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Exploring Substances: Acidic, Basic, and Neutral

NCERT Class 7 · Science Based on NCERT Class 7 Science textbook · Free CBSE study kit

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Chapter Notes

Overview
This chapter introduces students to the classification of substances around us into acidic, basic, and neutral based on their chemical nature. It explains how natural and synthetic indicators like litmus, red rose extract, and turmeric can be used to test the nature of substances. The chapter also covers the process of neutralisation, where an acid and a base react to form salt and water with the release of heat, and its applications in daily life such as treating ant bites, correcting soil pH, and managing industrial waste. The one big idea a student must walk away with is that the world around us can be understood and managed by knowing whether substances are acidic, basic, or neutral, and that indicators help us identify them safely.
Key Concepts (12)
Acidic substances
Substances that turn blue litmus paper to red are acidic in nature.
e.g. Lemon juice, amla juice, tamarind water, vinegar.
Basic substances
Substances that turn red litmus paper to blue are basic in nature.
e.g. Soap solution, baking soda solution, lime water, washing powder solution.
Neutral substances
Substances that do not change the colour of either red or blue litmus paper are neutral; they are neither acidic nor basic.
e.g. Tap water, sugar solution, salt solution.
Litmus
A natural substance obtained from lichens, used as an acid-base indicator. It is available as a solution and in the form of paper strips (blue and red).
e.g. Blue litmus paper turns red in acid; red litmus paper turns blue in base.
Acid-base indicator
A substance that shows different colours in acidic and basic solutions.
e.g. Litmus, red rose extract, turmeric, red hibiscus, purple cabbage, beetroot, jamun.
Red rose extract as indicator
The extract of red rose petals gives red colour in acidic solutions and green colour in basic solutions.
e.g. Adding lemon juice (acid) to red rose extract turns it red; adding soap solution (base) turns it green.
Turmeric paper
Filter paper dipped in turmeric paste and dried; it turns red in basic substances but remains yellow in acidic and neutral substances.
e.g. Soap solution turns turmeric paper red; lemon juice does not change its colour.
Olfactory indicator
A substance whose odour changes in an acidic or basic medium.
e.g. Onion — its strong smell changes when acidic or basic substances are added.
Neutralisation reaction
A reaction in which an acid and a base react to form salt and water with the evolution (release) of heat.
e.g. Lemon juice (acid) + lime water (base) → salt + water + heat.
Formic acid
The acidic liquid injected into the skin by a red ant bite, causing stinging pain and redness.
e.g. Ant bite injects formic acid; applying moist baking soda (base) neutralises it.
Lime water
A solution of calcium hydroxide in water, prepared by mixing lime (chuna, calcium oxide) in water and filtering it. It is basic in nature.
e.g. Lime water turns red litmus blue and is used to neutralise acidic soil.
Lichens
Natural organisms formed by the association of a fungus and an alga, growing on rocks and trees in regions with abundant rainfall and clean air. Litmus is obtained from lichens.
e.g. Lichens found on tree trunks in clean, rainy areas.
How It Works
Testing substances with litmus paper
  1. Take a strip of blue litmus paper and cut it into small pieces.
  2. Place the pieces on a clean, dry white tile.
  3. Using a dropper, put one drop of each sample on separate pieces of blue litmus paper.
  4. Observe and record any colour change — blue to red indicates acidic.
  5. Repeat the same procedure using red litmus paper — red to blue indicates basic.
  6. If no colour change occurs on either paper, the substance is neutral.
💡 Blue litmus turns red in acid; red litmus turns blue in base. Students often reverse this — remember: 'Blue to Red = Acid' (BRA).
Preparing red rose extract as an indicator
  1. Collect fallen red rose petals (do not pluck flowers).
  2. Wash the petals with water.
  3. Crush the petals using a mortar and pestle.
  4. Place the crushed petals in a glass tumbler and pour hot water to immerse them completely (under adult supervision).
  5. Cover the tumbler and wait for 5–10 minutes until the water becomes coloured.
  6. Filter the liquid — the filtrate is the red rose extract indicator.
💡 Red rose extract turns red in acid and green in base. Students often forget the green colour for bases.
Preparing and using turmeric paper
  1. Take a spoonful of turmeric (haldi) in a container and add a little water to make a paste.
  2. Dip a piece of filter paper in the turmeric paste until it turns yellow.
  3. Take it out and allow it to dry.
  4. Cut the yellow paper into thin strips — these are turmeric paper strips.
  5. Using a dropper, put a drop of each sample on separate turmeric paper strips.
  6. Observe colour change — yellow to red indicates basic; no change indicates acidic or neutral.
💡 Turmeric paper only detects bases (turns red). It cannot distinguish between acids and neutral substances — both leave it yellow.
Neutralisation reaction (acid + base)
  1. Take one drop of lemon juice (acid) in a test tube and add about twenty drops of water.
  2. Add a drop of blue litmus solution — the solution turns red, confirming it is acidic.
  3. Slowly add drops of lime water (base) using a dropper and swirl.
  4. Continue adding lime water until the colour changes from red to blue — this indicates the acid has been neutralised.
  5. The resulting solution is neither acidic nor basic; salt and water are formed with the release of heat.
💡 Neutralisation always produces salt + water + heat. The heat released is often forgotten by students.
Using olfactory indicators (onion test)
  1. Take finely chopped onions in a container with strips of clean cotton cloth or filter paper.
  2. Tightly close the container and leave it overnight so the strips absorb the onion odour.
  3. Take two strips and check their odour.
  4. Put a few drops of tamarind water (acid) on one strip and baking soda solution (base) on the other.
  5. Check the odour again — note any change in smell.
💡 Olfactory indicators change smell, not colour. Students often confuse them with colour-change indicators.
Exam Traps — Don't Make These Mistakes
⚠ Reversing litmus colour changes
✗ Blue litmus turns red in base; red litmus turns blue in acid.
→ The student has reversed the actual colour changes. Blue litmus turns red only in acids, and red litmus turns blue only in bases.
✓ Blue litmus turns red in acidic substances; red litmus turns blue in basic substances.
⚠ Confusing lime water with lime fruit
✗ Lime water is made from lime fruit (like lemon).
→ Lime water is a solution of calcium hydroxide in water, prepared from lime (chuna, calcium oxide), not from the fruit lime.
✓ Lime water is a basic solution made by mixing lime (chuna, calcium oxide) in water and filtering it. It is not related to the fruit lime.
⚠ Turmeric paper for all substances
✗ Turmeric paper can be used to test both acidic and basic substances.
→ Turmeric paper only changes colour (yellow to red) in basic substances. It remains yellow in both acidic and neutral substances, so it cannot distinguish between them.
✓ Turmeric paper is an indicator only for basic substances. It turns red in bases but shows no change in acids or neutral substances.
⚠ Neutralisation product missing heat
✗ Neutralisation reaction produces salt and water only.
→ The student forgets that heat is also released (exothermic reaction). The full equation includes heat.
✓ Neutralisation reaction produces salt, water, and heat. Acid + Base → Salt + Water + Heat.
⚠ All sour things are acidic, all bitter things are basic
✗ All substances that taste sour are acidic, and all substances that taste bitter are basic.
→ While acids generally taste sour, not all sour things are safe acids (e.g., some are toxic). Also, bases generally taste bitter, but not everything bitter is a base — for example, bitter gourd (karela) is bitter but not basic.
✓ Substances that taste sour tend to contain acids and are acidic in nature. Bases generally taste bitter, but everything that tastes bitter may not contain a base (e.g., bitter gourd).
Compare & Contrast
Effect on litmus paper
Acidic substances
Basic substances
Acidic substances turn blue litmus paper to red; basic substances turn red litmus paper to blue.
Colour change in acidic and basic solutions
Red rose extract
Turmeric paper
Red rose extract turns red in acid and green in base; turmeric paper turns red only in base and shows no change in acid or neutral substances.
Exam Focus
Marks pattern: 2 MCQs (1 mark each), 1 Very Short Answer (1 mark), 1 Short Answer (3 marks), 1 Long Answer (5 marks) — total approximately 10 marks.
Most asked: Identification of acidic, basic, and neutral substances using litmus paper · Neutralisation reaction and its applications in daily life (ant bite, soil treatment, industrial waste) · Natural indicators: litmus, turmeric, red rose extract · Properties of acids (sour taste) and bases (bitter taste, slippery feel)
Don't skip: Explain neutralisation reaction with an example and its application in any one daily life situation.; How would you test whether a given solution is acidic, basic, or neutral using litmus paper?; Describe how to prepare a natural indicator from red rose petals and use it to test lemon juice and soap solution.
One-Line Revision (read aloud before the exam)
  1. Substances that turn blue litmus red are acidic; those that turn red litmus blue are basic; those that cause no change are neutral.
  2. Litmus is a natural indicator obtained from lichens.
  3. Acids generally taste sour; bases generally taste bitter and feel slippery.
  4. Red rose extract turns red in acidic solutions and green in basic solutions.
  5. Turmeric paper turns red only in basic substances; it remains yellow in acidic and neutral substances.
  6. Neutralisation reaction: Acid + Base → Salt + Water + Heat.
  7. Ant bite injects formic acid (acidic); applying moist baking soda (base) neutralises it.
  8. Acidic soil is treated with lime (base); basic soil is treated with organic matter (which releases acids).
  9. Factory waste that is acidic can be neutralised by adding basic substances before releasing it into water bodies.
  10. Lime water is a basic solution made from calcium oxide (chuna) and water, not from the fruit lime.
  11. Olfactory indicators like onion change smell in acidic or basic media.
  12. Hydrangea flowers are blue in acidic soil and pink or red in basic soil.
  13. Lichens are formed by the association of a fungus and an alga.
  14. Do not taste unknown substances in the laboratory; use indicators to test nature.
  15. Bitter gourd (karela) tastes bitter but is not basic in nature.

MCQs — 10 Questions with Answers

Q1. Which of the following is obtained from lichens?

  • A. Litmus ✓
  • B. Turmeric
  • C. Rose extract
  • D. Baking soda

Answer: A — Litmus is a natural substance extracted from lichens and is used as an acid-base indicator.

Q2. What happens when lemon juice is added to blue litmus paper?

  • A. It turns green
  • B. It turns red ✓
  • C. It remains blue
  • D. It turns yellow

Answer: B — Lemon juice is acidic and turns blue litmus paper red because it contains citric acid.

Q3. Which colour change indicates a basic substance when tested with red litmus paper?

  • A. Red to blue ✓
  • B. Red to green
  • C. Blue to red
  • D. Yellow to brown

Answer: A — Red litmus paper turns blue in the presence of a basic substance, indicating the substance is basic.

Q4. How does soap solution feel when rubbed between fingers?

  • A. Rough and grainy
  • B. Soapy and slippery ✓
  • C. Hot and burning
  • D. Sticky and thick

Answer: B — Soap solution is basic in nature and basic substances feel slippery to touch.

Q5. You are given four liquids: lemon juice, baking soda solution, tap water, and vinegar. Which one will not change the colour of either blue or red litmus paper?

  • A. Lemon juice
  • B. Baking soda solution
  • C. Tap water ✓
  • D. Vinegar

Answer: C — Tap water is neutral and does not change the colour of blue or red litmus paper because it is neither acidic nor basic.

Q6. Seema made rose extract indicator for testing substances. She added a drop of soap solution to the extract and observed the colour change. What colour did the extract turn?

  • A. Red
  • B. Green ✓
  • C. Yellow
  • D. Brown

Answer: B — Soap solution is basic; red rose extract turns green in basic solutions, so the extract turned green.

Q7. A student prepared turmeric paper and tested it with both acidic and basic substances. With which substance will the turmeric paper show a colour change?

  • A. Lemon juice only
  • B. Baking soda solution only ✓
  • C. Both lemon juice and baking soda
  • D. Neither lemon juice nor baking soda

Answer: B — Turmeric paper is yellow and only changes colour (to brown) when basic substances are added; it does not change colour with acidic substances.

Q8. At the science fair, a white sheet became visible after a liquid was sprayed on it. This demonstrates that the sprayed liquid most likely contained:

  • A. A neutral substance
  • B. An acid or base that reacted with invisible ink ✓
  • C. Only water
  • D. Turmeric powder

Answer: B — The liquid revealed hidden text, suggesting it reacted chemically with an invisible ink, likely because the liquid was acidic or basic.

Q9. A vegetable extract changes from pink to colourless when acid is added and turns blue-green when base is added. Which vegetable can produce such an extract?

  • A. Beetroot
  • B. Purple cabbage ✓
  • C. Potato
  • D. Onion

Answer: B — Purple cabbage contains anthocyanins that act as natural pH indicators, changing colour from pink in acid to blue-green in bases.

Q10. Why is Hydrangea flower colour considered an indicator of soil nature in the Himalayan region?

  • A. Because it absorbs water
  • B. Because its colour depends on whether the soil is acidic or basic ✓
  • C. Because it needs sunlight to grow
  • D. Because it produces acids naturally

Answer: B — Hydrangea flowers produce blue flowers in acidic soil and pink/red flowers in basic soil, making them natural indicators of soil pH.

Flashcards

What is litmus and where does it come from?

Litmus is a natural substance obtained from lichens, available as paper strips (blue and red) or solution.

What colour does blue litmus turn when an acidic substance is added?

Blue litmus turns red when an acidic substance is added to it.

What colour does red litmus turn when a basic substance is added?

Red litmus turns blue when a basic substance is added to it.

Which of these is an example of an acidic substance: lemon juice, baking soda, or tap water?

Lemon juice is acidic because it contains citric acid and turns blue litmus paper red.

Name two properties of basic substances.

Basic substances feel slippery to touch and taste bitter.

What colour does red rose extract turn when added to an acidic solution?

Red rose extract turns to a shade of red when added to an acidic solution.

What colour does red rose extract turn when added to a basic solution?

Red rose extract turns to a shade of green when added to a basic solution.

What is a neutral substance? Give one example.

A neutral substance is neither acidic nor basic and does not change the colour of litmus paper; tap water is an example.

Can turmeric paper be used as an indicator for acidic substances?

No, turmeric paper cannot be used for acidic substances because it only changes colour with basic substances.

Why is litmus called an acid-base indicator?

Litmus is called an acid-base indicator because it shows different colours in acidic and basic solutions.

Important Board Questions

What is an acid-base indicator? Name one example. [1 mark]

Definition: substance that shows different colours in acidic and basic solutions. Example: litmus paper, rose extract, or turmeric.

What colour changes occur when blue litmus paper and red litmus paper are added to an acidic substance like tamarind water? [2 marks]

Blue litmus turns red in acidic solution. Red litmus does not change colour in acidic solution — no change means it stays red.

Describe the steps to prepare red rose extract as an acid-base indicator. [3 marks]

Steps: collect fallen red rose petals → wash → crush with mortar and pestle → place in glass → pour hot water → cover and wait 5-10 minutes → filter → red rose extract ready. (Do not pluck flowers from plants.)

Compare acidic and basic substances using five differences based on their properties, indicators, and everyday examples. [5 marks]

Make a table with: (1) Taste — sour vs bitter; (2) Feel — not slippery vs slippery; (3) Blue litmus — red vs no change; (4) Red litmus — no change vs blue; (5) Examples — lemon vs soap. Include: Definition of each, how litmus detects them, and one Indian food/household example for each.

Assertion-Reason Questions for Exploring Substances: Acidic, Basic, and Neutral

Q1. Assertion (A):

Litmus paper is an acid-base indicator because it shows different colours in acidic and basic solutions.

Reason (R):

Litmus changes colour due to the presence of specific chemical compounds in acidic and basic substances that react with litmus dye.

Answer: B — Both A and R are true, but R is NOT the correct explanation of A — Both statements are true. The assertion correctly defines litmus as an indicator based on colour changes. The reason explains why colour changes occur (chemical reaction with compounds in solutions). However, the reason is not the direct explanation of the assertion—the assertion simply defines litmus's function based on observed behaviour, while the reason provides a deeper chemical mechanism. Thus, they're both true but reason doesn't directly explain why litmus IS an indicator in the way intended by the assertion.

Q2. Assertion (A):

Substances that turn blue litmus paper red are classified as acidic.

Reason (R):

Acidic substances contain hydrogen ions that change the colour of blue litmus paper to red.

Answer: A — Both A and R are true, and R is the correct explanation of A — Both assertion and reason are true, and the reason correctly explains why blue litmus turns red—it's due to the chemical nature of acids (hydrogen ions). This is the correct explanation for the colour change observed in Activity 2.1, making this a straightforward application of acid-base chemistry concepts.

Q3. Assertion (A):

Neutral substances do not change the colour of either blue or red litmus paper.

Reason (R):

Neutral substances are neither acidic nor basic, so they contain neither hydrogen nor hydroxide ions.

Answer: C — A is true but R is false — The assertion is true—as shown in Table 2.2, Group C substances (tap water, sugar solution, salt solution) don't affect litmus. However, the reason is oversimplified and incomplete. Neutral substances can contain various ions; it's not just about the absence of H+ and OH- ions. The reason doesn't fully explain the assertion's truth in an accurate chemical way.

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Case-Based Questions (CBQ) for Exploring Substances: Acidic, Basic, and Neutral

Case Study:

Meera conducted an experiment in her science class to test various household substances using litmus paper. She tested eight substances: milk, pickle juice, baking soda solution, dish soap, honey, plant fertilizer, distilled water, and apple juice. She created a table showing which samples turned blue litmus red (Group A), red litmus blue (Group B), or neither (Group C). The teacher noted that Meera's results correctly classified the substances, but asked her to identify which substance could be used as a natural indicator, similar to how litmus paper functions.

Q1. According to the experiment pattern, which of Meera's tested substances would most likely belong to Group A (acidic)?

  • A. Baking soda solution and dish soap
  • B. Pickle juice and apple juice ✓
  • C. Honey and distilled water
  • D. Plant fertilizer and milk

Answer: B

Q2. If Meera wanted to use a natural indicator from the substances she tested, which would be the most appropriate choice?

  • A. Apple juice, because it's acidic and contains natural pigments ✓
  • B. Baking soda solution, because it's basic and colourless
  • C. Milk, because it's neutral and widely available
  • D. Plant fertilizer, because it's specifically designed for testing

Answer: A

Q3. What would happen if Meera tested her apple juice with blue litmus paper?

  • A. The blue litmus would turn red because apple juice is acidic ✓
  • B. The blue litmus would turn blue because apple juice is basic
  • C. The litmus paper would not change colour because apple juice is neutral
  • D. The litmus paper would turn purple because apple juice is both acidic and basic

Answer: A

Q4. Based on the litmus testing method, which substance from Meera's experiment would NOT affect either blue or red litmus paper?

  • A. Pickle juice or apple juice
  • B. Distilled water or honey ✓
  • C. Milk or plant fertilizer
  • D. Baking soda solution or dish soap

Answer: B

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