Litmus, phenolphthalein and methyl orange

An indicator is a substance (usually a dye) that is one colour in an acid and a different colour in an alkali. By adding a few drops to a solution — or dipping in indicator paper — you can tell whether the solution is acidic or alkaline from the colour it turns.

For Double Award Chemistry, spec point 2.38 asks you to recall the colours of three named indicators in acidic and alkaline solutions:

• litmus
• methyl orange
• phenolphthalein

These are pure recall marks. The examiner has a colour written on the mark scheme — write the exact colour and you score; write the wrong colour (or a vague answer like "it changes") and you score zero. The three indicators are easy to muddle, so learn them carefully.

Two jobs indicators do:

• Distinguish acid from alkali — any of these indicators does this.
• Give a sharp end-point for a titration — methyl orange and phenolphthalein do this well because they switch colour over a narrow range; litmus does not.

This single table is the whole subtopic. Memorise every box.

Indicator	In acid	Neutral	In alkali
Litmus	Red	Purple	Blue
Methyl orange	Red	Orange	Yellow
Phenolphthalein	Colourless	Colourless	Pink

Memory hooks that stop you muddling them:

• Litmus has the most familiar pair: red = acid, blue = alkali ("Acid → bRight Red"). Neutral litmus is purple (a blend of red and blue).
• Methyl orange is red in acid, yellow in alkali — its neutral colour is orange (red + yellow), which is also its name, an easy reminder.
• Phenolphthalein is the odd one out — it is colourless in acid and turns a dramatic pink in alkali. Many students wrongly say "white"; the answer is colourless.

Key contrast to remember: litmus and methyl orange are red in acid; phenolphthalein is colourless in acid. Only phenolphthalein gives a colour in alkali that isn't shared with the others (pink).

Different indicators are useful for different jobs.

Litmus (paper or solution) only tells you acid or alkali — red or blue. It changes colour over a wide range of pH, so the change is gradual and it cannot pinpoint the exact moment a reaction is complete. It is perfect for a quick "is this acidic or alkaline?" test, but not for a titration.

Methyl orange and phenolphthalein switch colour over a narrow range, so adding a single drop too much of acid (or alkali) causes a definite, sharp colour change. That sharp change marks the end-point of a titration precisely, which is why these two are the standard titration indicators.

Job	Best choice	Why
Quick "acid or alkali?" test	Litmus paper	Cheap, simple, just need red/blue
Sharp end-point in a titration	Methyl orange or phenolphthalein	Sudden, single colour change
Estimating a pH value	Universal indicator	Range of colours across the whole pH scale

Universal indicator is a mixture of several indicators. Its colour blends through a rainbow (red → green → purple) as pH rises, so it gives an approximate pH value — but the colour changes gradually, with no sharp switch, so it is not used to find a titration end-point.

So the rule is:

• Want an estimate of pH? → universal indicator (many colours).
• Want a sharp end-point for a titration? → a single indicator (methyl orange or phenolphthalein).

Indicator solution. Add a few drops of the indicator to the solution you are testing and read the colour. In a titration the indicator goes into the conical flask with the solution being tested, and the colour change tells you when the reaction is complete.

Indicator paper. Litmus paper comes as red and blue strips:

• Blue litmus paper turns red in an acid.
• Red litmus paper turns blue in an alkali.
• A neutral solution leaves both papers unchanged.

This is a quick way to identify a solution without measuring pH.

Identifying an unknown solution. If you are given indicator results, work backwards using the colour table. For example:

• Litmus stays red and phenolphthalein is colourless → the solution is acidic.
• Litmus turns blue and phenolphthalein turns pink → the solution is alkaline.

Using two indicators that agree makes the identification more reliable, and is a common exam-style task.

Watch your wording. In the exam, write the precise colour (e.g. "yellow", "pink", "colourless"). Vague answers — "it changes colour", "it goes dark", "it stays the same" — earn no marks because they don't name the colour the mark scheme is looking for.