CAIE IGCSE Chemistry Alkali Indicator Colours

Cambridge IGCSE Chemistry 0620 syllabus for exams in 2026, 2027 and 20282026–2028

Practise recalling indicator colours in alkaline solution and distinguishing litmus, methyl orange and thymolphthalein results without mixing their endpoints.

How this is tested

state that red litmus changes to blue and blue litmus remains blue in an alkaline solution
state that methyl orange appears yellow when it is added to an alkaline solution
state that thymolphthalein is blue in alkali but colourless in acidic solution

[Maximum number: 1]

Sulfuric acid has many uses.

In a titration, a student added $25.0 \mathrm{~cm}^{3}$ of $0.200 \mathrm{~mol} / \mathrm{dm}^{3}$ aqueous sodium hydroxide to a conical flask. The student then added a few drops of methyl orange to the solution in the conical flask.

Dilute sulfuric acid is then added from a burette to the conical flask. The volume of dilute sulfuric acid needed to neutralise the aqueous sodium hydroxide was $20.0 \mathrm{~cm}^{3}$ .

The reaction is shown by the equation.

2 \mathrm{NaOH}+\mathrm{H}_{2} \mathrm{SO}_{4} \rightarrow \mathrm{Na}_{2} \mathrm{SO}_{4}+2 \mathrm{H}_{2} \mathrm{O}

State the colour of methyl orange in aqueous sodium hydroxide.

How this is tested

Question 4(c)(i)