Year 10 · Science · Chemical sciences
Acids and bases
Students use a homemade red cabbage indicator to classify household substances as acidic, neutral or basic, and investigate neutralisation.
Awaiting educator review. This material was drafted by our content team with AI assistance and is waiting for review by a qualified educator. Please check it suits your child before using it. Created 10 October 2026.
These lessons were written around Australian Curriculum strands and matched to New Zealand year levels by age (NZ Year 1 is the first year of school). They are not yet mapped to The New Zealand Curriculum, so check them against your own learning programme.
Learning goals
Learners will
- Describe the properties of acids and bases and their place on the pH scale.
- Use a natural indicator to classify household substances.
- Describe neutralisation and write a general word equation for it.
Success looks like
- I can state whether a pH value is acidic, neutral or basic.
- I can make and use red cabbage indicator safely.
- I can write: acid + base → salt + water.
The big idea
Acids taste sour (never taste in experiments) and include vinegar, lemon juice and the acid in your stomach. Bases feel slippery and include bicarbonate of soda, soap and many cleaning products.
The pH scale measures acidity. Values below 7 are acidic, 7 is neutral and above 7 is basic. Each step on the scale is a tenfold change: a solution of pH 3 is ten times more acidic than one of pH 4.
An indicator changes colour depending on pH. Red cabbage contains natural pigments called anthocyanins. Its juice is roughly red or pink in acids, purple near neutral, and blue, green or yellow in bases.
When an acid and a base react, they can cancel each other out. This is neutralisation: acid + base → salt + water. (When the base is a carbonate or hydrogen carbonate, carbon dioxide gas is also produced.)
Neutralisation is used every day. Antacid tablets neutralise excess stomach acid, and Australian farmers spread lime on acidic soils to raise the pH so crops grow better.
Strong acids and bases are corrosive and can burn skin and eyes. This lesson uses only weak, food-grade substances.
Worked examples
Making red cabbage indicator
How do you make and use a red cabbage indicator?
- An adult chops the cabbage into small pieces and covers it with hot water in a heatproof bowl.
- Leave for 10–15 minutes until the water is deep purple, then let it cool.
- Strain the liquid into a jug.
- Pour a little indicator into each cup, then add a small amount of a test substance.
- Record the colour and compare it with the colour guide.
Answer: Red/pink means acidic, purple means about neutral, blue/green/yellow means basic.
Interpreting results
Indicator turns pink with lemon juice, purple with tap water and green with bicarbonate of soda solution. Classify each.
- Pink indicates an acid, so lemon juice is acidic.
- Purple indicates about neutral, so tap water is close to neutral.
- Green indicates a base, so bicarbonate of soda solution is basic.
Answer: Lemon juice: acidic. Tap water: about neutral. Bicarbonate of soda solution: basic.
Neutralising vinegar
What happens when you slowly add bicarbonate of soda solution to vinegar coloured with indicator?
- The vinegar starts pink (acidic).
- Add bicarbonate of soda solution a spoonful at a time. It fizzes as carbon dioxide forms.
- The colour shifts towards purple as the acid is neutralised, then to blue-green if you add too much base.
Answer: The mixture becomes less acidic and the colour moves from pink towards purple: neutralisation.
Practice check
Have a go, then check your answers. Each answer comes with an explanation.
Answer guide for parents
A solution has a pH of 3. It is…
Acidic — Any pH below 7 is acidic.
Which of these is a base?
Bicarbonate of soda solution — Bicarbonate of soda dissolves to form a weakly basic solution.
What are the products of neutralisation between an acid and a hydroxide base?
Salt and water — Acid + base → salt + water.
Why do some Australian farmers spread lime on paddocks?
To raise the pH of acidic soil — Lime is basic and neutralises acidic soils.
How many times more acidic is pH 2 than pH 3?
10 times — Each pH unit is a tenfold change.
What is the pH of a neutral solution at 25 °C? (number)
7 — Neutral is pH 7 at 25 °C.
What colour does red cabbage indicator usually turn in vinegar?
pink — In acids, red cabbage indicator turns red or pink.
A substance that changes colour depending on pH is called an i________.
indicator — This is an indicator.
Watch out for
- Students often think all acids are dangerous and burn. Many acids, like those in fruit and vinegar, are weak and found in food.
- Some think neutralisation makes substances disappear. New substances (a salt and water) form.
- Some think a pH of 1 is 'less acidic' than 6 because the number is smaller. Lower pH means more acidic.
Tips for parents
- An adult should handle the hot water; let the cabbage liquid cool before use.
- Keep a white sheet of paper behind the cups so colours are easier to judge.
- Safety: wear eye protection, use only food-grade substances and mild dishwashing liquid, never taste the results, and never use bleach, drain cleaner or oven cleaner, which are dangerous.
Go further
- Paint with red cabbage indicator on paper, then brush on lemon juice and bicarbonate solution to make colour-changing art.
- Test rainwater, pool water (with an adult) and tank water and compare.
- Research ocean acidification and its effect on corals of the Great Barrier Reef.