NCERT Solutions Class 7 Science Chapter 5 Acids Bases and Salts

NCERT Solutions for Class 7 Science for chapter 5 Acids Bases and Salts

Question.1

State differences between acids and bases.

Answer.

Acids	Bases
Acids are sour in taste.	Bases are bitter in taste
Acids are non-soapy to touch.	Bases are soapy to touch.
Acids turn blue litmus red.	Bases do not change colour of blue litmus.
They do not change colour of red litmus.	They turn red litmus blue.
Acids do not change the colour of turmeric indicator.	Bases turn the colour of turmeric indicator to red.
Acids give magenta colour with china rose indicator.	Bases give green colour with china rose indicator.
Examples of acids are citric acid, lactic acid, tartaric acid, etc.	Examples of bases are sodium hydroxide, magnesium hydroxide, calcium hydroxide, etc.

Question.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 red litmus blue. Bases turn red litmus blue.

Question.3

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

Answer.

Litmus is obtained from Lichens.

Uses of Litmus solution

• Litmus solution is used as an indicator. It is used to find the nature (acidic/basic/neutral) of the solution.
• Acid solution turns blue litmus solution red.
• Basic solution turns red litmus solution blue.

Question.4

Is the distilled water acidic/basic/neutral? How would you verify it?

Answer.

Distilled water is neutral in nature. The same can be tested by using red and blue litmus paper. Acidic solution turns blue litmus paper red while basic solution turns red litmus paper blue. Distilled water neither turns blue litmus paper red nor red litmus paper blue. Therefore, distilled water is a neutral.

Question.5

Describe the process of neutralisation with the help of an example:

Answer.

The reaction between the acid and base is known as the neutralisation reaction. In the neutralisation reaction, salt and water are produced with the evolution of heat.

Acid + Base -> Salt + Water + Heat

Example: Hydrochloric acid (acid) reacts with sodium hydroxide (base) to form sodium chloride (salt) and water. Heat is evolved in this process.

 

Question.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) 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 turns red litmus blue. (F)

(ii) Sodium hydroxide turns blue litmus red. (F)

(iii) Sodium hydroxide and hydrochloric acid neutralise each other and form salt and water. (T)

(iv) Indicator is a substance which shows different colours in acidic and basic solutions. (T)

(v) Tooth decay is caused by the presence of a base. (F)

Question.7

Dorji has a few bottles of soft drink in his restaurant. But unfortunately, these are not labelled. He has to serve the drink 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.

Since, soft drinks are edible; Dorji can decide acidic, basic and neutral soft drinks by tasting them. Acidic soft drink will be sour in taste whereas basic soft drink will be bitter in taste. Neutral soft drink will has neither sour taste nor bitter.

Acidic, basic and neutral soft drinks can also be decided by using blue and red litmus paper.

Dorji can pour a few drops of one soft drink on red and blue litmus paper. If red litmus turns blue, then it will be a basic soft drink and if blue litmus changes to red, then it will be an acidic soft drink. If colour of both the litmus papers remains same, then it will be a soft drink.

The same process can be repeated with the other soft drinks also.

Question.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) Hydrochloric acid present in our stomach helps in the digestion of food. But excess of it causes acidity. To relieve from acidity, antacid tablets are taken as it contains base such as magnesium hydroxide that neutralises the acid present in the stomach.

(b) Formic acid is present in ant’s sting. When an ant bites, it injects the solution of formic acid into skin. The effect of sting can be neutralised by rubbing calamine solution on the stung area. Calamine solution being basic in nature neutralises the formic acid.

(c) The factory wastes contain acids. If such water is released into a water body, it can harm aquatic animals like fishes. Factory wastes should be treated with calcium hydroxide to neutralise acid before it is disposed off in water.

Question.9

The 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.

Put a drop each of hydrochloric acid, sodium hydroxide and sugar solution on the turmeric indicator. The solution which changes the colour of turmeric indicator to red will be basic that is sodium hydroxide.

Now add a few drops of sodium hydroxide solution in remaining two solutions i.e. both in hydrochloric acid and sugar solution. After that, put some drops of these mixtures on turmeric indicator separately.

Drop that changes the colour of turmeric indicator will be neutral in nature i.e. sugar solution. Mixture of basic solution and neutral solution will be basic in nature and change the colour of turmeric indicator to red.

Drop that does not change the colour of turmeric indicator will be acidic in nature i.e. hydrochloric acid. This is because hydrochloric acid being acidic in nature neutralises the sodium hydroxide solution.

Question.10

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

Answer.

The nature of the solution will be either basic or neutral. This is because both basic solution and neutral solution do not change the colour of blue litmus paper.

Question.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 change colour 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 and c (iv) only d

Answer.

The correct option is (iv) only d.