CBSE – 7th Standard Science


Question Bank – Chapter 5 – Acids, Bases and Salts




Q.1. What are Acids?

Ans. Curd, lemon juice, orange juice and vinegar taste sour. These substances taste sour because they contain acids. The chemical nature of such substances is acidic. The word acid comes from the Latin word acere which means sour. The acids in these substances are natural acids.


Q.2. What are Bases?

Ans. Since baking soda does not taste sour it means, that it has no acids in it. It is bitter in taste. If we rub its solution between fingers, it feels soapy. Substances like these which are bitter in taste and feel soapy on touching are known as bases. The nature of such substances is said to be basic.


Q.3. What are Indicators?
Ans. Special type of substances are used to test whether a substance is acidic or basic. These substances are known as indicators. The indicators change their colour when added to a solution containing an acidic or a basic substance. Turmeric, litmus, china rose petals (Gudhal), etc., are some of the naturally occurring indicators.


Q.4. What is Litmus paper?

Ans. The most commonly used natural indicator is litmus. It is extracted from lichens. It has a mauve (purple) colour in distilled water. When added to an acidic solution, it turns red and when added to a basic solution, it turns blue. It is available in the form of a solution, or in the form of strips of paper, known as litmus paper. Generally, it is available as red and blue litmus paper.


Q.5. What do you mean by Acid rain?

Ans. 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.


Q.6. What happens when an acidic solution is mixed with a basic solution?

Ans. When an acidic solution is mixed with a basic solution, both the solutions neutralise the effect of each other. When an acid solution and a base solution are mixed in suitable amounts, both the acidic nature of the acid and the basic nature of the base are destroyed. The resulting solution is neither acidic nor basic.


Q.7. What is Neutralisation?

Ans. In neutralisation reaction a new substance is formed. This is called salt. Salt may be acidic, basic or neutral in nature. Thus, neutralisation can be defined as follows: The reaction between an acid and a base is known as neutralisation. Salt and water are produced in this process with the evolution of heat.


Q.8. What is Indigestion?

Ans. Our stomach contains hydrochloric acid. It helps us to digest food, too much of acid in the stomach causes indigestion. Sometimes indigestion is painful. To relieve indigestion, we take an antacid such as milk of magnesia, which contains magnesium hydroxide. It neutralises the effect of excessive acid.


Q.9. How can we neutralize the effect of an Ant bite?

Ans. 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.


Q.10. What do you understand by Soil treatment?

Ans. Excessive use of chemical fertilisers makes the soil acidic. Plants do not grow well when the soil is either too acidic or too basic. 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 is added to it. Organic matter releases acids which neutralises the basic nature of the soil.


Q.11. What are Factory wastes?

Ans. 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.

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