CLASS 7 SCIENCE · CHAPTER 2 · STUDY NOTES
Exploring Substances: Acidic, Basic, and Neutral
Read indicator changes, compare evidence and understand neutralisation through labelled textbook experiments.
01 / EXPLORE & UNDERSTAND
A colour change can reveal a hidden message
The writing appears because substances react with an indicator.
1. Look for a chemical explanation
At the chapter’s science fair, spraying a liquid makes a message appear on a sheet. One possible explanation is a basic solution used for writing and a turmeric indicator used to reveal it. The colour change marks where the writing was.
2. Separate a possibility from proof
Seeing coloured words does not identify the exact liquids by itself. To test the explanation, we need to know the materials and compare how they behave. The chapter introduces indicators so we can connect an observation with evidence.
Your turn
Question
Why is “the paper is magic” less useful than an explanation involving the liquids used?
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02 / EXPLORE & UNDERSTAND
Use both red and blue litmus
A colour change tells us more than a colour that stays the same.
One drop on a fresh paper
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1. Learn the two changes
An acidic sample turns blue litmus red. A basic sample turns red litmus blue. Acid does not turn red litmus blue, and base does not turn blue litmus red. Start with the paper’s original colour before describing the result.
2. Why one unchanged strip is not enough
If red litmus stays red, the sample could be acidic or neutral in the chapter’s tests. Try blue litmus too. If neither paper changes in these prepared examples, we classify the sample as neutral. Use fresh pieces and clean droppers so samples do not mix.
| Prepared sample | Blue litmus | Red litmus |
|---|---|---|
| Acidic | Turns red | Stays red |
| Basic | Stays blue | Turns blue |
| Neutral | Stays blue | Stays red |
Your turn
Question
A sample leaves blue litmus blue. Does that observation alone prove it is basic?
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03 / EXPLORE & UNDERSTAND
Acidic, basic and neutral samples
Use known examples to understand the three groups.
1. Connect familiar samples to the groups
Lemon juice, vinegar and tamarind water are acidic examples. Soap solution, baking soda solution and lime water are basic examples. The chapter uses common salt and sugar solutions as neutral examples. Lime water means calcium hydroxide solution, not lime fruit juice.
2. Use evidence rather than taste
Sour foods often contain acids, but we never taste unknown substances to test them. Bitterness alone does not identify a base: bitter gourd is the chapter’s counterexample. Tap water can vary, so record the tested result instead of assuming every water sample is neutral.
Your turn
Question
Why must lime water and lemon juice not be put in the same group just because their names sound similar?
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04 / EXPLORE & UNDERSTAND
Familiar foods contain different acids
The name of an acid can connect with a food you already know.
Foods and some acids they contain
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1. Build a small set of connections
Lemon is associated with citric acid; tamarind with tartaric acid; vinegar with acetic acid; curd with lactic acid. Amla contains vitamin C, or ascorbic acid. Foods are mixtures, so these names do not mean the food contains only one substance.
2. Keep a food example separate from a laboratory chemical
A familiar edible food and a concentrated chemical are different situations. The presence of an acid does not tell us whether an unknown material is safe to eat or touch. In school investigations, use only approved samples with teacher guidance.
Your turn
Question
Which familiar pair goes together: vinegar and acetic acid, or vinegar and lime water?
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05 / EXPLORE & UNDERSTAND
Red rose extract gives another colour clue
An extract brings coloured substances from petals into water.
Compare tube A with tube B
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1. Understand the preparation
The chapter crushes washed red-rose petals, uses warm or hot water under adult supervision and filters the coloured liquid. Filtering separates petal pieces from the liquid extract. The liquid is then used as the indicator.
2. Compare the two test tubes
In the textbook’s examples, lemon juice gives a red shade with rose extract, while soap solution gives a green shade. The important comparison is the indicator’s response to an acidic and a basic sample. Actual shades depend on the extract and sample.
Your turn
Question
Why filter the petal mixture before using the coloured liquid?
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06 / EXPLORE & UNDERSTAND
Turmeric detects a base, but has a limit
Yellow can stay yellow in more than one kind of sample.
Preparing yellow indicator paper
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1. Notice the colour change
The chapter makes yellow turmeric paper and tests prepared samples. A basic solution changes it to red or reddish-brown. Acidic and neutral samples leave it yellow in these tests. This explains why soap can change a turmeric stain’s colour.
2. Know what yellow cannot tell you
An unchanged yellow strip does not separate an acid from a neutral sample. Use another suitable indicator to find that difference. A good scientist notices both what a test reveals and what it cannot decide.
Your turn
Question
An unknown prepared sample leaves turmeric yellow. Can you choose between acidic and neutral from this result alone?
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07 / EXPLORE & UNDERSTAND
Read several tests together
Combine observations instead of guessing from one unchanged colour.
One sample, three indicator papers
Compare the paper before and after one drop.
Blue litmus turns red. Yellow turmeric alone cannot distinguish this acid from a neutral sample.
1. Use a comparison that keeps the test fair
Test separate portions of the same sample with fresh indicator papers. Do not pour all indicators into the same container. Clean tools and clear labels help ensure that a colour change belongs to the sample being tested.
2. Explain the conclusion with the change
For a basic result, “red litmus turned blue” is stronger reasoning than “the paper looked blue” without saying its starting colour. For turmeric, red-brown supports a basic result, while yellow leaves two possibilities in these examples.
Your turn
Question
Why should a record include the paper’s starting colour?
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08 / EXPLORE & UNDERSTAND
Some indicators involve smell
Not every indicator works through a visible colour change.
1. Connect onion with the idea
The chapter uses onion-scented strips to compare acidic and basic samples. Such indicators are called olfactory indicators. The test concerns a prepared indicator’s smell, not a rule that all acids or bases have one common smell.
2. Keep an observation separate from an unsafe guess
This is a teacher-guided investigation using known materials. Do not sniff unknown laboratory liquids or household cleaners. Learning that smell can be an indicator does not make direct smelling a general method for identifying chemicals.
Your turn
Question
Does “olfactory” mean an indicator must change colour?
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09 / EXPLORE & UNDERSTAND
An acid and a base can react
The reaction changes the acidic and basic substances.
1. Learn the chapter’s basic pattern
For the acid–hydroxide reaction pattern introduced here: acid + base → salt + water, with heat released. “Salt” is a chemical group name; it does not always mean the table salt used in food.
2. Amounts matter
Adding any small amount of base does not automatically make an acidic mixture neutral. The substances react according to their amounts. One may remain in excess. A teacher uses indicators and a controlled procedure to follow the change.
A little deeper: Not every base follows exactly the same word equation
Carbonates and bicarbonates can release carbon dioxide when they react with acids. Baking soda is a bicarbonate. The simple salt-and-water pattern is useful for the hydroxide example, but it should not hide additional products in other reactions.
Your turn
Question
If much acid remains after a little base is added, must the final mixture be neutral?
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10 / EXPLORE & UNDERSTAND
Blue at the end means basic conditions
Distinguish “the acid has reacted” from “the mixture is exactly neutral”.
First: acidic conditions
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After enough lime water: basic conditions
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1. Follow the lemon–lime-water example
The chapter first adds blue litmus solution to diluted lemon juice; it turns red. Lime water is then added gradually. If the mixture becomes blue, the indicator shows that it has reached basic conditions.
2. Explain why more acid can reverse the result
Adding enough acid again can react with the base and eventually make the mixture acidic. The final colour depends on what remains and the indicator response. Blue is not evidence of an exactly neutral endpoint.
Your turn
Question
A student says a blue litmus mixture is “exactly neutral”. What should be corrected?
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11 / EXPLORE & UNDERSTAND
Test soil before deciding what it needs
Poor plant growth can have more than one cause.
Treatment follows the soil test
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1. Connect treatment with the tested problem
The chapter explains that some fertiliser use can make soil too acidic. Appropriate lime treatment can reduce acidity. It also describes adding organic matter to some basic soils. The suitable treatment and quantity depend on soil testing and agricultural guidance.
2. Do not diagnose from one symptom
Even soil near neutral can lack nutrients. Water, light and other conditions affect plant health too. “The plant looks weak” is a reason to investigate, not enough evidence to add an acid or a base.
Your turn
Question
Two plants look weak. Must both soils be acidic?
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12 / EXPLORE & UNDERSTAND
Neutralising waste is only one treatment step
A corrected acid-base condition does not remove every pollutant.
1. Connect acidic waste with neutralisation
The chapter considers acidic factory waste harming life in a lake. Controlled treatment can neutralise acidity before discharge. This is work for proper treatment systems, not an instruction to add household chemicals to a lake.
2. Look for other pollutants too
Waste may contain harmful materials even after its acidity is corrected. Treatment and checking must address the actual pollutants. “Neutral” describes acid-base nature, not a guarantee that a liquid is clean or safe.
Your turn
Question
Can a neutral liquid still be polluted?
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13 / EXPLORE & UNDERSTAND
Use the idea carefully in new situations
A familiar example helps learning when we also understand its limits.
1. Compare natural examples
The book connects hydrangea flower colour with soil conditions and uses an ant-bite story to introduce acid–base reactions. Real flower colour also depends on the plant and available substances; bite reactions vary. These examples should prompt questions rather than automatic treatment rules.
2. Connect chemistry with careful investigation
P.C. Ray, introduced in the chapter, helped advance chemistry research and education in India. The habit to practise here is careful comparison: name the sample, record the test and explain the evidence. Ask a teacher or trusted adult about any real exposure or bite.
A little deeper: Why vinegar can make carbonate bubble
Eggshell and marble contain calcium carbonate. An acid can react with carbonate and release carbon dioxide gas, producing bubbles. This is an additional reaction idea, not a reason to test unknown substances at home.
Your turn
Question
Why should an example not become a rule for every real-life situation?
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Take these ideas with you
- Acid turns blue litmus red; base turns red litmus blue.
- Turmeric turns red-brown with a base, but yellow does not separate acid from neutral.
- Rose extract is another natural indicator; compare the observed shades.
- Neutralisation is a reaction. The final mixture can have acid or base left over.
- A neutral liquid is not automatically safe or unpolluted.
- Use known samples, teacher guidance and careful records.
Use your understanding in a new situation
Question
A sample turns blue litmus red and leaves turmeric yellow. Explain why these results agree.
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After acid and base are mixed, red litmus turns blue. Is the remaining mixture acidic, basic or proved exactly neutral?
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