Acids, Bases and pH
Senior 1 · Chemistry
Acids, Bases and pH
Lesson 3 of 5
25 min
Squeeze a slice of lemon into a cup of black tea and the dark amber drink turns paler and more orange. Stir in a pinch of baking soda instead and it turns a deep red-brown. The tea is behaving like an indicator. Many plants contain coloured substances that change colour with acidity, so you can make indicators from things in a kitchen or garden when the school has no litmus.
The best-known natural indicators, red cabbage, hibiscus flowers and dark berries such as blackberries, all contain purple, red or blue pigments called anthocyanins. Anthocyanins are red in acids and turn purple, blue, green and finally yellow as a solution becomes more and more alkaline.
Definition
Natural indicator: a coloured substance from a plant (or other living thing) that changes colour in acids and alkalis. Examples: red cabbage juice, hibiscus flower extract, blackberry juice, black tea, turmeric.
You can make hibiscus or blackberry indicator in the same way, from a handful of red hibiscus petals or crushed ripe berries.
Figure 3.1: Making red cabbage indicator and using it to test household liquids.
Warning
Safety. Take care with hot water and knives. Test only foods and mild household liquids such as lemon juice, vinegar, salt water, baking soda and soap. Never test bleach, drain cleaner or toilet cleaner yourself: they are corrosive. Do not drink anything used in an experiment.
Figure 3.2: An indicator colour chart. The swatches show typical colours; real colours vary with the plant and how strong the extract is.
Table 3.1: Natural and household indicators
| Indicator | Coloured substance | In acid | In alkali |
|---|---|---|---|
| Red cabbage juice | anthocyanins | red to pink | blue, green, then yellow |
| Hibiscus flower extract | anthocyanins | pink-red | green |
| Blackberry or raspberry juice | anthocyanins | red | blue-green |
| Black tea | brown tea pigments | paler, orange | darker, red-brown |
| Turmeric (a yellow spice) | curcumin | yellow | red-brown |
Worked example
Problem. Umutoni tests five liquids with red cabbage indicator. Use Figure 3.2 to decide whether each is acidic, neutral or alkaline, and estimate its pH: lemon juice (red); ikivuguto (pink); tap water (purple); baking soda solution (blue); wood-ash water (green).
Solution. Match each colour with the red cabbage scale in Figure 3.2.
| Liquid | Colour | Estimated pH | Type |
|---|---|---|---|
| Lemon juice | red | about 2 | strongly acidic |
| Ikivuguto | pink | about 4 | weakly acidic (lactic acid) |
| Tap water | purple | about 7 | neutral |
| Baking soda solution | blue | about 8 to 9 | weakly alkaline |
| Wood-ash water | green | about 11 | alkaline |
Red cabbage shows not only whether a liquid is acidic or alkaline, but roughly how acidic or alkaline it is.
Worked example
Problem. Which indicator would you choose to tell apart (a) vinegar and water, (b) soap solution and water? Explain why turmeric would fail in (a).
Solution. (a) Use red cabbage, hibiscus or litmus: all of them change colour in an acid (red or pink) and show a different colour in water. Turmeric is yellow in both acids and neutral solutions, so vinegar and water would look the same. Phenolphthalein would fail too, being colourless in both. (b) Any of them works for an alkali: turmeric turns red-brown, phenolphthalein turns pink, red cabbage turns green.
Important
A good indicator has different colours on the two sides of the change you want to detect. Red cabbage is the most useful household indicator because it changes colour many times across the whole pH range.
Tip
Purple plants are pH detectors. If a flower, fruit or leaf is red, purple or blue, it very probably contains anthocyanins and will change colour in acids and alkalis.
Harvard University: Measuring pH with Red Cabbage (2 min). Watch the colour of each liquid and try to place it on the red cabbage scale in Figure 3.2.
Practice
Sources and credits
This lesson adapts CC BY-SA material and is shared under CC BY-SA 4.0.
