NCERT Solutions Science Chapter 5 Acids, Bases and Salts for Class 7th
Introduction
- Acids: Substances that taste sour, turn blue litmus red, and can react with metals and carbonates.
- Bases: Substances that taste bitter, feel slippery, turn red litmus blue, and can react with acids to form salts and water.
- Salts: Formed when acids react with bases.
Properties of Acids and Bases
- Physical Properties:
- Acids: Sour taste, corrosive, conduct electricity when dissolved in water.
- Bases: Bitter taste, slippery feel, also conduct electricity when dissolved.
- Chemical Properties:
- Acids react with metals to produce hydrogen gas and a salt.
- Bases can neutralize acids, producing salt and water.
Indicators
- Litmus Paper: A common indicator that changes color in the presence of an acid or a base.
- Other Indicators: Turmeric, phenolphthalein, and universal indicators are also used to determine the pH of substances.
pH Scale
- The pH scale ranges from 0 to 14:
- pH < 7: Acidic solution
- pH = 7: Neutral (pure water)
- pH > 7: Basic (alkaline) solution
- The pH value indicates the strength of an acid or a base.
Neutralization Reaction
- When an acid reacts with a base, they neutralize each other, resulting in the formation of salt and water. This reaction is crucial in many practical applications, such as treating acid spills or in digestion.
Everyday Examples
- Acids: Vinegar (acetic acid), citrus fruits (citric acid).
- Bases: Baking soda (sodium bicarbonate), soap (sodium or potassium hydroxide).
- Salts: Table salt (sodium chloride).
Applications
- Acids and bases have numerous applications in daily life, industry, and biology. For example, citric acid in food, hydrochloric acid in digestion, and sodium hydroxide in cleaning agents.
NCERT Solutions Science Chapter 5 Acids, Bases and Salts for 2024-2025
Q.1. State differences between acids and bases.
Ans.
| Acids | Bases |
| Acids are sour in taste | Bases are bitter in taste |
| Acids turn blue litmus paper into red color | The base does not change the colour of blue litmus paper |
| Acids does not change the colour of the red litmus | Bases turn red litmus paper to blue color |
| Acids do not change the colour of turmeric | Bases turn turmeric to red |
| Acids contains Hydrogen (H+) ions. | Bases contains Hydroxyl (OH–) ions. |
Q.2. Ammonia is found in many household products, such as window cleaners. It turns red litmus blue. What is its nature?
Ans. Ammonia has basic nature.
Q.3. Name the source from which litmus solution is obtained. What is the use of this solution?
Ans. Litmus solution is extracted from lichens. Litmus solution is used as an indicator to find acidic and basic nature of a solution.
Q.4. Is the distilled water acidic/basic/neutral? How would you verify it?
Ans. Distilled water will be neutral. We can verify it by showing that neither blue nor red litmus paper changes its colour when dipped in it.
Q.5. Describe the process of neutralisation with the help of an example.
Ans. 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.
Antacids like milk of magnesia (magnesium hydroxide), baking soda, etc. which contain a base are used for reducing acidity in stomach when excessive acid released by glands.
Q.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 neutralise each other and form salt and water. (T/F)
(id) 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)
Ans.(1) F (ii) F (iii) T (iv) T (V) F
Q.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 acidic drink, another wants basic and third one wants neutral drink. How
will Dorji decide which drink is to be served to whom?
Ans.Dorji can decide with the help of litmus paper:
(i) The drink which would turn a red litmus blue would be basic.
(ii) If the drink turns a blue litmus to red would be acidic.
(iii) The drink which would not affect both red and blue litmus would be neutral.
Q.8.Explain why:
(a) An antacid tablet is taken when you suffer from acidity.
(b) Calamine solution is applied on the skin when an ant bites.
(c) Factory waste is neutralised before disposing it into the water bodies.
Ans.(a) We take an antacid such as milk of magnesia to neutralises the excessive acid released in stomach.
(b) Ant injects an acidic liquid (Formic acid) into the skin on biting which causes inflammation, to the skin. The effect of the acid can be neutralised by rubbing. Calamine solution which contains zinc carbonate which is very weak base and causes no harm to the skin.
(c) The wastes of factories contain acids. If acids are disposed off in the water body, the acids will harm the organisms. So factory wastes are neutralised by adding basic substances.
Q.9. Three liquids are given to you. One is hydrochloric acid, another is sodium hydroxide and third is a sugar solution. How will you identify them? You have only turmeric indicator.
Ans.Name of the substances Effect on turmeric indicator
1. Hydrochloric acid Yellow to blue
2. Sodium hydroxide Yellow to red
3. Sugar solution No change
Q.10. Blue litmus paper is dipped in a solution. It remains blue. What is the nature of the solution? Explain.
Ans. (i) It can be identified on the basis of the following observations : Bases change the colour of litmus paper to blue. As the colour of blue litmus paper is not affected, the solution must be basic.
(ii) If the solution is neutral, even then colour of litmus will not change.
Q. 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) and (c) (iv) only (d)
Ans. (ii) (a) and (d)