Exploring Substances: Acidic, Basic, and Neutral
Class 7 · Science
6 ideas in Class 7 Science, explained in plain words, with games and quizzes to practise.
Litmus and the Three Groups
Part of: Acidic, basic and neutral
Substances around us can be acidic, basic or neutral. Litmus, a natural substance from lichens, helps us tell which. Blue litmus paper turns red in an acid, red litmus paper turns blue in a base, and a neutral substance changes neither. Because litmus shows different colours in acids and bases, it is called an acid-base indicator.
Why it matters. Sorting things by a test result is how scientists work. Children learn to trust an observation instead of a guess, and they see that everyday liquids fall into three clear groups.
Try it in two minutes
- Ask your child to name four liquids at home, such as tap water, lemon juice, sugar water and a little soap in water.
- Ask them to guess which might be acidic, basic or neutral, and why.
- Explain that litmus paper would settle it: blue to red means acidic, red to blue means basic, no change means neutral.
- Let them make a three-column chart and place each liquid in a group.
Try at home. Draw a three-column chart (Acidic, Basic, Neutral) and, with an adult, fill it with the kitchen liquids you already know about. Do not taste unknown liquids.
Taste, Touch and Safe Testing
Part of: Acidic, basic and neutral
Everyday clues hint at whether a substance is acidic or basic. Many acids taste sour, like lemon juice, and many bases feel soapy or slippery, like baking soda solution. But these clues are only hints: bitter karela is not basic, and we must never taste unknown substances.
Why it matters. Children learn that a hint is not proof, and that good scientists use safe tests rather than tasting or touching mystery liquids.
Try it in two minutes
- Name a few sour foods at home and ask your child what they have in common.
- Explain that sour foods tend to contain acids.
- Rinse a pinch of baking soda in water with an adult and let your child feel how slippery it is.
- Ask: why should we never taste an unknown liquid to test it?
Try at home. With an adult, feel a little soap lather and a little plain water between your fingers. Talk about which feels slippery and what that hint suggests.
Red Rose and Other Natural Indicators
Part of: Natural indicators
Litmus is not the only indicator. Extract made from red rose petals turns red in an acidic solution and green in a basic solution, and shows no change in a neutral one. Beetroot, purple cabbage, jamun and red hibiscus can work in a similar way.
Why it matters. Children see that science tools can be made from things around us, and that the same idea (a colour change) can be tested in different ways.
Try it in two minutes
- Pick up a few fallen red petals with your child instead of plucking flowers.
- With an adult, soak the crushed petals in warm water and strain the coloured liquid.
- Put a few drops in two clear glasses and add a little lemon juice to one and a little soap solution to the other.
- Ask your child to describe each colour change.
Try at home. With an adult handling the warm water, make red rose or purple cabbage extract and test lemon juice, soap solution and plain water on it.
Turmeric Paper and Smell Indicators
Part of: Natural indicators
Turmeric paper is yellow paper coated with turmeric. It turns red in a basic solution but stays yellow in both acidic and neutral ones, so it tests only for bases. Some substances, like chopped onion, change their smell in acidic or basic solutions, and are called olfactory indicators.
Why it matters. Children learn that every tool has limits. A test that cannot tell two things apart needs a partner test, which is an important habit in science.
Try it in two minutes
- Ask your child why a curry stain turns reddish when soap touches it.
- Explain that turmeric turns red in a base and stays yellow otherwise.
- Ask: if turmeric stays yellow, can we say the liquid is acidic? Why not?
- Talk about which second test, such as blue litmus, would settle it.
Try at home. With an adult, put a small spot of soap solution on a turmeric-stained cloth or plate and watch the colour. Compare with a spot of lemon juice.
Neutralisation: Acid Meets Base
Part of: Neutralisation
When an acid and a base are mixed in enough quantity, each cancels the effect of the other. The new solution is neither acidic nor basic. This is neutralisation, and it forms salt and water while releasing heat: acid plus base gives salt plus water plus heat.
Why it matters. Neutralisation is the idea behind many everyday fixes. Understanding it helps children reason about why certain remedies work instead of memorising them.
Try it in two minutes
- Ask your child to imagine a see-saw with acid on one side and base on the other.
- Explain that adding enough of the opposite substance brings the see-saw to the middle: neutral.
- Ask what two things form besides heat: salt and water.
- Ask why the amount matters: a tiny bit of base may not be enough to cancel a lot of acid.
Try at home. Use the see-saw drawing to show what happens as you add more base to an acid. Draw the three stages: acidic, neutral, basic.
Neutralisation in Daily Life
Part of: Neutralisation
Neutralisation helps solve real problems. A red ant bite leaves formic acid on the skin, and moist baking soda, a base, eases it. Acidic soil can be treated with lime, basic soil with manure or composted leaves, and acidic factory waste can be neutralised with a base before it enters a lake.
Why it matters. Children see science working in gardens, kitchens and rivers. They learn that the fix depends on the problem, and that a neutral soil can still have other problems.
Try it in two minutes
- Ask your child: what do we do when an ant bites? Why might a base help?
- Discuss why a farmer would add lime to acidic soil.
- Ask what could be added to basic soil, and why.
- Discuss why factory waste should be treated before it reaches a lake.
Try at home. Ask a grandparent or neighbour which home remedies they use for ant bites, and talk about whether they use an acid or a base.
What your child will practise
- Litmus and the Three Groups: Space Quest (Collect the Answer)
- Neutralisation: Acid Meets Base: Ocean Puzzle (Clue Puzzle)
- Neutralisation in Daily Life: Jungle Quick Fire (Quick Fire)
- Red Rose and Other Natural Indicators: Desert Quick Fire (Quick Fire)
- Taste, Touch and Safe Testing: Ocean Quest (Collect the Answer)
- Quiz: Litmus and the Three Groups (8 questions)
- Quiz: Neutralisation in Daily Life (8 questions)
- Quiz: Neutralisation: Acid Meets Base (8 questions)
- Quiz: Red Rose and Other Natural Indicators (8 questions)