Chapter 2 — Exploring Substances: Acidic, Basic and Neutral
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Class 7 Science Chapter 2 — Exploring Substances: Acidic, Basic, and Neutral — teaches students how to identify whether substances are acidic, basic, or neutral using natural and synthetic acid-base indicators such as litmus, red rose extract, turmeric, and onion.
This chapter introduces students to the three categories of substances — acidic, basic, and neutral — and the indicators used to distinguish them. Students learn that litmus (derived from lichens) turns red in acidic solutions and blue in basic ones, while red rose extract turns red in acid and green in base, and turmeric paper turns red only in basic solutions. The chapter also covers neutralisation reactions, in which an acid and a base react to form salt, water, and heat, and explores everyday applications such as treating ant bites, correcting acidic or basic soil, and neutralising industrial waste before it enters water bodies.
Key points & formulas
- 01Substances are classified as acidic, basic, or neutral based on how they affect indicators like litmus paper.
- 02Litmus, obtained from lichens, is an acid-base indicator available in two forms — blue and red paper strips. Acids turn blue litmus red; bases turn red litmus blue.
- 03Substances that do not change the colour of either blue or red litmus paper are neutral (e.g., tap water, sugar solution, salt solution).
- 04Acidic substances (e.g., lemon juice, vinegar, tamarind water) generally taste sour; basic substances (e.g., baking soda solution) feel soapy or slippery and generally taste bitter.
- 05Red rose extract is a natural acid-base indicator: it turns red in acidic solutions and green in basic solutions.
- 06Turmeric paper turns red (or reddish-brown) in basic solutions but does not change colour in acidic or neutral solutions — so it cannot distinguish between acidic and neutral substances.
- 07Substances whose odours change in an acidic or basic medium are called olfactory indicators (e.g., onion).
- 08When an acid and a base react in sufficient quantity, a neutralisation reaction occurs: Acid + Base → Salt + Water + Heat.
- 09Ant bites inject formic acid (an acidic liquid) into the skin; applying moist baking soda (a base) neutralises the acid and relieves pain.
- 10Acidic soil can be treated with lime (a base); if soil is basic, organic matter such as manure and composted leaves (which release acids) can neutralise it.
Frequently asked questions
01What is Class 7 Science Chapter 2 about?
Chapter 2 is about exploring substances that are acidic, basic, and neutral. It teaches students how to classify common substances using natural indicators like litmus, red rose extract, and turmeric, and explains what happens when an acid and a base are mixed (neutralisation).
02What is an acid-base indicator?
An acid-base indicator is a substance that shows different colours in acidic and basic solutions. Litmus, red rose extract, red hibiscus, purple cabbage, turmeric, and beetroot are examples of natural acid-base indicators mentioned in the chapter.
03Where does litmus come from, and how is it used?
Litmus is a natural substance obtained from lichens — organisms formed by the association of a fungus and an alga. It is available as a solution or as paper strips in two colours. Blue litmus turns red in acidic substances, and red litmus turns blue in basic substances.
04What colour does red rose extract show in acidic and basic solutions?
Red rose extract turns red (a shade of red) when added to an acidic solution and turns green when added to a basic solution. Substances that do not change its colour are neutral.
05Why can turmeric paper not distinguish between acidic and neutral substances?
Turmeric paper turns red in basic solutions but does not change colour in either acidic or neutral solutions. Because both acidic and neutral substances leave the yellow turmeric paper unchanged, it cannot be used to tell acidic from neutral — it can only detect the presence of a base.
06What is a neutralisation reaction?
A neutralisation reaction occurs when an acid and a base react in sufficient quantity so that the resulting solution is neither acidic nor basic. The products are salt and water, and heat is also released: Acid + Base → Salt + Water + Heat.
07Why does applying baking soda relieve the pain of an ant bite?
When an ant bites, it injects an acidic liquid called formic acid into the skin. Baking soda is a base. Applying moist baking soda to the affected area neutralises the formic acid, which relieves the stinging pain.
08How can acidic or basic soil be corrected to help plants grow?
If the soil is too acidic due to excessive use of chemical fertilisers, it can be treated with lime, which is a base, to neutralise the acidity. If the soil is basic, organic matter such as manure and composted leaves can be added; organic matter releases acids that neutralise the basic nature of the soil.
09What are olfactory indicators?
Olfactory indicators are substances whose odour changes when they come into contact with an acidic or basic medium. Onion is an example used in the chapter — strips of cloth or filter paper infused with onion odour are tested with acidic and basic samples to observe whether the smell changes.
10How can you prepare a natural indicator from red rose petals?
Collect and wash fallen red rose petals, crush them with a mortar and pestle, place them in a glass tumbler, pour hot water over them until the petals are fully immersed, cover for 5–10 minutes until the water is coloured, then filter. The coloured filtrate is the red rose extract, which can be used as an acid-base indicator.
11Can turmeric be used to make indicator paper strips like litmus paper?
Yes. Mix turmeric powder with a little water to make a paste, dip strips of filter paper in the paste until they turn yellow, let them dry, then cut into strips. These 'turmeric paper' strips turn red in basic solutions but remain yellow in acidic and neutral solutions.
12Is the Class 7 Science Chapter 2 PDF free to download? Do I need to sign up?
Yes — the NCERT PDF for Class 7 Science Chapter 2 is completely free to read and download on cbseprepmaster.com. No account or sign-up is required.
More chapters in Curiosity
This is the complete Curiosity Chapter 2 as published by NCERT — every diagram, solved example, and exercise included, free. Browse all NCERT Class 7 textbooks.
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