NCERT Solutions Class 7 Science
The NCERT Solutions in English Language for Class 7 Science Chapter – 5 (Acids, Bases and Salts) has been provided here to help the students in solving the questions from this exercise.
Chapter – 5 (Acids, Bases and Salts)
1. State differences between acids and bases.
Answer –
| Acids | Bases |
| Acids are sour to taste | Bases are bitter to taste |
| Acids turn blue litmus paper into red color | The base does not change the colour of blue litmus paper |
| Acids does not change the colour of the red litmus | Bases turn red litmus paper to blue color |
| Acids do not change the colour of turmeric | Bases turn turmeric to red |
2. Ammonia is found in many household products, such as window cleaners. It turns red litmus blue. What is its nature?
Answer – Ammonia is basic in nature as it turns the colour of red litmus paper to blue.
3. Name the source from which litmus solution is obtained. What is the use of this solution?
Answer – Litmus solution is extracted from lichens. Litmus solution is used as an indicator to find acidic and basic nature of a solution.
4. Is the distilled water acidic/basic/neutral? How would you verify it?
Answer – Distilled water is neutral in nature, and this can be tested by using red and blue litmus paper. In either of the cases, colour remains unchanged.
5. Describe the process of neutralisation with the help of an example.
Answer – Neutralisation is a reaction between an acid and a base. Here both acids and bases get neutralised. For example, when sodium hydroxide (NaOH) is added to hydrochloric acid (HCl), sodium chloride (NaCl) and water (H2O) are obtained.
NaOH + HCl → NaCl + H2O + Heat
6. Mark ‘T’ if the statement is true and ‘F’ if it is false:
(i) Nitric acid turn red litmus blue. (T/F)
(ii) Sodium hydroxide turns blue litmus red. (T/F)
(iii) Sodium hydroxide and hydrochloric acid neutralise each other and form salt and water. (T/F)
(iv) Indicator is a substance which shows different colours in acidic and basic solutions. (T/F)
(v) Tooth decay is caused by the presence of a base. (T/F)
Answer –
(i) Nitric acid turn red litmus blue. (False)
(ii) Sodium hydroxide turns blue litmus red. (False)
(iii) Sodium hydroxide and hydrochloric acid neutralise each other and form salt and water. (True)
(iv) Indicator is a substance which shows different colours in acidic and basic solutions. (True)
(v) Tooth decay is caused by the presence of a base. (False)
7. Dorji has a few bottles of soft drink in his restaurant. But, unfortunately, these are not labelled. He has to serve the drinks on the demand of customers. One customer wants acidic drink, another wants basic and third one wants neutral drink. How will Dorji decide which drink is to be served to whom?
Answer – Dorji can taste a few drops out of soft drinks bottles, acidic solution is sour in taste, basic solution is bitter in taste, and the neutral solution has no taste. Along with tasting, Dorji can use litmus paper to test the nature of soft drink. He should use blue litmus paper to test the acidic solution. Dorji has to put a drop of solution on blue litmus. If it turns red then the solution will be acidic in nature.
Similarly, he can use red litmus paper to test the basic solution. He has to put a drop of solution on red litmus. If it turns blue then the solution will be basic in nature.
8. Explain why:
(a) An antacid tablet is taken when you suffer from acidity
(b) Calamine solution is applied on the skin when an ant bites.
(c) Factory waste is neutralised before disposing it into the water bodies.
Answer –
(a) This is because during acidity, an excess of acids is produced in the stomach. An antacid contains base, such as milk of magnesia. These bases react with excess of acids and neutralize their effect, thus giving us relief.
(b) When an ant bites, it injects formic acid into the skin. Calamine solution contains zinc carbonate which is basic in nature. Therefore, it is applied on the skin to neutralize the effect of formic acid.
(c) Factory wastes contain acids. Therefore, these wastes, when thrown directly to water bodies, harm aquatic lives. Hence, these wastes are neutralised with basic chemicals before disposing to water bodies.
9. Three liquids are given to you. One is hydrochloric acid, another is sodium hydroxide and third is a sugar solution. How will you identify them? You have only turmeric indicator.
Answer – Turmeric indicator is yellow in colour.
→ Take a few drops from each solution and test it with a turmeric solution. If the solution turns pink then the solution is basic i.e., sodium hydroxide. Mark the beaker (containing solution) as base.
→ Take a test tube and add a few drops of base solution and the second solution. Check if the test tube becomes warm and then add turmeric solution to it. If the colour does not change, it means the solution is acidic i.e., hydrochloric acid. The test tube becomes warm because of the neutralization effect.
→ If the test tube of the above solution does not become warm and it does show pink colour when turmeric solution is added, the second solution is neutral.
10. Blue litmus paper is dipped in a solution. It remains blue. What is the nature of the solution? Explain.
Answer – Above solution may be neutral or basic in nature as both of them will not change the colour of the blue litmus paper.
11. Consider the following statements:
(a) Both acids and bases change colour of all indicators.
(b) If an indicator gives a colour change with an acid, it does not give a change with a base.
(c) If an indicator changes colour with a base, it does not change colour with an acid.
(d) Change of colour in an acid and a base depends on the type of the indicator.
Which of these statements are correct?
(i) All four
(ii) a and d
(iii) b, c and d
(iv) only d
Answer – (iv) Only d is correct
