Tuesday , July 5 2022
NCERT 7th Class (CBSE) Science: Acids, Bases and Salts

NCERT 7th Class (CBSE) Science: Acids, Bases and Salts

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

Answer: If a blue litmus paper when dipped in a solution, remains blue, it implies the solution is either basic or neutral.

Question: What are salts? Are they acidic, basic or neutral? Give examples.

Answer: Salts are the ionic compounds generally formed by the neutralization of an acid with a base. They can be acidic, basic or neutral.

Examples of salts are:

(a) Acidic Salts: sodium bicarbonate
(b) Basic Salts: magnesium chloride
(c) Neutral Salts: Sodium chloride, potassium chloride
(d) Mixed Salts: bleaching powder, potash alum

Question: Distinguish between organic acids and mineral acids.

Answer: The difference is as follows:

Organic Acids
Mineral Acids
1. Acids that are naturally obtained from plant and animal sources are called organic acids.
1. Acids that are derived from an inorganic material or source are called mineral acids
2. For example: Lactic acid – Curd, Milk; Citric acid – Oranges, lemon
2. For example: nitric acid, hydrochloric acid

Question: What are universal indicators and what are their uses?

Answer: Universal indicators are compounds that show a gradual change in colour to indicate the acidity or basicity for pH values from 1-14. These colours indicate the basicity and acidity of a chemical. These indicators are available both as strip of paper called pH paper and as solution.

To find the pH value of unknown solution, the change in colour of these substances can be compared with that of the standard colour chart.


Question: Distinguish between strong and weak bases.

Answer: The difference is as follows:

Strong Bases
Weak Bases
1. Some of the bases are corrosive in nature and may result in skin burns, such bases are called strong bases.
1. Some of the bases are not corrosive in nature, such bases are called weak bases.
2. Sodium hydroxide, Calcium hydroxide and Potassium hydroxide
2. Magnesium hydroxide, ammonium hydroxide and copper hydroxide.

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

Answer: Since it turns red litmus blue, Ammonia is basic in nature.

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

Answer: Litmus is extracted from plant lichens. It has a mauve (purple) colour when dissolved in distilled water. Litmus solution is used to detect acidic or basic nature of any substance. When added to an acidic solution, it turns red and when added to a basic solution, it turns blue.

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One comment

  1. Asutosh Panigrahy

    Such a nice app and I have learnt a lot of thing from it.