NCERT Solutions for Class 7 Science Chapter 4 Acids Bases and Salts

1. State differences between acids and bases.

Answer: Differences between acids and bases are given below:

2. Ammonia is found in many household products, such as window cleaners. It turns red litmus blue. What is its nature?

Answer: Bases turn the colour of red litmus to blue, and ammonia also turns the colour of red litmus to blue.
Therefore, ammonia is basic in nature

3. Name the source from which litmus solution is obtained. What is the use of this solution?

Answer: Litmus solution is commonly obtained from lichens. A natural dye, which is extracted from lichen,s is dissolved in distilled water to form a litmus solution. Litmus solution is used as an indicator to distinguish between acids and bases.

When we add an acidic solution to it, it turns red, and when we add a basic solution to it, it turns blue.

4. Is the distilled water acidic/basic/neutral?

How would you verify it?

Answer: The Nature of distilled water is neutral. Because when we test it with a litmus paper, it does not change the colour of it and turns neither red nor blue.
Therefore, we can say that distilled water is neutral in nature.

5. Describe the process of neutralisation with the help of an example.

Answer: The reaction between an acid and a base is known as neutralization reaction. In such type of reactions, acid and base reacts with each other and cancel out each other's effect and formation of salt and water takes place and energy in the form of heat is evolved.

$Acid+Base\to Salt+Water+Heat$

For eg.
Sodium hydroxide $(NaOH)$ reacts with hydrochloric acid $(HCI)$ to give sodium chloride $(NaCl)$ and water $(H_{2}0)$ with evo of some heat..

$NaOH+HCl\to NaCl+H_{2}O+Heat$

6. Mark ‘T’ if the statement is true and ‘F’ if it is false:

(i) Nitric acid turns 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) An 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) FALSE

(ii) FALSE

(iii) TRUE

(iv) TRUE

(v) 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: If Dorji has litmus paper or solution then with the help of that he can distinguish between them by putting one drop of each drink on litmus paper. If the colour of the litmus paper changes to blue, then it is a basic drink and if the colour of the litmus paper changes to red, then it is an acidic drink and if no change in colour then it is neutral.
Another method is he can distinguish between them by tasting the drinks. Drink with sour taste is acidic whereas drink with bitter taste is basic and neutral drinks will have no taste.

8 (a). Explain why:

An antacid tablet is taken when you suffer from acidity.

Answer: This is because antacids are the substance that neutralizes excess acids, which are produced in the stomach during acidity, as they contain a base, such as milk of magnesia.

These bases react with the excess amount of acids and neutralize their effect and giving us relief.

8 (b). Explain why:

Calamine solution is applied to the skin when an ant bites.

Answer: The sting of an ant contains formic acid. Therefore, when an ant bites, it injects formic acid into our skin and due feel a burning sensation. Calamine solution contains zinc carbonate, which is basic in nature.

Therefore, when we applied it to the skin, it neutralised the effect of formic acid.

8 (c). Explain why:

Factory waste is neutralised before being it into the water bodies.

Answer: Wastes materials in factories contain acids. Therefore, if we threw them directly into water bodies then the water becomes acidic and harm aquatic lives.

Therefore, these factory wastes are neutralized with basic chemicals before being disposed of into water bodies.

9. Three liquids are given to you.

One is hydrochloric acid, another is sodium hydroxide, and a third is a sugar solution. How will you identify them? You have only a turmeric indicator.

Answer: When we place turmeric solution in contact with bases it turns into red colour and generally It is not affected by acids and neutral substances.

Now, we can distinguish between hydrochloric acid, sodium hydroxide and a sugar solution by applying turmeric solution to three test tubes, we will observe that

(1) test tube A (Hydrochloric acid): No change in colour will take place because turmeric is not affected by acids.

(2) test tube B (sodium hydroxide): Turmeric solution will change its colour to red because turmeric solution changes its colour to red when we place it in contact with bases.

(3) test tube c (sugar solution): No change in color will take place because sugar solution is neither basic nor acidic in nature
Therefore, we are only able to identify a basic solution using turmeric as an indicator

10. Blue litmus paper is dipped in a solution. It remains blue. What is the nature of the solution? Explain.

Answer: We know that blue litmus paper turns red colour when coming in contact with an acidic solution, and basic and neutral solutions do not change the colour of blue litmus paper. Since blue litmus paper remains blue after it is dipped in a solution.

Therefore, the solution is either basic or neutral in nature.

11. Consider the following statements:

(a) Both acids and bases change the 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 is correct?

(i) All four

(ii) a and d

(iii) b, c and d

(iv) only d

Answer: Out of all these, only

(iv) only (d) is correct