A. Fill in the blanks

1. Acids which are present in plants and animals are called organic acids.
2. Bases taste bitter and have a soapy feel.
3. Acids turn the colour of blue litmus paper to red.
4. The products of neutralisation reaction are salt and water.
5. Salts of nitric acid (HNO₃) are named as nitrates.
6. Sodium acetate (CH₃COONa) is a basic salt formed by the reaction of acetic acid and sodium hydroxide.

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B. Match the following

Column A	Column B
1. Lemon juice	(c) Citric acid
2. Tamarind	(e) Tartaric acid
3. Vinegar	(d) Acetic acid
4. Red ants	(f) Formic acid
5. Sour milk	(b) Lactic acid
6. Guava	(a) Oxalic acid

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C. Tick (✓) the correct option

1. Bases have a—

✅ bitter taste and a soapy feel

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2. An example of a natural indicator is—

✅ litmus

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3. An acid, that contributes to the sour taste of some fruits, is—

✅ citric acid

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4. Which of the following is a strong acid?

✅ nitric acid

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5. Substances, produced through a chemical reaction between acids and bases, are known as—

✅ salts

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6. An indicator, that turns red in a basic medium, is—

✅ hibiscus

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7. The general taste, of acids and bases, is respectively—

✅ sour and bitter

D. Answer the following questions in brief

1. What are mineral acids?

Mineral acids are acids prepared from minerals and are generally strong acids. They are mostly made in laboratories and industries and are highly corrosive in nature.

Examples:

• Hydrochloric acid (HCl)
• Sulphuric acid (H₂SO₄)
• Nitric acid (HNO₃)

These acids are widely used in industries, batteries, fertilizers and chemical manufacturing.

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2. Give two examples each of mineral acids and organic acids.

Mineral acids:

1. Hydrochloric acid (HCl)
2. Sulphuric acid (H₂SO₄)

Organic acids:

1. Citric acid (present in lemon)
2. Acetic acid (present in vinegar)

Organic acids are usually found in plants and animals and are generally weaker than mineral acids.

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3. Name any two substances that can be used as indicators.

Indicators are substances that help us identify whether a substance is acidic or basic by changing colour.

Two examples of indicators:

1. Litmus
2. Phenolphthalein

Other examples include turmeric, methyl orange and hibiscus indicator.

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4. Write the meaning of the term ‘neutralisation reaction’.

A neutralisation reaction is a chemical reaction in which an acid reacts with a base to form salt and water.

General form:

$Acid + Base \rightarrow Salt + Water$Acid+Base→Salt+Water

Example:

$HCl + NaOH \rightarrow NaCl + H_2O$HCl+NaOH→NaCl+H2​O

In this reaction:

• Hydrochloric acid is the acid.
• Sodium hydroxide is the base.
• Sodium chloride and water are formed.

This reaction reduces the acidic and basic properties of the substances.

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5. Give any two properties of salts.

Properties of salts:

1. Salts are formed by the reaction between acids and bases.
2. Many salts are soluble in water and can conduct electricity in solution form.

Some salts are neutral, while others may be acidic or basic in nature.

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6. Classify the following salts as neutral, acidic or basic. Also, write their names.

(a)

$Na_3PO_4$Na3​PO4​

• Name: Sodium phosphate
• Type: Basic salt

Reason: It is formed from a strong base (NaOH) and a weak acid (H₃PO₄).

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(b)

$K_2CO_3$K2​CO3​

• Name: Potassium carbonate
• Type: Basic salt

Reason: It is formed from a strong base (KOH) and a weak acid (H₂CO₃).

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(c)

$NH_4NO_3$NH4​NO3​

• Name: Ammonium nitrate
• Type: Acidic salt

Reason: It is formed from a weak base (NH₄OH) and a strong acid (HNO₃).

E. Answer the following questions

1. ‘All alkalies are bases but all bases are not alkalies.’ Justify this statement.

Bases are substances that neutralise acids and usually have a bitter taste and soapy feel. However, only those bases that dissolve in water are called alkalies.

This means:

• Every alkali is a base because it shows basic properties.
• But some bases do not dissolve in water, so they are not called alkalies.

Examples:

• Sodium hydroxide (NaOH) and potassium hydroxide (KOH) dissolve in water, so they are alkalies.
• Copper oxide (CuO) and zinc oxide (ZnO) are bases but do not dissolve in water, so they are not alkalies.

Therefore, all alkalies are bases, but all bases are not alkalies.

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2. Suggest an activity that can help one to decide whether a given solution is acidic or basic in nature.

Activity using litmus paper

Materials required:

• Red litmus paper
• Blue litmus paper
• Given solution

Procedure:

1. Take a few drops of the given solution in a test tube or bowl.
2. Dip blue litmus paper into the solution.
3. Observe the colour change.
4. Now dip red litmus paper into the solution and observe again.

Observation:

• If blue litmus turns red, the solution is acidic.
• If red litmus turns blue, the solution is basic.
• If there is no colour change, the solution may be neutral.

Conclusion:

With the help of indicators like litmus paper, we can easily identify whether a solution is acidic or basic.

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3. Write chemical equations for the following reactions.

(a) Calcium hydroxide reacts with nitric acid

$Ca(OH)_2 + 2HNO_3 \rightarrow Ca(NO_3)_2 + 2H_2O$Ca(OH)2​+2HNO3​→Ca(NO3​)2​+2H2​O

This is a neutralisation reaction in which calcium nitrate and water are formed.

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(b) Acetic acid reacts with calcium hydroxide

$2CH_3COOH + Ca(OH)_2 \rightarrow Ca(CH_3COO)_2 + 2H_2O$2CH3​COOH+Ca(OH)2​→Ca(CH3​COO)2​+2H2​O

In this reaction, calcium acetate and water are produced.

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(c) Hydrochloric acid reacts with sodium hydroxide

$HCl + NaOH \rightarrow NaCl + H_2O$HCl+NaOH→NaCl+H2​O

This reaction forms common salt and water.

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(d) Ammonium hydroxide reacts with sulphuric acid

$2NH_4OH + H_2SO_4 \rightarrow (NH_4)_2SO_4 + 2H_2O$2NH4​OH+H2​SO4​→(NH4​)2​SO4​+2H2​O

This reaction forms ammonium sulphate and water.

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4. State the difference between neutral, acidic and basic salts. Give one example of each.

Type of Salt	Description	Example
Neutral salt	Formed by reaction of a strong acid and a strong base.	Sodium chloride (NaCl)
Acidic salt	Formed by reaction of a strong acid and a weak base.	Ammonium chloride (NH₄Cl)
Basic salt	Formed by reaction of a weak acid and a strong base.	Sodium acetate (CH₃COONa)

Explanation:

• Neutral salts do not change the colour of litmus paper.
• Acidic salts show acidic properties in water.
• Basic salts show basic properties in water.

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5. Describe an activity to show that solutions of salts in water can conduct electricity.

Aim:

To show that salt solutions conduct electricity.

Materials required:

• Beaker
• Water
• Common salt
• Battery
• Bulb
• Two wires
• Two graphite rods or metal strips

Procedure:

1. Take water in a beaker and dissolve some common salt in it.
2. Connect the battery, bulb and graphite rods using wires to make a simple circuit.
3. Dip the graphite rods into the salt solution carefully.
4. Observe the bulb.

Observation:

The bulb glows when the rods are dipped into the salt solution.

Conclusion:

The glowing bulb shows that electric current passes through the salt solution. Therefore, solutions of salts in water can conduct electricity because they contain ions that carry electric current.

Also Read: Class 7 Living Science Chapter 2 Nutrition in Living Organisms – Animals and Man Solutions | Tata DAV Science

Chemical Substances and Processes Class 7 DAV Chapter 3 Solutions, Notes, Questions & Answers