Acids, Alkalis and Titrations
5 marks · EasyThe table below shows the colours of three indicators in different solutions.
| Solution | Litmus | Phenolphthalein | Methyl orange |
|---|---|---|---|
| A | Red | Colourless | Red |
| B | Blue | Pink | Yellow |
| C | Purple | Colourless | Yellow |
Using the table, state which solution is acidic, which is alkaline, and which is neutral. Explain your answers using indicator colours.
For solution B, state what the pH is likely to be, and justify your answer.
Work this out on paper, then compare with the answer and worked solution below. With a free account you can submit written answers for marking.
Show answer
(a) A is acidic (red litmus, red methyl orange, colourless phenolphthalein); B is alkaline (blue litmus, pink phenolphthalein, yellow methyl orange); C is neutral (purple litmus, colourless phenolphthalein, yellow methyl orange).
(b) B is alkaline, so its pH is likely to be between 8 and 14; pink phenolphthalein suggests pH > 8, yellow methyl orange confirms alkaline.
Common mistakes: swapping neutral and alkaline indicator results; misinterpreting purple litmus
Show mark scheme and worked solution
Marking points
A1 (a) · 1 mark · Correctly identifies A as acidic, B as alkaline, C as neutral.
M1 (a) · 1 mark · Supports answer for acidic with at least one indicator colour (e.g. red litmus or methyl orange).
M2 (a) · 1 mark · Supports answer for alkaline and neutral with correct indicator colour(s).
A2 (b) · 1 mark · States B’s pH is above 7 or in correct alkaline range (8–14).
M3 (b) · 1 mark · Justifies using the indicator colour(s) (e.g., pink phenolphthalein = alkaline).
Worked solution
a. Solution A is acidic: litmus is red, methyl orange is red, phenolphthalein is colourless – all are colours in acid.
Solution B is alkaline: litmus is blue, phenolphthalein is pink, methyl orange is yellow – all indicate alkali.
Solution C is neutral: litmus is purple (neutral), phenolphthalein is colourless (neutral or acid), methyl orange is yellow (neutral or alkali), supporting neutrality.
b. Solution B has a pH above 7 (alkaline). The pink colour in phenolphthalein suggests the pH is above 8. Yellow methyl orange also confirms it is alkaline. So the pH is likely to be between 8 and 14.