MAK LABS — Std 8 General Science Chapter 12 Introduction to Acid and Base

1Identify the following solutions, whether they are acid or alkali.

Solution	Litmus	Phenolphthalein	Methyl orange	Acid / Alkali
1.	–	No change	–	Acid
2.	–	–	Red	Acid
3.	Red litmus turns blue	–	–	Alkali

Colours of the common laboratory indicators in acid and in alkali.

2Write chemical names from given formulae.

H₂SO₄ – Sulphuric acid
Ca(OH)₂ – Calcium hydroxide
HCl – Hydrochloric acid
NaOH – Sodium hydroxide
KOH – Potassium hydroxide
NH₄OH – Ammonium hydroxide

3Sulphuric acid has highest importance in chemical industry. Why?

Sulphuric acid is called the ‘king of chemicals’. It is used in many industries.

It is used to make chemical fertilizers.
It is used to make explosives, dyes, paints and medicines.
It is used in batteries.
It is used to purify petroleum.

Hence, sulphuric acid has highest importance in chemical industry.

4Give answers.

(a) Which acid is used for getting chloride salt?

Hydrochloric acid (HCl) is used for getting chloride salts.

(b) By squeezing lemon on a piece of rock the gas liberated turned lime water milky. Which compound is present in the rock?

The gas is carbon dioxide. Acid gives carbon dioxide when it reacts with a carbonate. So the rock contains calcium carbonate (CaCO₃).

(c) The label on the bottle of chemical is spoiled. How will you find whether the chemical is acidic or not?

Test a few drops of the chemical with indicators:

If blue litmus turns red, it is an acid.
If methyl orange turns red, it is an acid.
If phenolphthalein remains colourless, it may be an acid.

5Answer the following questions.

(a) Explain the difference between acid and alkali.

Acid	Alkali
It is sour in taste.	It is bitter in taste.
It gives H⁺ ions in water.	It gives OH⁻ ions in water.
It turns blue litmus red.	It turns red litmus blue.
Phenolphthalein remains colourless.	Phenolphthalein turns pink.
Examples: HCl, H₂SO₄	Examples: NaOH, KOH

(b) Why does indicator not get affected by salt?

Indicators change colour due to H⁺ ions or OH⁻ ions. A salt solution is neutral. It does not have free H⁺ or OH⁻ ions. Hence, indicators are not affected by salt.

(c) Which substances are produced by neutralization process?

Salt and water are produced by neutralization.

Acid + Base → Salt + Water
HCl + NaOH → NaCl + H₂O

(d) Which are the industrial uses of acids?

To make chemical fertilizers.
To make explosives, dyes, paints and medicines.
Hydrochloric acid is used to make chloride salts.
Sulphuric acid is used in batteries.
To make white paper from wood pulp.

6Select proper word given in bracket and fill in the blanks.

(a) Main constituent of acid is H⁺ ion.

(b) Main constituent of alkali is OH⁻ ion.

(c) Tartaric acid is a natural acid.

7Match the pairs.

Group A	Group B
1. Tamarind	c. Tartaric acid
2. Curd	d. Lactic acid
3. Lemon	b. Citric acid
4. Vinegar	a. Acetic acid

8State true or false.

(a) Oxides of metals are alkaline in nature. True

Metal oxides like CaO and MgO form bases with water.

(b) Salt is acidic. False

Salt is neutral.

(c) Metal corrodes due to salts. False

Metals corrode due to air and moisture.

(d) Salts are neutral. True

Salts are formed by neutralization of acid and base.

9Classify following substances into acidic, basic and neutral group.

Acidic: HCl, H₂SO₄, HNO₃
Basic: MgO, CaO, Na₂CO₃
Neutral: NaCl, KCl, H₂O

12. Introduction to Acid and Base

Exercises