Exploring Substances: Acidic, Basic, and Neutral — Class 7 Science
Curiosity · Chapter 2
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7- Mathematics
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- + 3 more
- Space weighted by chapter size
- Charts from the book's own data
- Traps corrected
2- Mathematics
- Social Science
- Space weighted by chapter size
- Charts from the book's own data
- Traps corrected
- Space weighted by chapter size
- Charts from the book's own data
- Traps corrected
Chapter summary
Key ideas & terms
- Acid-base indicator
- A substance that shows different colours in acidic and basic solutions, such as litmus, red rose extract, or turmeric.
- Litmus
- A natural dye obtained from lichens, available as blue and red paper; acids turn it red and bases turn it blue.
- Acidic substance
- A substance that turns blue litmus red and generally tastes sour, such as lemon juice, amla, or vinegar.
- Basic substance
- A substance that turns red litmus blue, feels slippery, and generally tastes bitter, such as soap or baking soda solution.
- Neutral substance
- A substance that is neither acidic nor basic and does not change the colour of litmus, such as sugar or salt solution.
- Neutralisation
- The reaction in which an acid and a base react to form salt and water with the release of heat.
Extra questions & answers
What is litmus and from where is it obtained?
Litmus is a natural acid-base indicator obtained from lichens; it is available both as a solution and as blue and red paper strips.
How can you tell whether a substance is acidic using litmus?
An acidic substance turns blue litmus paper red.
How can you tell whether a substance is basic using litmus?
A basic substance turns red litmus paper blue.
What happens to a neutral substance when tested with litmus?
A neutral substance such as tap water, sugar solution, or salt solution does not change the colour of either red or blue litmus paper.
What colours does red rose extract show in acidic and basic solutions?
Red rose extract turns a shade of red in acidic solutions and a shade of green in basic solutions.
Why can turmeric paper not distinguish acidic from neutral substances?
Turmeric turns red only in basic solutions and stays yellow in both acidic and neutral solutions, so it can detect bases but not tell acids from neutrals.
What is an olfactory indicator? Give an example.
An olfactory indicator is a substance whose odour changes in an acidic or basic medium, such as chopped onion strips.
Write the general word equation for a neutralisation reaction.
Acid + Base gives Salt + Water + Heat — the acid and base cancel each other to form a neutral product with the release of heat.
Why is moist baking soda applied to a red ant bite?
An ant injects acidic formic acid into the skin; baking soda, being a base, neutralises this acid and relieves the pain.
How can acidic soil be treated so that plants grow well?
Acidic soil can be treated by adding lime (a base), which neutralises the excess acidity so the plants grow better.
Let Us Enhance Our Learning
1. A solution turns the red litmus paper to blue. Excess addition of which of the following solution would reverse the change?
(i) Lime water
(ii) Baking soda
(iii) Vinegar
(iv) Common salt solution
A solution that turns red litmus to blue is basic. To reverse the change the blue colour must turn back to red, which needs an acidic solution. Among the options, lime water and baking soda are basic, common salt solution is neutral, and vinegar is acidic. So excess addition of vinegar (iii) will neutralise the base and make the solution acidic, turning blue litmus back to red.
2. You are provided with three unknown solutions labelled A, B, and C, but you do not know which of these are acidic, basic, or neutral. Upon adding a few drops of red litmus solution to solution A, it turns blue. When a few drops of turmeric solution are added to solution B, it turns red. Finally, after adding a few drops of red rose extract to solution C, it turns green.
Based on the observations, which of the following is the correct sequence for the nature of solutions A, B, and C?
(i) Acidic, acidic, and acidic
(ii) Neutral, basic, and basic
(iii) Basic, basic, and acidic
(iv) Basic, basic, and basic
Red litmus turning blue indicates a basic solution, so A is basic. Turmeric remaining yellow in acidic and neutral solutions but turning red in basic solutions, so B turning red means B is basic. Red rose extract gives red in acidic and green in basic solutions, so C turning green means C is basic. Hence A, B and C are all basic. The correct sequence is (iv) Basic, basic, and basic.
3. Observe and analyse Figs. 2.13, 2.14, and 2.15, in which red rose extract paper strips are used. Label the nature of solutions present in each of the containers.
Red rose extract is red in acidic medium, green in basic medium and shows no significant change in neutral medium. Therefore the container where the rose extract strip turns to a shade of red contains an acidic solution, the container where it turns to a shade of green contains a basic solution, and the container where there is no colour change contains a neutral solution.
4. A liquid sample from the laboratory was tested using various indicators:
Indicator Red litmus Blue litmus Turmeric
Change No change Turned red No change in colour
Based on the tests, identify the acidic or basic nature of the liquid and justify your answer.
The liquid is acidic. Blue litmus turning red is the characteristic test for an acid, while red litmus shows no change in an acidic solution. Turmeric paper turns red only with bases and stays yellow with acids and neutral substances; no change with turmeric therefore also supports that the liquid is not basic. All three observations together confirm acidic nature.
5. Manya is blindfolded. She is given two unknown solutions to test and determine whether they are acidic or basic. Which indicator should Manya use to test the solutions and why?
Manya cannot see a colour change, so she should use an olfactory indicator such as onion extract. Olfactory indicators change their odour in acidic or basic media. For example, onion has a characteristic smell which is retained in acidic solutions but is lost or reduced in basic solutions, so she can distinguish acidic and basic by smell without needing to see.
6. Could you suggest various materials which can be used for writing the message on the white sheet of paper (given at the beginning of the chapter) and what could be in the spray bottle? Make a table of various possible combinations and the colour of the writing obtained.
The trick is invisible ink using an acid-base indicator. Examples:
Writing ink (acidic or basic, colourless when dry) | Spray (indicator) | Colour of writing revealed
Soap solution or baking soda solution or lime water (basic) | Turmeric solution | Red
Lemon juice, amla juice, tamarind water or vinegar (acidic) | Red rose extract or red hibiscus extract | Red tint
Soap solution or baking soda solution (basic) | Red rose extract | Green
Turmeric solution can be used as the paper coating and soap solution as the ink also gives red writing. Any combination where one partner is an indicator and the other is acidic or basic that causes a colour change will work.
7. Grape juice was mixed with red rose extract; the mixture got a tint of red colour. What will happen if baking soda is added to this mixture? Justify your answer.
Red tint with rose extract shows grape juice is acidic. Baking soda solution is basic. On adding a basic substance to the acidic mixture, neutralisation occurs:
8. Keerthi wrote a secret message to her grandmother on her birthday using orange juice. Can you assist her grandmother in revealing the message? Which indicator would you use to make it visible?
Orange juice contains citric acid and is acidic, so it is invisible when dry. To reveal it a basic indicator solution should be sprayed. Red rose extract is suitable because it turns red with acids, so spraying dilute red rose extract will make the writing appear red. Alternatively a basic spray like soap solution or baking soda solution on turmeric-coated paper would also reveal acidic writing, but turmeric itself does not show a change with acids, so red rose extract is the better visual indicator for an acidic ink. Lightly spray the paper and the hidden message will become visible.
9. How can natural indicators be prepared? Explain by giving an example.
Natural indicators are prepared from coloured plant parts. For example, red rose indicator: Collect fallen petals of red roses, wash them, crush them with a mortar and pestle, place the crushed petals in a glass tumbler, add hot water enough to immerse them, cover and wait for 5 to 10 minutes till water becomes coloured, then filter. The filtrate is the rose extract indicator. Similarly turmeric paper is made by making a paste of turmeric with water, dipping filter paper in it, drying and cutting into strips. Beetroot, purple cabbage, hibiscus and jamun can be used in the same way.
10. Three liquids are given to you. One is vinegar, another is a baking soda solution, and the third is a sugar solution. Can you identify them only using turmeric paper? Explain.
No, all three cannot be identified with only turmeric paper. Turmeric paper stays yellow with acidic vinegar and with neutral sugar solution, and turns red only with basic baking soda solution. So turmeric can identify the baking soda solution as the one that turns it red, but it cannot distinguish between vinegar (acidic) and sugar solution (neutral) because neither changes its colour. Another indicator like litmus or red rose extract is needed to differentiate those two.
11. The extract of red rose turns the liquid X to green. What will the nature of liquid X be? What will happen when excess of amla juice is added to liquid X?
Green colour with red rose extract indicates a basic solution, so liquid X is basic. Amla juice is acidic. Adding excess amla juice will neutralise the base:
12. Observe and analyse the information given in the following flowchart. Complete the missing information.
Which indicator can be used to test the nature of the soil? _________________
Imagine a garden with plants showing signs of poor health.
The basic soil can be treated with ________________________.
The acidic soil can be treated with ________________________.
The soil can be ______ in nature. The soil can be ______ in nature.
Which indicator can be used to test the nature of the soil? Any acid-base indicator such as blue and red litmus, red rose extract, turmeric paper or other natural indicator.
The soil can be acidic in nature. The soil can be basic in nature.
The acidic soil can be treated with a base such as lime, i.e. calcium oxide or lime water, calcium hydroxide, to neutralise the acidity.
The basic soil can be treated with organic matter such as manure, compost or composted leaves which release acids and neutralise the basic nature.
DIVE DEEPER
Aman accidentally spilt vinegar on some pieces of an eggshell or marble and noticed bubbling. He then poured a soap solution on another piece of eggshell or marble, but no bubbles appeared. Why did bubbles occur with vinegar but not with soap solution?
Eggshell and marble are made mainly of calcium carbonate