Class 7 Science Chapter 2 Exploring Substances Notes 2026-27

The NCERT Notes for Class 7 Science Chapter 2, Exploring Substances: Acidic, Basic, and Neutral, bring together every indicator rule, reaction, and real-life example the 2026-27 Curiosity textbook covers. These notes explain how litmus, red rose extract, and turmeric paper each react to acids and bases, and how neutralisation turns a sting or a sour spill into something harmless.

• Covers the full chapter: litmus, red rose extract, turmeric paper, and the neutralisation reaction with everyday examples.
• Includes quick-reference tables, key terms, and a common-mistakes list for fast revision before a test.
• Free PDF download, matched to the 2026-27 NCERT Curiosity textbook.

These Class 7 Science notes for Exploring Substances: Acidic, Basic, and Neutral are curated by subject experts and checked line by line against the 2026-27 NCERT Curiosity textbook.

What Makes a Substance Acidic, Basic, or Neutral

Every liquid around us, lemon juice, soap water, plain tap water, belongs to one of three natural groups. You do not need a lab to find out which. Nature gives us simple tools called indicators that change colour depending on the substance they touch.

• Acidic substances: lemon juice, amla juice, tamarind water, vinegar. Most taste sour.
• Basic substances: soap solution, baking soda solution, lime water, washing powder solution. Many feel slippery.
• Neutral substances: tap water, sugar solution, salt solution. Neither acidic nor basic.

An acid-base indicator is any substance, natural or synthetic, that shows a different colour in acidic and basic solutions. Litmus is not the only one; red rose extract and turmeric work just as well, and both are covered later in these notes.

Tip: never test an unknown substance by taste or touch. Bitter gourd tastes bitter but contains no base at all, so an indicator test is always the safer, more reliable check.

The Litmus Test and the Blue-to-Red, Red-to-Blue Rule

Litmus comes from lichens, organisms formed by a fungus and an alga growing together on rocks and tree bark. It is sold as a solution or as coloured paper strips soaked in litmus and dried, in two forms: blue litmus and red litmus.

The rule behind Activity 2.1 of the NCERT chapter is the one fact this whole section rests on: blue litmus paper turns red in an acid, and red litmus paper turns blue in a base. A neutral sample changes neither strip. Testing samples like lemon juice, soap solution, and sugar solution this way sorts them cleanly into Group A (acidic), Group B (basic), and Group C (neutral).

A common mix-up: lime water is not the lime fruit. It is a solution of calcium hydroxide, made by mixing lime (calcium oxide) with water and filtering it. The two share a name but nothing else.

Acids and Bases in Everyday Chemistry: A Quick Video Recap

Source: Khan Academy India on YouTube

Red Rose Extract and Turmeric Paper as Natural Indicators

Litmus is not the only plant-based indicator in this chapter. Crushed red rose petals soaked in hot water yield a coloured extract, and turmeric paste soaked into filter paper strips gives another. Both react differently to acids and bases, and neither reacts the same way as litmus.

Red rose extract turns a shade of red in an acidic sample and green in a basic sample; a neutral sample leaves it unchanged. Turmeric paper only turns red in a basic sample; it cannot tell an acidic sample from a neutral one, since both leave it unchanged. This is a favourite trick question: if turmeric shows no colour change, the sample could still be acidic or neutral, so a second indicator like litmus is needed to decide.

Also Check: a soap stain on a turmeric-yellow curry mark often turns reddish-brown, because the soap is basic and reacts with the turmeric pigment, a real-life version of the same rule.

How to Prepare a Natural Indicator at Home

Both red rose extract and turmeric paper follow the same broad recipe, and trying it once makes the whole chapter easier to remember.

1. Collect the material: fallen petals, or a spoonful of turmeric powder.
2. Wash petals with water, or mix turmeric with a little water to form a paste.
3. Crush petals with a mortar and pestle, or dip filter paper into the turmeric paste.
4. Soak crushed petals in hot water for 5 to 10 minutes, under adult supervision.
5. Filter the liquid, or dry the dipped paper, to get the finished indicator.

Beetroot, purple cabbage, jamun, and hibiscus (gudhal) all work the same way. Hydrangea plants show a related trick directly in soil: acidic soil produces blue flowers, while basic soil produces pink or red ones, a living indicator a gardener can read at a glance. Never taste any extract or paste made this way, even though the raw materials are otherwise edible.

Common Edible Acids You Already Know

Many foods taste sour because they contain a specific, named acid. Curd contains lactic acid and vinegar contains acetic acid, even though neither appears in the NCERT figure for this table.

Neutralisation: What Happens When an Acid Meets a Base

When an acid and a base are mixed in the right amounts, both cancel each other's effect. The result is neither acidic nor basic. This is called neutralisation, one of the most useful ideas in this chapter.

Every neutralisation reaction follows the same word equation: Acid + Base → Salt + Water + Heat. In Activity 2.7, diluted lemon juice turns blue litmus solution red; adding lime water slowly shifts the colour back to blue, showing the acid has been neutralised. Students often remember the salt and the water but forget that heat is a product too.

Neutralisation at Work: Ant Bites, Soil, and Factory Waste

Neutralisation is not confined to a test tube. It solves everyday problems at very different scales.

• Ant bites: an ant injects formic acid, which stings. Moist baking soda, a base, neutralises it and eases the pain.
• Farm soil: lime treats soil that has become too acidic; organic manure treats soil that has turned too basic.
• Factory waste: acidic waste is treated with a base before it reaches a lake, protecting the fish living in it.

Once you spot "acidic problem," the fix is almost always the same: add a base.

Quick Revision: Indicator Colour Chart and Common Mistakes

• Litmus is not the only acid-base indicator; red rose extract and turmeric work too.
• Lime water is calcium hydroxide solution, not the lime fruit.
• A bitter taste does not always mean a substance is basic; bitter gourd is proof.
• Turmeric paper cannot separate an acidic sample from a neutral one; only litmus can.
• Heat is a product of neutralisation too, alongside salt and water.

Key terms to know cold: an acid-base indicator shows different colours in acids and bases; an olfactory indicator changes odour instead of colour; neutralisation is the acid-plus-base reaction that forms salt and water and releases heat; a neutral substance changes none of the three indicators in this chapter.

One Worked Example: Identifying an Unknown Solution

Three unknown solutions, P, Q, and R, are tested with different indicators. Red litmus turns blue when added to P. Turmeric solution turns red when added to Q. Red rose extract turns green when added to R. What can you say about each one?

Red litmus turning blue means P is basic. Turmeric turning red also means Q is basic. Red rose extract turning green means R is basic too. All three results point the same way, even though three different indicators were used to reach the same answer. This is exactly the kind of reasoning question that shows up in a Class 7 test on this chapter.

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Other Class 7 Science Chapter 2 Resources on Collegedunia

Class 7 Science Curiosity Notes for All Chapters