm{N_2O}\), NO, \(\mathrm{N_2O_3}\), \(\mathrm{NO_2}\), \(\mathrm{N_2O_4}\) and \(\mathrm{N_2O_5}\), covering oxidation states from \(+1\) to \(+5\). This unusually wide range follows from nitrogen's five valence electrons and its ability to form multiple bonds to oxygen.

Answer: C

Metallurgy is the process of extracting a metal in pure form from its ore.

It runs in three stages: concentration of the ore, reduction to the crude metal by smelting or electrolysis, and refining. The method chosen depends on the metal's position in the reactivity series - carbon reduction for iron and zinc, electrolysis for aluminium and sodium.

Answer: B

Gelatine is used to slow the melting of ice.

Its long protein chains form a gel that traps water and hinders both heat transfer into the ice and the flow of meltwater away from it, so the block holds together far longer. It is used the same way in ice creams and sorbets.

Answer: A

Acids taste sour.

The word acid comes from the Latin acidus, meaning sour, and the taste is that of the hydrogen ion in solution - citric acid in lemon, acetic in vinegar, lactic in curd. Bases taste bitter and feel soapy.

Answer: A

Vinegar is an aqueous solution of acetic acid.

Table vinegar is 4–8 per cent \(\mathrm{CH_3COOH}\), made by the bacterial oxidation of ethanol. The pure acid freezes at 17 °C to an ice-like solid, which is why it is called glacial acetic acid.

Answer: C

Rusting is corrosion, and it is a chemical change.

Iron combines with oxygen and water to form hydrated iron(III) oxide, a new substance with different properties from the metal. The change cannot be undone by physical means, which is what marks it as chemical rather than physical.