4.8 Core practical: pH change in neutralisation
5 exam questions worth 8 marks. Write your answer, open the mark scheme, then give yourself the marks you earned. Your answers are saved on this device.
0 / 8
What happens to the pH as calcium hydroxide is added to hydrochloric acid?
Choose one answer.
Correct Not quite: the answer is It increases
Mark scheme
A
Model answer
A: the pH rises as the acid is neutralised and then the alkali is in excess.
Give the independent variable and the dependent variable in this investigation.
Mark scheme
- independent: mass of calcium hydroxide added (1)
- dependent: pH (1)
Model answer
The independent variable is the mass of calcium hydroxide added; the dependent variable is the pH.
Explain why the pH stays almost constant at about 12 at the end of the experiment.
Mark scheme
- all the acid has been neutralised / calcium hydroxide is in excess (1)
- calcium hydroxide is only slightly soluble, so adding more does not change the pH much (1)
Model answer
All the acid has reacted, so calcium hydroxide is in excess; it is only slightly soluble, so the pH stops rising.
Give two variables that must be controlled.
Mark scheme
any two from:
- volume of acid (1)
- concentration of acid (1)
- temperature (1)
- mass added each time / stirring (1)
Model answer
The volume and the concentration of the hydrochloric acid.
Suggest why a pH probe gives better results than universal indicator paper.
Mark scheme
- more accurate / gives a numerical value / no judging of colour (1)
Model answer
A pH probe gives an accurate numerical reading, while indicator paper relies on matching a colour.
Found a mistake?