TEACHGYAN • CLASS 7 SCIENCE • CURIOSITY
Chapter 2 — Exploring Substances: Acidic, Basic, and Neutral
Indicators • Acids • Bases • Neutralisation • Everyday Chemistry
🌟 Chapter at a Glance
Many substances around us can be described as acidic, basic or neutral. Indicators help us identify these properties without tasting or touching unknown substances.
| Category | Typical property | Examples |
|---|---|---|
| Acidic | Turns blue litmus red | Lemon juice, vinegar |
| Basic | Turns red litmus blue | Soap solution, baking-soda solution |
| Neutral | Neither acidic nor basic in the simple indicator test | Pure water |
🧪 1. Indicators
An indicator is a substance that changes colour in response to acidic or basic conditions. Litmus is a common natural indicator. Turmeric and some flower or plant extracts can also act as indicators.
Safety: Never taste an unknown substance to decide whether it is acidic or basic.
🔴🔵 2. Acidic and Basic Substances
Acids and bases are not limited to laboratories. Citrus fruits contain acids; soap solutions are commonly basic. The observable indicator response gives evidence about their nature.
⚖️ 3. Neutralisation
When an acidic substance reacts with a basic substance, their properties can be reduced or changed. This process is called neutralisation. Neutralisation is useful in everyday life, agriculture and treatment of some acidic effects.
| Situation | Basic idea |
|---|---|
| Acidic soil | A suitable basic material may be added to reduce acidity |
| Antacid | A basic substance can help neutralise excess stomach acid |
| Insect sting | The appropriate neutralising substance depends on the nature of the sting; treatment should follow safe guidance |
🔎 4. Comparing Substances Experimentally
Instead of guessing, place small samples on separate indicator papers or use a suitable indicator solution. Record the colour change in a table. The same procedure should be used for each sample.
🧠 Worked Examples
- Blue litmus turns red in a sample → the sample shows acidic behaviour.
- Red litmus turns blue → the sample shows basic behaviour.
- No litmus colour change in the simple test does not mean “safe to taste”; it only describes that particular indicator response.
🎯 Concept Check
- Why are indicators useful?
- Why should unknown substances never be tasted?
- What is neutralisation?
- Give two examples of acidic and two examples of basic substances.
🧩 Interactive MCQs
- Blue litmus in an acidic solution generally becomes: A red B blue C green D yellow
- Red litmus in a basic solution becomes: A blue B red C black D colourless
- A substance used to identify acidic/basic nature is called: A indicator B solvent C fuel D catalyst
- Reaction between an acid and a base is called: A neutralisation B filtration C evaporation D freezing
Answers: 1-A, 2-A, 3-A, 4-A
📌 Case-Based Learning — Garden Soil
A gardener tests two soil samples with a suitable indicator. Sample A shows acidic behaviour; Sample B shows less acidic behaviour.
- Why is testing better than guessing?
- What kind of substance might be used to reduce excessive soil acidity?
- Why should the gardener follow recommended quantities?
- How could repeated measurements improve confidence?
🔥 HOTS / Competition Corner
- Design a safe test to classify five household liquids using an indicator.
- Why is “sour taste” not an acceptable scientific method for identifying an unknown acid?
- A student gets different indicator colours on different days. List possible experimental causes.
📝 Worksheet
| Level | Task |
|---|---|
| 1 | Classify examples as acidic, basic or neutral. |
| 2 | Predict litmus changes. |
| 3 | Interpret indicator observations. |
| 4 | Explain neutralisation in real situations. |
| 5 | Design a safe investigation. |
🏆 Mastery Checklist
- ☐ I can explain what an indicator does.
- ☐ I can interpret litmus results.
- ☐ I understand acidic, basic and neutral behaviour.
- ☐ I can explain neutralisation.
- ☐ I can design a safe indicator investigation.
🔁 One-Minute Revision
Indicators give evidence about acidic/basic nature. Acids and bases have different indicator responses, and neutralisation is a reaction in which their characteristic effects are reduced or changed.