Acids and alkalis · Model
The pH scale and indicators
A dye squeezed out of a boiled cabbage can tell you something about a liquid that no amount of looking at it will. What is it actually reading?
Start here
Boil a red cabbage, keep the purple water, and throw away the cabbage. You have made a scientific instrument.
A few drops of that purple liquid go into six beakers. It comes out red in one, pink in another, purple in the third, then blue, then green, then yellow. Nothing else was added and the cabbage water was the same in every beaker.
What is the cabbage dye responding to?
How acidic or alkaline each beaker is. The dye is a molecule that changes shape depending on the acid around it, and a different shape reflects a different colour. Six beakers, six positions on one scale — and because the colours come in a fixed order, the colour is a reading, not a decoration. That is what an indicator is.
An indicator is a dye that changes colour depending on whether it is in an acid or an alkali. Litmus is the simple one: red in acid, blue in alkali, and nothing in between. It answers one question — which side of neutral is this? — and answers it instantly.
Universal indicator is a mixture of several dyes, and its colour changes gradually all the way along. Match that colour to a printed chart and you get a number: the pH. Below 7 is acidic, above 7 is alkaline, and 7 exactly is neutral.
Your turn · the testing bench
Pick a sample, guess its pH, then drop the indicator in.
A guess you commit to is worth ten you keep to yourself.
Battery acid
Lemon juice
Rainwater
Pure water
Baking soda solution
Oven cleaner
A few drops from a car battery, handled with gloves and a face shield.
Squeezed straight from the fruit, no water added.
Collected clean, in the open, well away from a road.
Distilled, straight from the still, kept sealed.
A spatula of sodium hydrogencarbonate stirred into water.
Sprayed into a beaker, in a fume cupboard, gloves on.
Which band is it in?
Strongly acidic.
Litmus: red
Sulfuric acid, and about as far down the scale as anything you will ever meet. It is about a thousand times more acidic than the vinegar on a chip shop counter.
Strongly acidic.
Litmus: red
Citric acid. The same pH as the acid in your own stomach, and safe to drink because your mouth and stomach are built for it — your tooth enamel less so.
Weakly acidic.
Litmus: red
Slightly acidic even when perfectly clean, because carbon dioxide from the air dissolves into it. Genuinely pure water at pH 7 is a laboratory object, not a weather event.
Neutral.
Litmus: no change
The definition of neutral. Leave the bottle open for an hour and it will drift towards 6 as air dissolves in.
Weakly alkaline.
Litmus: blue
A mild alkali — mild enough to eat, which is why it is in cakes and in some toothpastes, where it works against the acid that attacks enamel.
Strongly alkaline.
Litmus: blue
Sodium hydroxide. It is as far from neutral as the battery acid was, in the other direction, and it does comparable damage — but it feels soapy instead of stinging.
Key fact
Litmus tells you which side of neutral you are on. Universal indicator gives you a number on the pH scale — and every step down that scale is ten times more acidic.
Three jobs · pick the right indicator
Litmus, universal indicator, or neither
0 of 3 decided
Commit to each. The right tool is the one that answers the question being asked — no more than that.
A technician needs to know whether a spilled solution is acid or alkali, immediately, before deciding what to clean it up with.
Litmus. One question, one answer, no chart to consult: red means acid, blue means alkali. A pH number would be more information than the decision needs, and it would take longer to read.
A fish farmer must keep pond water between pH 6.5 and 8, and needs to know when it drifts out of range.
Universal indicator — or better, a pH meter. Litmus cannot help here at all: both ends of the acceptable range are close to neutral, and litmus only tells you which side of it you are on. This job needs a number.
A student wants to know how much acid is in a bottle of vinegar compared with a bottle of lemon juice.
Neither, on its own. pH tells you how acidic the solution is, not how much acid is dissolved in it — a weak acid at high concentration and a strong acid at low concentration can read the same. Answering this properly means neutralising each with a measured alkali and comparing how much it took.
Think again
“pH 2 is twice as acidic as pH 4 — it is half the number, so it is double the strength.”
It looks like an ordinary number line. Commit before you read on.
Each step of one on the pH scale is a factor of ten. pH 3 is ten times more acidic than pH 4; pH 2 is a hundred times more acidic than pH 4; pH 1 is a thousand times. The numbers are close together because the scale squashes an enormous range into fifteen steps — that is the entire reason it exists.
Which is why the gap between lemon juice at pH 2 and rainwater at pH 6 is not four small steps. It is ten thousand times. Two pH numbers that look near each other can describe liquids that behave nothing alike.
Mastery ladder
Not started yet.
Rungs 3 and 4 you mark yourself.
Rung 1 · Recall
What does universal indicator do that litmus does not?
Rung 2 · The one that catches people
A solution turns universal indicator green. A student adds more indicator and the green gets deeper. What has changed about the solution?
Rung 3 · Explain
Explain how you would find the pH of a soil sample from a garden, and why the answer would be useful to the gardener.
Rung 4 · Take it somewhere new
A drinks company claims its new drink is “gentle on teeth”. Enamel starts to dissolve below about pH 5.5. Design a fair test that would check the claim against an ordinary cola, and say what result would support it.
Key note
An indicator is a dye whose colour depends on how acidic or alkaline a solution is. Litmus is red in acid and blue in alkali. Universal indicator changes gradually and is matched against a chart to give a pH number: below 7 acidic, 7 neutral, above 7 alkaline. Each step on the scale is a factor of ten.
Going further
Litmus is not a manufactured chemical. It is squeezed out of lichens — the crusty growths on gravestones and stone walls — and has been used as a dye since long before anyone knew what pH was. Red cabbage, beetroot, blackberries and the petals of hydrangeas all work for the same reason: the pigment that makes them coloured is a molecule that changes shape in acid. Hydrangeas go further and read the soil, coming up blue in acid ground and pink in alkaline.
Nobody in a working laboratory judges pH by eye any more. A pH meter puts a thin glass bulb into the solution and measures a voltage across it, giving two decimal places in a second and no argument about whether the colour is more yellow-green than green. Colour charts survive because they are cheap, need no power and cannot go out of calibration halfway through a lesson.
Before this lesson
At GCSE this becomes
- pH as a measure of hydrogen ion concentration, the logarithmic scale, and choosing indicators by the range over which they change.
Where to next
Ask Mr Badmus AI
Still not sure why the scale jumps by ten?
Two of the six samples on that bench are corrosive. They are tested a few drops at a time, in a beaker, with gloves and eye protection — never by dipping a strip into the bottle, which contaminates what is left in it.
Lesson content © MrBadmusAI.