Textbook Solution: Acids, Bases and Salts | Class 7 Oxford Science: Chapter Notes, Worksheets & Tests PDF Download

A. Choose the correct option.

1.
Ans: (d)

2.
Ans: (c)

3.
Ans: (a)

4.
Ans: (a)

5.
Ans: (b)

6.
Ans: (b)

B. Fill in the blanks with the correct words.

1.
Ans: Acetic acid, formic acid, and tartaric acid are called organic acids.

2.
Ans: Acids are sour in taste.

3.
Ans: The chemical name of common salt is sodium chloride.

4.
Ans: When you use phenolphthalein as an indicator, it gives the colour pink for bases.

5.
Ans: Litmus is used as an indicator.

6.
Ans: Acidic salt is formed when a strong acid reacts with a weak acid.

C. Write T for true and F for false.

1.
Ans: Nitric acid is also called king of chemicals. - False

2.
Ans: Universal indicator is a mixture of indicators. - True

3.
Ans: Acid with pH 1 is the weakest acid. - False

4.
Ans: Hydrated salt is one that has lost its water of crystallization. - False

5.
Ans: Salts are generally liquids with low melting and boiling points. - False

D. Name the following.

1.
Ans: Substances containing an acid are called acidic substances.

2.
Ans: Acids that occur naturally in plant and animal materials are called Organic Acids.

3.
Ans: Acids that have a greater quantity of water than acid are called Dilute Acids.

4.
Ans: Substances that do not show a change in colour in any of the indicators are Neutral Substances.

5.
Ans: The reaction in which an acid reacts with a base to form salt is called Neutralization Reaction.

6.
Ans: The water molecules present in solid crystals are called Water of Crystallization.

7.
Ans: Salts which have lost their water of crystallization are called Anhydrous Salts.

E. Answer the following questions.

1.
Ans: Differentiate between the following:
(a) Acidic and basic substances: Acidic substances are those that contain acids, have a sour taste, and are highly reactive. Examples include lemon and raw mango. Basic substances contain bases, are generally bitter in taste and feel slippery to touch. Examples include soap and baking soda.
(b) Mineral and organic acids: Mineral acids are made from inorganic minerals and are commonly used in the laboratory. Examples include hydrochloric acid and sulphuric acid. Organic acids naturally occur in plants and animals. Examples include acetic acid and citric acid.
(c) Dilute and concentrated acids: Dilute acids are those where the quantity of water is more than the acid. Concentrated acids are those where the quantity of water is less than the acid.

2.
Ans: Most acids are not stored in metal containers because they are highly reactive and corrosive in nature. They can corrode a wide range of substances, including metals.

3.
Ans: The acid called the oil of vitriol is sulphuric acid. Its two uses include its use in car batteries and in the manufacture of paints, drugs, and dyes.

4.
Ans: Vinegar acts as a preservative because it checks the growth of microorganisms within the preserves. Acidic conditions slow down microbial activities and can also kill them.

5.
Ans:

• Hydrochloric acid: It is used to remove deposits from the inner surfaces of large industrial boilers and to purify common salt.
• Nitric acid: It is used to clean silver and gold jewellery and to manufacture fertilizers.
• Sodium hydroxide: It is used in the manufacture of soaps and detergents and in the manufacture of paper and textiles.
• Calcium hydroxide: It is used for neutralizing the acidity in soils and for whitewashing.

6.
Ans: Alkalis are bases that can dissolve in water. Two examples of alkalis are sodium hydroxide and ammonium hydroxide.

7.
Ans:
(a) Magnesium hydroxide is used as an antacid because it helps to reduce excessive acidity in the stomach.
(b) Strong bases should be handled carefully because they can destroy human tissue, causing severe burns if accidentally spilled on the skin.
(c) Calamine lotion is applied on bee stings because it contains zinc carbonate, a base, that neutralizes the acidic liquid in the sting.
(d)  A base is added to factory waste before releasing it to neutralize the acidic substances in the waste, preventing harm to aquatic life.

8.
Ans: Indicators are chemical substances that show a change in colour when in contact with acids and bases. Two examples are litmus and phenolphthalein.

9.
Ans: The effect of dilute sulphuric acid on the following is: (a) Red Litmus - turns blue (b) Blue Litmus - remains blue (c) Phenolphthalein - becomes colourless.

10.
Ans: Natural indicators are prepared from substances such as flowers, roots, stems, and leaves. With red cabbage juice, it gives a deep red colour in an acidic solution, purple in a neutral solution, and green or yellow in a basic solution.

11.
Ans: A neutralization reaction is a chemical reaction in which an acid reacts with a base to form salt and water. An example is the reaction of hydrochloric acid with sodium hydroxide to form sodium chloride and water.

12.
Ans: Water of crystallization is the water present in the crystal structure of a salt. A hydrated salt can be converted into an anhydrous salt by heating it to lose its water of crystallization.

13.
Ans: The chemical formula and one use for the following salts are:

• Silver nitrate (AgNO3) - Used in the preparation of photographic films.
• Ammonium nitrate (NH4NO3) - Used in the manufacture of fertilizers.

14.
Ans: Baking soda is sodium bicarbonate and is used in cooking for baking. Washing soda is sodium carbonate and is used in laundry as a water softener.