Acids, Bases and Salts Class 7 Notes NCERT and MCQs

30-08-2023 18:38 PM IST Priyanka Chaudhary

The chapter defines various terms such as acids, bases, indicators and neutral solution. It discusses neutralization and its applications in daily life.

Acids and Bases

Acids are substances that are sour in taste. Curd, lemon juice, orange juice and vinegar are some substances that are sour and contain acids. Generally, bases are bitter in taste and soapy to touch. Names of some acids and bases are given in the following table.

Indicators

They are the substances used to test whether a substance is acidic or basic. They change their colour when added to a solution containing an acidic or a basic substance. Some examples of naturally occurring indicators are Turmeric, litmus, China rose petals (Gudhal), etc.

Litmus: It is a natural indicator. It is extracted from lichens.  Acid turns blue litmus red. Bases turn red litmus blue.

Bases turn turmeric from yellow to red and Acids turn turmeric from red to yellow.

China rose indicator turns acidic solutions to dark pink (magenta) and basic solutions to green.

Neutral solutions/substances

Substances that are neither acidic nor basic are called neutral. They do not change the colour of either red or blue litmus.

Neutralisation

When an acidic solution is mixed with a basic solution, both solutions neutralise the effect of each other. As a result, acidic nature of the acid and the basic nature of the base get destroyed. This is called neutralisation. In neutralisation reaction a new substance is formed. This is called salt.  A salt may be acidic, basic or neutral in nature. In neutralisation reaction, heat is always produced, or evolved.

Acid+Base → Salt+Water (Heat is evolved)

The following reaction is an example: Hydrochloric acid (HCl) + Sodium hydroxide (NaOH) →  Sodium chloride (NaCl) + Water (H₂O)

Applications of neutralisation in everyday life

1. Relieving indigestion

Excessive amount of hydrochloric acid in stomach causes indigestion. When we take antacid such as milk of magnesia to relieve indigestion, magnesium hydroxide contained in milk of magnesia neutralises the effect of excessive acid.

2. Ant bite

When an ant bites, it injects the acidic liquid (formic acid) into the skin. The effect of the acid can be neutralised by rubbing moist baking soda (sodium hydrogen carbonate) or calamine solution, which contains zinc carbonate.

3. Soil treatment

Excessive use of chemical fertilisers makes the soil acidic. When the soil is too acidic, it is treated with bases like quick lime (calcium oxide) or slaked lime (calcium hydroxide). If the soil is basic, organic matter (compost) is added to it. The acids in organic matter neutralise the basic nature of the soil.

4. Factory wastes

The factory wastes are neutralised by adding basic substances as the wastes of many factories contain acids. If factory wastes are allowed to flow into the water bodies, the acids will kill fish and other organisms.

What is Acid Rain?

The rain containing excess of acids is called an acid rain. The rain becomes acidic because carbon dioxide, sulphur dioxide and nitrogen dioxide (which are released into the air as pollutants) dissolve in rain drops to form carbonic acid, sulphuric acid and nitric acid respectively. Acid rain can cause damage to buildings, historical monuments, plants and animals.

MCQs based on NCERT Class 7 Science Chapter 5: Acids, Bases and Salts

1. Match the names of acids given in column A with their natural sources given in column B and choose correct code.

          A        B        C       D

a.       1        3         2       4

b.       3        4         2       1

c.       3        1        2        4

d.       4        3        2        1

Ans c

Explanation: Acids are substances that are sour in taste. Curd, lemon juice, orange juice and vinegar are some substances that are sour and contain acids. Names of some acids and their natural sources are given in the following table.

2. Petals of which of the following are used to test whether a substance is acidic or basic?

a. China rose

b. Rose

c. Sunflower

d. Catharanthus

Ans. a

Explanation: Indicators are the substances used to test whether a substance is acidic or basic. They change their colour when added to a solution containing an acidic or a basic substance. Some examples of naturally occurring indicators are Turmeric, litmus, China rose petals (Gudhal), etc.

3. Which of the following statements is correct with reference to salts?

a. Salts are always acidic in nature.

b. Salts are always basic in nature.

c. Salts are always neutral in nature.

d. Salts may be acidic, basic or neutral in nature.

Ans d

Explanation: When an acidic solution is mixed with a basic solution, both the solutions neutralise the effect of each other. As a result, acidic nature of the acid and the basic nature of the base get destroyed. This is called neutralisation. In neutralisation reaction a new substance is formed. This is called salt. A salt may be acidic, basic or neutral in nature.

4. 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

Ans. (iv)

Explanation: Change of colour in an acid and a base depends on the type of the indicator.