Science STD 7 Chapter 4: Acids, Bases and Salts - Exercises

Science STD 7 Chapter 4: Acids, Bases and Salts - Exercises

1. State differences between acids and bases.

Acids Bases
They are sour in taste. They are bitter in taste.
They turn blue litmus paper red. They turn red litmus paper blue.
They do not feel soapy to touch. They feel soapy to touch.
They are corrosive and harmful to skin. They are also corrosive and harmful to skin.

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

Since ammonia turns red litmus paper blue, its nature is basic.

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

Litmus solution is obtained from lichens. This solution is used as an indicator to test whether a substance is acidic or basic. It turns red in an acidic solution and blue in a basic solution.

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

Distilled water is neutral. To verify this, you can use litmus paper. If you test distilled water with red and blue litmus paper, there will be no change in colour. This indicates that it is neither acidic nor basic.

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

Neutralisation is a chemical reaction between an acid and a base. When an acidic solution and a basic solution are mixed in suitable amounts, they neutralise the effect of each other. The resulting solution is neither acidic nor basic. In this process, a new substance called salt is formed along with water and the evolution of heat.

The reaction can be represented as:
Acid + Base → Salt + Water

Example: When hydrochloric acid (an acid) is mixed with sodium hydroxide (a base), sodium chloride (salt) and water are formed along with the release of heat.

6. Mark 'T' if the statement is true and 'F' if it is false:

  1. (i) Nitric acid turn red litmus blue. (T/F)

    F

  2. (ii) Sodium hydroxide turns blue litmus red. (T/F)

    F

  3. (iii) Sodium hydroxide and hydrochloric acid neutralise each other and form salt and water. (T/F)

    T

  4. (iv) Indicator is a substance which shows different colours in acidic and basic solutions. (T/F)

    T

  5. (v) Tooth decay is caused by the presence of a base. (T/F)

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

Dorji can use natural indicators like litmus paper, turmeric paper, or China rose indicator to test the drinks. He should not taste them as they could be harmful.

  • For the acidic drink: He would use a blue litmus paper. The drink that turns blue litmus red is acidic.
  • For the basic drink: He would use a red litmus paper. The drink that turns red litmus blue is basic.
  • For the neutral drink: He would test the remaining drink with both red and blue litmus papers. If there is no change in the colour of either paper, the drink is neutral.

8. Explain why:

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

    When we suffer from acidity, there is an excess of hydrochloric acid in our stomach. An antacid tablet contains a base like magnesium hydroxide, which neutralises the effect of this excessive acid, providing relief.

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

    When an ant bites, it injects an acidic liquid (formic acid) into the skin. Calamine solution contains zinc carbonate, which is a base. Applying it helps to neutralise the effect of the acid, reducing the irritation and pain.

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

    Many factory wastes contain acids. If these wastes are released directly into water bodies, the acids can harm or kill fish and other aquatic organisms. Therefore, they are neutralised by adding basic substances before being discharged, protecting the environment.

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.

Turmeric indicator turns red when a basic substance is added to it, but its colour remains yellow with acidic and neutral substances.

  1. First, take a small amount of each liquid and add a few drops of the turmeric indicator. The liquid that turns the yellow indicator to red is the **sodium hydroxide** (base).
  2. Now, take the sodium hydroxide solution and add it drop by drop to the remaining two liquids separately.
  3. The liquid that shows a neutralisation reaction (a salt and water are formed, and the mixture becomes warm) is the **hydrochloric acid**.
  4. The remaining liquid that does not react is the **sugar solution**, which is a neutral substance.

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

If blue litmus paper remains blue when dipped in a solution, the solution is either basic or neutral. An acidic solution would turn blue litmus paper red. To be sure, you would need to test the solution with red litmus paper as well. If it turns red litmus blue, it is basic. If it shows no change with either red or blue litmus paper, it is neutral.

11. Consider the following statements:

  1. (a) Both acids and bases change colour of all indicators.
  2. (b) If an indicator gives a colour change with an acid, it does not give a change with a base.
  3. (c) If an indicator changes colour with a base, it does not change colour with an acid.
  4. (d) Change of colour in an acid and a base depends on the type of the indicator.

Which of these statements are correct?

  1. All four
  2. a and d
  3. b, c and d
  4. only d

Suggested Activities and Projects

1. Using the knowledge of acids and bases, write a secret message with the help of baking soda and beet root. Explain how it works. (Hint: Prepare baking soda solution in water. Use this solution to write the message on a sheet of white paper with a cotton bud. Rub a slice of fresh beet root over the message.)

Outline: This activity uses the principle of acid-base indicators. The beet root juice acts as a natural indicator. When you write a message with the basic baking soda solution on white paper, it remains invisible. However, when you rub the beet root slice (which contains a natural colour pigment that changes colour with a base) over the message, the message becomes visible as a different color, revealing the secret message.

2. Prepare red cabbage juice by boiling a piece of red cabbage in water. Use it as an indicator and test the acidic and basic solutions with it. Present your observations in the form of a table.

Outline: This is a hands-on experiment to create a natural indicator. The red cabbage juice turns a different color in acidic, basic, and neutral solutions. In an acidic solution (like lemon juice), it would turn reddish-pink. In a neutral solution (like distilled water), it remains purple/blue. In a basic solution (like baking soda solution), it turns bluish-green. You would present these color changes in a table to show the results clearly.

3. Bring the soil sample of your area, find out if it is acidic, basic or neutral. Discuss with farmers if they treat the soil in any manner.

Outline: This activity involves testing soil samples to determine their pH level (acidic, basic, or neutral). You could mix the soil with water and use litmus paper or another indicator to test it. Then, by talking to local farmers, you can learn about their farming practices. Farmers often add substances like quicklime (calcium oxide) to acidic soil or organic matter to basic soil to neutralise it and make it suitable for crop growth.

4. Visit a doctor. Find out the medicines, he prescribes to treat acidity. Ask him how acidity can be prevented.

Outline: This project is about understanding how acidity is treated and prevented. A doctor would explain that antacids, which are bases, are prescribed to neutralise the excess stomach acid. The doctor would also suggest lifestyle changes and dietary modifications to prevent acidity, such as avoiding spicy food, not overeating, and eating on time.