NCERT Solutions for Class 7 Science Curiosity Chapter 2 Exploring Substances: Acidic, Basic and Neutral

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
See Answer(iii) Vinegar.
Red litmus turning blue indicates a basic solution. To reverse this, an acidic solution is needed. Vinegar is acidic.

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
See Answer(iv) Basic, basic and basic.
A turns red litmus blue -> Basic. B turns turmeric red -> Basic. C turns red rose extract green -> 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.

See AnswerFig. 2.13: Acidic (Red rose extract turns red in acid).
Fig. 2.14: Basic (Red rose extract turns green in base).
Fig. 2.15: Neutral (Red rose extract color is unchanged in neutral solutions).

4. A liquid sample from the laboratory was tested using various indicators:

Based on the tests, identify the acidic or basic nature of the liquid and justify your answer.
See AnswerThe liquid is acidic. Justification: It turned blue litmus red, which is characteristic of acids. The lack of change with red litmus and turmeric is also consistent with an acidic substance.

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?
See AnswerManya should use an olfactory indicator, such as onion strips. Because she is blindfolded, she cannot rely on visual color changes. Olfactory indicators change their smell in acidic or basic media, which she can detect.

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.
Answer:
The effect uses an acid-base indicator reaction.

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.
See AnswerThe mixture will likely turn green.
Justification: Grape juice turning the red rose extract red indicates it’s acidic. Baking soda is basic. Adding a base (baking soda) neutralizes the acid. If enough baking soda is added to make the solution basic, the red rose extract indicator will turn green.

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?
See AnswerUse an indicator that changes color in acidic conditions, as orange juice is acidic (contains citric acid). Suitable indicators are Blue litmus solution (will turn red) or Red rose extract (will turn red). Apply the indicator solution to the paper.

9. How can natural indicators be prepared? Explain by giving an example.
See AnswerNatural indicators can be prepared from parts of plants like flowers or vegetables. Example (Red Rose Extract): Collect fallen red rose petals, wash them, crush them, place them in a beaker and add hot water. Let it stand until the water gets colored, then filter the liquid. The colored filtrate is the indicator.

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.
See AnswerYou can identify the baking soda solution, but not distinguish between vinegar and sugar solution. Explanation: Turmeric paper turns red in basic solutions. It shows no color change in acidic or neutral solutions. Vinegar is acidic, baking soda solution is basic, and sugar solution is neutral. Only the basic baking soda solution will turn the yellow turmeric paper red. The other two will cause no change.

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?
See AnswerLiquid X is basic (red rose extract turns green in basic solutions). Amla juice is acidic. Adding excess acidic amla juice will neutralize the base and make the solution acidic. In the resulting acidic solution, the red rose extract indicator will turn red.

12. Observe and analyse the information given in the following flowchart. Complete the missing information.
(Flowchart shows poor plant health due to soil condition)
See AnswerThe soil can be Acidic in nature. –> The acidic soil can be treated with bases like lime.
The soil can be Basic in nature. –> The basic soil can be treated with organic matter (e.g., manure, compost).
Which indicator can be used to test the nature of the soil? –> An acid-base indicator (like litmus used on a soil-water mixture, or a soil testing kit) can be used.

1. What surprising thing happened with Ashwin and Keerthi at the science fair?
See AnswerTheir white sheets showed the words “Welcome to the Wonderful World of Science” after being sprayed with a liquid. This made them very excited and curious​.

2. What is litmus obtained from?
See AnswerLitmus is a natural substance obtained from lichens. It is available as a solution or as paper strips in two colours – blue and red​.

3. What does a substance turning blue litmus paper red indicate?
See AnswerIf a substance turns blue litmus paper red, it means the substance is acidic in nature​.

4. What does a basic substance feel like when touched?
See AnswerBasic substances usually feel slippery or soapy when touched, like the baking soda solution​.

5. What is an olfactory indicator?
See AnswerOlfactory indicators are substances that change their smell when mixed with an acidic or basic medium, like onion​.

1. What do we learn by grouping samples into Group A, B, and C in the litmus test?
See AnswerGroup A substances turn blue litmus red (acidic), Group B substances turn red litmus blue (basic) and Group C substances do not affect either (neutral). This helps classify substances easily​.

2. How does red rose extract work as an indicator?
See AnswerRed rose extract gives a red shade in acidic solutions and a green shade in basic solutions. It remains unchanged with neutral substances, making it a natural acid-base indicator​.

3. How is turmeric used as an indicator?
See AnswerTurmeric is made into a paste and applied on paper strips. In basic substances, turmeric paper changes from yellow to red, but it stays yellow with acidic and neutral substances​.

4. What happens in a neutralisation reaction?
See AnswerWhen an acid reacts with a base, they neutralise each other to form salt and water, along with the release of heat​.

5. Why did moist baking soda help Ashwin after an ant bite?
See AnswerThe ant injects formic acid during a bite. Baking soda, being a base, neutralises the acid and relieves the stinging pain.

1. Explain how soil treatment helps plants grow better.
See AnswerIf soil becomes too acidic, lime (a base) is added to neutralise it. If soil is too basic, organic matter like compost is added. Maintaining soil’s pH balance is important for healthy plant growth and good nutrient availability​.

2. How does neutralisation help in industrial waste treatment?
See AnswerFactories sometimes release acidic waste into lakes, harming aquatic life. Before releasing, the waste can be treated with basic substances to neutralise it. This protects fish and other organisms living in the water​.

3. Why can’t turmeric be used to test acidic substances?
See AnswerTurmeric changes colour only in the presence of a base (turning red). It doesn’t show any change with acidic or neutral substances. Therefore, turmeric can only detect basicity, not acidity​.

4. What does the hydrangea flower’s colour tell about soil?
See AnswerHydrangea flowers turn blue in acidic soil and pink or red in basic soil. Gardeners can adjust soil acidity or basicity to change the colour of the flowers​.

5. Why should we not taste unknown substances during experiments?
See AnswerUnknown substances might be harmful or poisonous. To stay safe, students must avoid tasting anything unless clearly instructed by the teacher during science experiments.

1. How could Ashwin’s “secret greeting card” trick work?
See AnswerAshwin coated paper with turmeric paste and used a soap solution to write the message. When sprayed with water or acid, the letters appeared in a different colour​.

2. How can a gardener change the colour of hydrangea flowers?
See AnswerBy changing the soil’s pH. Making the soil more acidic by adding organic matter makes flowers blue; making it basic by adding lime turns flowers pink or red​.

3. How can you identify vinegar, baking soda solution, and sugar solution using turmeric paper?
See AnswerBaking soda turns turmeric paper red (basic). Vinegar and sugar solution do not change turmeric paper colour. Among them, vinegar will taste sour (acidic) and sugar solution will taste sweet (neutral)​.

4. Why did bubbles form when Aman spilled vinegar on eggshells?
See AnswerVinegar is acidic and reacts with the calcium carbonate in eggshells, producing carbon dioxide gas. This causes the bubbles. Soap solution is basic and does not react​.

5. Why is neutralisation useful in everyday life?
See AnswerNeutralisation helps in treating ant bites, balancing soil for farming, treating factory waste and even controlling acidity problems in the stomach with antacids. It protects both living beings and the environment​.