Clarification details
Outcome 1
ER 1.1
Solutions used are limited to solutions of acids, alkalis and salts. This means that molecular species that do not react with water do not provide suitable evidence. The intent of ER 1.1 is to make judgements based on experimental evidence rather than prior knowledge of specific substances.
Outcome 2
ER 2.1
When sketching pH curves, the curve must be drawn with an appropriate shape as well as pass through approximated pH values at various sections of the curve. For a weak acid-strong base, or weak base-strong acid titration, the shape given for the initial section of the curve must reflect the weak acid, or base, which will result in an initial, more rapid pH change. This will be followed by a less rapid change in pH as a buffer system is formed. In order for the buffer section to be appropriate it must be centred on the pH which is equal to the p K a of the weak acid and the volume halfway to the equivalence point. At the volume related to the equivalence point, the graph will show a sudden change in pH. While the range statement requires only two of the three aspects given to be correct, the overall shape of the graph must still be appropriate. It is appropriate to encourage students to round to whole numbers when determining the approximate pH at the different parts of the titration curve. This will result in a pH value within 1 unit of the actual pH.
It is recommended that examples used to sketch acid-base titration curves involve acids and bases of the same concentration.