Exploring Substances Acidic Basic and Neutral
Lemon juice is sour and soap feels slippery — these are clues to a substance's nature. This Class 7 Curiosity chapter explores acidic, basic and neutral substances, the indicators that reveal them, and what happens when an acid and a base react in neutralisation.
Learning objectives
- Identify acidic, basic and neutral substances.
- Use natural indicators to test substances.
- Describe properties of acids and bases.
- Explain neutralisation with examples.
Key concepts
Acids, bases and neutral substances
Acids generally taste sour, like lemon juice and vinegar, while bases taste bitter and feel slippery, like soap and baking soda solution. Substances that are neither acidic nor basic, such as pure water and salt solution, are neutral. We never taste laboratory chemicals to test them — we use indicators instead.
Indicators
An indicator is a substance that changes colour to show whether something is acidic or basic. Litmus turns red in acids and blue in bases. Many natural materials work too: turmeric stays yellow in acids but turns red in bases, and china rose and red cabbage juice also change colour.
Acids and bases around us
Acids and bases are common in daily life. Citrus fruits, curd and tamarind contain acids; soap, lime water and antacids are basic. Our stomach produces acid to digest food, and an antacid (a base) is taken to relieve the discomfort of too much stomach acid.
Neutralisation
When an acid and a base are mixed in the right amounts, they react and cancel each other's nature, forming a salt and water. This reaction is called neutralisation. It explains why an antacid soothes acidity and why lime is added to acidic soil, and it usually releases heat.
Important formulas
Neutralisation
acid + base → salt + water
Key definitions
- Acid
- A substance that is sour and turns blue litmus red.
- Base
- A substance that is bitter, slippery, and turns red litmus blue.
- Indicator
- A substance that changes colour to show acid or base.
- Neutralisation
- The reaction of an acid with a base to give salt and water.
Solved examples
Q1. What colour does blue litmus turn in lemon juice?
Solution: Red, because lemon juice is acidic.
Q2. What forms when an acid reacts with a base?
Solution: A salt and water (neutralisation).
Q3. Name a natural indicator other than litmus.
Solution: Turmeric (or china rose / red cabbage juice).
Common mistakes to avoid
- Tasting unknown substances to test them (never do this).
- Thinking litmus turns the same colour in acids and bases.
- Believing all liquids are either acidic or basic (some are neutral).
- Forgetting that neutralisation produces a salt and water.
Exploring Substances Acidic Basic and Neutral — MCQ Quiz
10 questions with instant feedback. Use number keys 1–4 to answer.
Acids generally taste:
Practice questions
Short answer
How does litmus behave in acids and bases?
It turns red in acids and blue in bases.
What is neutralisation?
The reaction of an acid with a base to form salt and water.
Why should we not taste lab chemicals?
They may be harmful; we use indicators instead.
Long answer
Compare the properties of acids and bases, and explain how indicators detect them.
Acids and bases have contrasting properties. Acids generally taste sour (like lemon and vinegar) and turn blue litmus red, while bases taste bitter, feel slippery (like soap and baking soda solution) and turn red litmus blue. Because tasting unknown substances is unsafe, we detect acids and bases using indicators — substances that change colour depending on the nature of what they touch. Litmus is the most common, but natural indicators also work: turmeric stays yellow in acids but turns red in bases, and china rose and red cabbage juice change colour too. A neutral substance, such as pure water, does not change the colour of litmus.
What is neutralisation, and where do we see it in daily life?
Neutralisation is the reaction that occurs when an acid and a base are mixed in suitable amounts: they cancel each other's nature and form a salt and water, usually releasing some heat. This idea explains several everyday situations. When the stomach produces too much acid and causes discomfort, an antacid — which is a base — neutralises the excess acid. Farmers add lime, a base, to soil that is too acidic to make it suitable for crops. The treatment of factory waste before it is released often involves neutralising acids or bases so the water is safe.
HOTS (Higher Order Thinking)
A gardener finds the soil is too basic for a plant. Should they add an acidic or basic material, and why?
An acidic material, to neutralise the excess base and bring the soil closer to neutral.
Why does an ant sting feel better after rubbing baking soda on it?
An ant injects an acid; baking soda is a mild base that neutralises the acid, easing the sting.
Quick revision
Revision notes
- Acids: sour, turn blue litmus red.
- Bases: bitter, slippery, turn red litmus blue.
- Neutral substances don't change litmus (e.g. water).
- Acid + base → salt + water (neutralisation).
Key takeaways
- Indicators reveal acidic, basic or neutral nature.
- Litmus turns red in acid, blue in base.
- Neutralisation makes salt and water.
Frequently asked questions
What is an indicator?
A substance that changes colour to show whether something is acidic or basic.
Is water acidic or basic?
Pure water is neutral.
What does an antacid do?
It is a base that neutralises excess stomach acid.