NCERT Solutions Std 7 Science Acids, Bases and Salts

Acids, Bases and Salts


1). State differences between acids and bases.

Acids Bases
1. Acids are substances that contain hydrogen ion.


1. Bases are substances that contain hydroxyl ion.
2. Acid turns blue litmus red.


2. Base turns red litmus blue.
3. Acids are sour to taste.


3. Bases are bitter to taste.

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

          Ammonia has basic nature.

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

          Litmus is extracted from lichens. It is used to determine whether the given solution is acidic or basic.

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

       The nature of distilled water is identified with the help of litmus. Litmus shows no change in distilled water. Therefore, distilled water is neutral.

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

          The reaction between an acid and a base is known as neutralization. Salt and water are produced in this process with the evolution of 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 neutralize 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)


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 an acidic drink, another wants a basic and the third one wants a neutral drink. How will Dorji decide which drink is to be served to whom?

          Dorji can decide with the help of the litmus paper.

i). if the blue litmus paper turns to red the given drink is acidic.

ii). if the red litmus paper turns to blue the given drink is basic.

iii). if there is no change in the litmus paper the given drink is neutral.

8). Explain why:

(a) An antacid tablet is taken when you suffer from acidity.

          To relieve indigestion, we take an antacid such as milk of magnesia, which contains magnesium hydroxide. It neutralizes the effect of excessive acid.

(b) Calamine solution is applied on the skin when an ant bites.

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

(c) Factory waste is neutralized before disposing it into the water bodies.

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

9). Three liquids are given to you. One is hydrochloric acid, another is sodium hydroxide and the third is a sugar solution. How will you identify them? You have only a turmeric indicator.

Hydrochloric acid       the yellow colour of turmeric turns blue.

Sodium hydroxide       the yellow colour of turmeric turns red.

Sugar solutions       no change in turmeric colour.

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

          In acid blue litmus turns to red, here there is no change in the colour of blue litmus therefore, the given solution may be base or neutral.

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: a and d


Additional Questions

Fill in the blanks.

1). Acids are sour in taste.

2). Bases are bitter in taste.

3). Too much of acid in the stomach causes indigestion.

4). Excessive use of chemical fertilizers makes the soil acidic.

5). Methyl orange is an indicator.

Write whether ‘True’ or ‘False’.

1). Acids are bitter to taste.


2). Bases are soapy on touching.


3). China rose indicator turns basic solutions to pink.


4). In a neutralization reaction, heat is always produced, or evolved.


5). In the neutralization reaction no new substance is formed.


6). Excessive use of chemical fertilizers makes the soil fertile.



1). Indicators

The substances that are used to test whether a substance is acidic or basic are known as indicators.

2). Neutral solutions

The solutions which do not change the colour of either red or blue litmus are known as neutral solutions.

3). Neutralization

The reaction between an acid and a base is known as neutralization.

Name the following.

1). Natural acids

Curd, lemon juice, orange juice and vinegar.

2). Natural indicators

Turmeric, litmus, china rose petals.

3). Source of litmus


Match the following.

Column A                                          Column B

i). Acetic acid                            curd

ii). Formic acid                          grapes

iii). Citric acid                           Vinegar

iv). Lactic acid                           Orange

v). Tartaric acid                         Ant’s sting



i). Acetic acid                            Vinegar

ii). Formic acid                          Ant’s sting

iii). Citric acid                           Oranges

iv). Lactic acid                           Curd

v). Tartaric acid                         grapes


Match the following.

Column A                                          Column B

i). Calcium hydroxide                       milk of magnesia

ii). Ammonium hydroxide                 soap

iii). Sodium hydroxide                      Window cleaner

iv). Magnesium hydroxide                 lime water



i). Calcium hydroxide                       lime water

ii). Ammonium hydroxide                 Window cleaner

iii). Sodium hydroxide                      soap

iv). Magnesium hydroxide                 milk of magnesia


Answer the following in one sentence each.

1). Give examples of natural acids.

Curd, lemon juice, orange juice and vinegar.

2). Write any two characteristics of acids.

i). Acids are sour in taste.

ii). Acids turn blue litmus paper to red.

3). Write any two characteristics of bases.

i). Bases are bitter in taste.

ii). Bases turn red litmus paper to blue.


Write a short note on acid rain.

          The rain containing excess acids is called 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.

Click here for the solutions of

1). Nutrition in Plants

2). Nutrition in Animals

3). Fibre to Fabric

4). Heat

5). Acids, Bases and Salts

6). Physical and Chemical Changes

7). Weather, Climate and Adaptations of Animals to Climate

8). Winds, Storms and Cyclones

9). Soil

10). Respiration in Organisms

11). Transportation in Animals and Plants

12). Reproduction in Plants

13). Motion and Time

14). Electric Currents and its Effects

15). Light

16). Water: A Precious Resource

17). Forests: Our Lifeline

18). Wastewater Story



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