CBSE Class 7 Science Chapter 5: Acids, Bases, and Salts - NCERT Solutions

Solution: The key differences between acids and bases are:

1. Taste: Acids have a sour taste (like lemon juice or curd), whereas bases have a bitter taste (like baking soda or soap).
2. Effect on Litmus: Acids turn blue litmus solution red. Bases turn red litmus solution blue.
3. Examples: Common acids include curd, lemon juice, and vinegar. Common bases include baking soda, soap, and lime water.

Solution: Substances that turn red litmus blue are basic in nature. Since ammonia turns red litmus blue, its nature is basic.

Solution: Litmus solution is obtained from lichens. It is a natural indicator. Litmus solution has a mauve (purple) colour in distilled water. Its use is to test the acidic or basic nature of a solution: it turns red in an acidic solution and blue in a basic solution.

Solution: Distilled water is neutral by nature. To verify this, we can perform a litmus test. Take a small amount of distilled water and test it with both red and blue litmus paper. If the water is neutral, it will not change the colour of either red or blue litmus paper.

Solution: Neutralisation is a chemical reaction in which an acid and a base react with each other to destroy their respective acidic and basic properties. The resulting solution is neither acidic nor basic, meaning it becomes neutral. This process typically forms salt and water.

Example: Let's consider the reaction between hydrochloric acid (an acid) and sodium hydroxide (a base).

1. Take about one-fourth of a test tube filled with dilute hydrochloric acid.
2. Add a few drops of litmus solution to it. The solution will turn red, indicating it is acidic.
3. Now, gradually add drops of sodium hydroxide solution to the acidic solution, stirring gently with a dropper.
4. Continue adding sodium hydroxide solution drop by drop until the red colour just disappears and the solution turns green (or the colour of the neutral litmus indicator).

At this point, the hydrochloric acid has been neutralised by the sodium hydroxide. The resulting solution contains salt (sodium chloride) and water, and it is neutral.

Solution:

1. (i) Nitric acid turns red litmus blue. (F) Nitric acid is an acid, and acids turn blue litmus red, not red litmus blue.
2. (ii) Sodium hydroxide turns blue litmus red. (F) Sodium hydroxide is a base, and bases turn red litmus blue, not blue litmus red.
3. (iii) Sodium hydroxide and hydrochloric acid neutralise each other and form salt and water. (T) This is a classic example of a neutralisation reaction where an acid (hydrochloric acid) and a base (sodium hydroxide) react to form salt (sodium chloride) and water.
4. (iv) Indicator is a substance which shows different colours in acidic and basic solutions. (T) Indicators are substances that change colour in the presence of an acid or a base, helping us to identify their nature.
5. (v) Tooth decay is caused by the presence of a base. (F) Tooth decay is caused by acids, which are produced by bacteria in the mouth from the sugars in food. These acids erode the tooth enamel.