Cambridge O Level Chemistry · Syllabus 5070 · Acids, Bases and Salts
Indicator
What is Indicator?
A substance that has one colour in acidic solution and a different colour in alkaline solution, so that its colour reports which of the two is present. Litmus, thymolphthalein and methyl orange are the three named in this syllabus.
This definition is part of the Acids, Bases and Salts chapter in Cambridge O Level Chemistry.
Common mistakes with Indicator
- “Methyl orange is purple in alkali.” Repair Methyl orange is red in acid and yellow in alkali. Purple belongs to litmus in a neutral solution. Learn each indicator as a complete pair; half a pair is worth nothing in a multiple-choice question.
- “Thymolphthalein is the pink one.” Repair That is phenolphthalein, a different indicator and not one of the three this syllabus requires. Thymolphthalein is colourless in acid and blue in alkali. The three required indicators are litmus, thymolphthalein and methyl orange.
- “Universal indicator just tells you acid or alkali.” Repair Litmus does that. Universal indicator gives a graded colour, so it lets you compare how acidic or alkaline two solutions are, and match the colour to an approximate pH. That is what statement 7.1.12 is asking for.
- “Titrate to the end point, then evaporate that flask to get the crystals.” Repair That flask contains indicator, which would be trapped in the crystals and colour the product. Record the volume from the titration, then repeat with the same measured volumes and no indicator, and crystallise that solution.
Examiner tips on Indicator
- The two sentences examiners are looking for “An excess of an alkali cannot be removed by filtration, because it is dissolved — so the volumes must be measured instead.” and “The titration is repeated without indicator so that the indicator does not contaminate the crystals.” Those two ideas are what separate a described method from a memorised one.
Questions students ask about Indicator
Why can't you make a soluble salt from an acid and an alkali by filtering?
Both the acid and the alkali are dissolved, and so is the salt formed, so there is no undissolved solid for a filter to remove. The exact volume of alkali that reacts with the acid must instead be found by titration, using an indicator, and then the same volumes are mixed again without indicator so the crystals are not contaminated.
What are the colours of the three required indicators in acid and in alkali?
Litmus is red in acid and blue in alkali. Thymolphthalein is colourless in acid and blue in alkali. Methyl orange is red in acid and yellow in alkali. Each indicator must be learned as a complete pair, since half a pair — such as remembering only that methyl orange is red — is not enough to answer a question correctly.