4.5 Acids, alkalis and the pH scale
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
Which ion is produced by all acids in water?
Choose one answer.
Correct Not quite: the answer is H+
Mark scheme
A
Model answer
A: acids produce hydrogen ions, H+.
A solution has a pH of 13.
Describe the solution and name the ion present in excess.
Mark scheme
- (strongly) alkaline (1)
- hydroxide ions / \mathrm{OH^-} (1)
Model answer
It is strongly alkaline, so it contains a high concentration of hydroxide ions.
Give one advantage of using a pH probe instead of universal indicator paper.
Mark scheme
- more accurate / more precise / gives a numerical value / not judging a colour (1)
Model answer
A pH probe gives an accurate numerical reading instead of relying on judging a colour.
Phenolphthalein is added to sodium hydroxide solution. Dilute acid is then added until the solution is acidic.
Describe the colour change.
Mark scheme
- pink at first (in alkali) (1)
- becomes colourless (in acid) (1)
Model answer
The solution starts pink and turns colourless when it becomes acidic.
Explain the difference between a base and an alkali.
Mark scheme
- a base neutralises an acid (1)
- an alkali is a base that dissolves in water (producing \mathrm{OH^-} ions) (1)
Model answer
A base is any substance that neutralises an acid; an alkali is a base that is soluble in water, forming hydroxide ions.
Found a mistake?