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Drip acid or alkali into water and watch the colour: how much does each drop change it?
pH Scale lab · Last updated: 16 October 2026
Drip acid or alkali into water and watch the colour change. In this 3D lab, you see how an indicator reveals pH and discover that each step on the scale is a big jump. It links kitchen ingredients, soaps, stomach acid and soil to a single number from 0 to 14.
What you'll discover
- Acids have a pH below 7, alkalis have a pH above 7, and pure water sits at 7.
- Indicators change colour with pH, so you can read how acidic or alkaline a liquid is by looking at it.
- Each step on the pH scale means a tenfold change, so the first few drops into pure water change it most.
The science in plain words
The pH scale measures how acidic or alkaline a solution is, usually running from 0 to 14. Pure water is neutral at pH 7. Acids such as lemon juice and vinegar have a pH below 7, and alkalis such as soap solution and baking soda solution have a pH above 7. What pH really tracks is the concentration of hydrogen ions in the water: the more hydrogen ions, the more acidic and the lower the number.
The scale is logarithmic, which means each whole step is a factor of ten. A solution at pH 3 has ten times as many hydrogen ions as one at pH 4, and a hundred times as many as one at pH 5. So a few drops of strong acid into pure water can shift the pH sharply, while later drops make less difference.
Indicators are substances that change colour with pH. Universal indicator runs from red for strong acids through green at neutral to violet for strong alkalis. Natural indicators include litmus, turmeric, which turns red in alkalis, and red cabbage juice. When an acid and an alkali are mixed in the right amounts they neutralise each other, forming a salt and water, which is how antacids soothe an upset stomach.
Where it fits
Common questions
What is the pH of water?
Pure water has a pH of 7, which is neutral: neither acidic nor alkaline. Tap water and rainwater often differ a little because of dissolved minerals and gases. Rain is usually slightly acidic because carbon dioxide from the air dissolves in it.
What does pH stand for?
pH is usually read as the 'power of hydrogen'. It is a measure of how many hydrogen ions are in a solution. Because the scale works in powers of ten, a lower pH means many more hydrogen ions and a more acidic solution.
How can I test pH at home?
You can make a natural indicator by boiling chopped red cabbage and keeping the purple juice. It turns pink or red in acids such as vinegar and green or yellow in alkalis such as baking soda solution. Turmeric paper turns reddish brown in alkalis such as soap solution.
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Try it yourself
The pH Scale lab is one of the interactive 3D science labs inside Taz. Change one thing, predict what will happen, then run it and watch. Labs open after you sign in.