Describe the acid-base character of the oxides of each of the period 3 elements, Na to Cl.

State one example of an acidic gas, produced by an industrial process or the internal combustion engine, which can cause large-scale pollution to lakes and forests.

Suggest one method, other than measuring pH, which could be used to distinguish between solutions of a strong acid and a weak acid of the same molar concentration. State the expected results.

Na, Mg: basic;

Al: amphoteric;

Do not accept amphiprotic.

Si to Cl: acidic;

Award [1] for stating oxides become more basic towards left/Na and more acidic towards right/Cl.

Do not penalize incorrect formulas of oxides.

\({\text{N}}{{\text{O}}_{\text{2}}}\)/nitrogen dioxide / \({{\text{N}}_{\text{2}}}{{\text{O}}_{\text{4}}}\)/dinitrogen tetroxide / \({\text{S}}{{\text{O}}_{\text{2}}}\)/sulfur dioxide / \({\text{S}}{{\text{O}}_{\text{3}}}\)/sulfur trioxide;

Do not accept NO/NOx/CO₂/CO.

measure electrical conductivity;

strong acids are good conductors/better conductors than weak acids / weak acids are poor conductors;

OR

react with magnesium or a named active metal/metal carbonate/hydrogen carbonate/bicarbonate;

Do not accept Na/K

strong acids react faster/more gas bubbles (per unit time)/more heat produced / weak acids react slower/less gas bubbles (per unit time)/less heat produced;

Do not accept answers based on titration curves as they are based on pH.

Accept Neutralization: weak acid would produce less energy/less temperature increase compared to a strong acid.

The majority of candidates gave the correct answers to (a), but a few were confused about the acid-base character of the oxides of aluminium and silicon.

Part (b) proved to be a difficult question. Not many candidates gave the name or formula of an acidic gas produced by an industrial process. Some wrong answers were: CO, SO, \({{\text{H}}_{\text{2}}}{\text{S}}{{\text{O}}_{\text{4}}}\), CFCs, Methane, \({\text{N}}{{\text{H}}_{\text{3}}}\).

There were a few good answers to (c); measuring the conductivity or the reaction with magnesium or calcium carbonate was a possible method for distinguishing between a strong and a weak acid of the same concentration.