Acids, Bases, and Salts | Science | Chapter 5 | Class 7th | Questions and Answers |

JANDKNCERT | Science | Class 7th |

Acids, Bases, and Salts | Science | Chapter 5 | Class 7th | Questions and Answers |

 

 

Chapter 5: Acids, Bases, and Salts

 

EXERCISES

 

(I). Fill in the blank spaces by choosing the correct words from the list, given below:

Bitter, ants, corrosive, citric, soapy, slaked lime

 

1. The acid found in lemon is _________ acid.

2. The bases have __________ taste and _________ touch.

3. The sting of the _________ contains formic acid.

4. Sulphuric acid is highly ___________ acid.

5. Acidic soils are neutralized with ____________.

Ans.

1. The acid found in lemon is citric acid.

2. The bases have a bitter taste and soapy touch.

3. The sting of the ant contains formic acid.

4. Sulphuric acid is a highly corrosive acid.

5. Acidic soils are neutralized with slaked lime.

 

(II). The statements given below, are incorrect. Write the correct statements.

1. Sulphuric acid is an example of an organic acid.

Ans. Sulphuric acid is an example of mineral acid.

2. Blue litmus paper turns red in the basic solution.

Ans. Blue litmus paper turns red in acidic solution.

3. China rose solution turns green in citric acid solutions.

Ans. China rose solution turns green in basic solutions.

4. Formic acid is found in the sting of a grasshopper.

Ans. Formic acid is found in the sting of an ant.

5. During neutralization, an acid reacts with a salt to form water and a base, as products.

Ans. During neutralization, an acid reacts with a base to form, salt, and water, as products.

(III). Write true or false in front of the following sentences.

1. Tooth decay is caused by the presence of a basic substance in the mouth. False

2. The substance which shows different colours in acids and bases are called indicators. True

3. Milk of magnesia and slaked lime are examples of neutral substances. True

4. Acid rain is caused by the excess of carbon dioxide in the air. False

5. Potassium hydroxide turns blue litmus red. False

6. Most of the fruits contain organic acids. True

Answer the following questions:

Q1. Name three organic and three inorganic acids.

Ans.

Examples of Organic Acids: Citric Acid, Tartaric Acid, Lactic Acid, Oxalic Acid, Acetic Acid

Examples of Inorganic Acids: Sulphuric Acid, Nitric Acid, Hydrochloric Acid, Phosphoric Acid, Boric Acid

Q2. What are the indicators? Name any three indicators and state the colour change which takes place in (i) acids (ii) bases.

Ans. The substances that change their colour on coming in contact with acid or base are called indicators. For example, turmeric, litmus solution, china rose petals and phenolphthalein solution.

(i) Litmus changes to red when added to an acidic solution.

(ii) Phenolphthalein changes to deep pink colour when added to the base solution.

Q3. What are neutral substances? Give examples of two neutral substances.

Ans. Neutral substances are those substances that are neither acidic nor basic and do not affect indicators. For example, alcohol, distilled water, common salt etc.

Q4. You are given three unlabelled bottles A, B, and C, containing colourless solutions, such that one of them is acid, the other being basic and neutral. How will you distinguish between them by using china rose as an indicator?

Ans. If the colour of china rose changes to green colour, we will say that the solution in the bottle is basic. If its colour changes to deep pink colour, then the solution in the bottle is acidic, and if no change takes place in the bottle then we will say that the solution in the bottle, is neutral.

Q5. What do you understand by the term neutralization? Describe an activity in which neutralization of hydrochloric acid takes place with sodium hydroxide, using phenolphthalein as an indicator.

Ans. The process due to which an acid completely reacts with a base with the evolution of heat to form salt and water as the only products, is called neutralization.

ACTIVITY

To show neutralization of hydrochloric acid with sodium hydroxide

Materials required: Sodium hydroxide solution, hydrochloric acid, beaker, phenolphthalein solution (indicator), and a dropper.

Method: Pour about 20cc of sodium hydroxide solution in the beaker. To this solution, add phenolphthalein solution. You will notice that the solution turns deep pink. Now pour some drops of hydrochloric acid into the beaker and stir completely. After some time, the solution becomes colourless. It is because the sodium hydroxide solution has completely reacted with hydrochloric acid to form salt and water. It also warms the beaker. Thus, we can say that heat is also evolved when an acid reacts with a base.

Acid + Base ---à Salt + Water + Heat

HCl + NaOH ---à NaCl + H₂O + Heat

Q6. Explain why:

(a) Farmers add slaked lime to acidic soils?

Ans. Farmers add slaked lime to acidic soils in order to covert the acidic soil into neutral soil so that to increase its fertility for better crop yield.

(b) Soap solution or baking soda paste is applied on the part of the body stung by a bee?

Ans. When a bee stings, it injects formic acid into our body which causes painful irritation and swelling. To reduce the effect of this acid, soap solution or baking soda solution is applied around the affected area. These solutions neutralize the effect of the acid and hence helps in relieving the pain.

(c) Factory wastes and city sewerage is neutralized before discharging in rivers?

Ans. To prevent the killing of aquatic animals, factory wastes and city sewerages are neutralized before discharging in rivers because these wastes contain acids and if directly discharged in rivers, may kill fishes and other living organisms in it.

(d) Antacids are used for relieving stomach activity?

Ans. Sometimes when we eat more spicy food, stomach walls more hydrochloric acid, causing acidity in the stomach and indigestion. To relieve this acidity and indigestion, antiacids are used. These antacids neutralize the excess hydrochloric acid and hence help in controlling the acidity of the stomach.

Q7. Give four differences between acids and alkalis.

Ans.

Acid	Alkali (Base)
i. They have a sour taste. ii. They turn blue litmus solution red. iii. They turn china rose solution to deep pink colour. iv. Most of the acids are soluble in water.	i. They have a bitter taste. ii. They turn red litmus solution blue. iii. They turn china rose solution green. iv. Most of the bases are insoluble in water.

 

Q8. Write an equation when hydrochloric acid neutralises sodium hydroxide.

Ans. Equation of neutralization of HCl and NaOH is given below:

HCl + NaOH ---à NaCl + H₂O + Heat

HCl = Hydrochloric acid

NaOH = Sodium hydroxide

NaCl = Sodium chloride

H₂O = Water

Recommended: Know more about your body

MCQ's: Choose the correct answer.

1. Name the acid which is present in the sting of ants:

(a) Acetic acid     (b) Formic acid   (c) Oxalic acid     (d) Tartaric acid

Ans. (b) Formic acid

2. Blue litmus turns red in which solution:

(a) Acidic solution       (b) Basic solution        (c) Both acidic and basic solution (d) Neutral solution

Ans. (a) Acidic solution

3. Which of the following is not a mineral acid:

(a) Hydrochloric acid  (b) Sulphuric acid        (c) Citric acid      (d) Nitric acid

Ans. (c) Citric acid

4. When a few drops of china rose solution are added to shampoo taken in a test tube the colour of the solution becomes:

(a) Blue      (b) Red      (c) Green   (d) Deep pink

Ans. (c) Green

 

Additional Questions

1. State differences between acids and bases.

Ans. See Q7 above.

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

Ans. Ammonia is basic in nature because only bases turn red litmus blue.

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

Ans. Litmus is a natural dye. It is obtained from lichen plants found in Tundra region.

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

Ans. Distilled water is neutral in nature. It does not affect indicators. We will verify it by putting red litmus or blue litmus papers in the solution. If any of the paper changes its colour, then it is not neutral. If no change takes place then it is neutral.

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

Ans. See Q5 above.

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

(i) Nitric acid turns red litmus blue. (T/F) False

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

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

(iv) The indicator is a substance that shows different colours in acidic and basic solutions. (T/F) True

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