ICSE Class 7 Chemistry Chapter 03 Acids Bases and Salts

Acids, Bases and Salts

Acids, bases and salts are important classes of compounds. Most compounds that we come across belong to one of these classes. For example, lemon contains an acid, toothpaste contains a base and table salt is a salt. Let us see what these compounds are and how they are important.

Acids

The term acid is derived from the Latin word acidus, meaning sour. In fact, everything that tastes sour, e.g., a lemon, an orange, curd or vinegar, contains an acid.

Acids In Common Foodstuff

Sour things we eat contain organic acids. It is interesting to know that vitamin C is ascorbic acid. Taste a tablet of vitamin C - it is sour.

Some inorganic acids are mentioned in Table 3.2. Hydrochloric, nitric and sulphuric acids - the common inorganic acids - are known as mineral acids. Inorganic acids have biological as well as industrial importance. Make sure you do not touch any of these acids (except carbonic acid) by mistake. They can cause what are known as acid burns.

Common Inorganic Acids

Teacher's Note

Explain to students why lemon juice stings when it gets into a cut on their skin - the citric acid reacts with the wound. This helps them understand acid properties viscerally.

How Are Acids Prepared?

Acids are generally prepared by the following methods.

By The Direct Combination Of Elements

\[\text{H}_2 + \text{Cl}_2 \xrightarrow{\text{sunlight}} 2\text{HCl}\]

Hydrogen chloride

By Dissolving The Oxide Of A Nonmetal In Water

\[\text{SO}_2 + \text{H}_2\text{O} \longrightarrow \text{H}_2\text{SO}_3\]

Sulphur dioxide - Sulphurous acid

\[\text{SO}_3 + \text{H}_2\text{O} \longrightarrow \text{H}_2\text{SO}_4\]

Sulphur trioxide - Sulphuric acid

By Heating A Salt Like Sodium Chloride Or Sodium Nitrate With Concentrated Sulphuric Acid

\[2\text{NaCl} + \text{H}_2\text{SO}_4 \xrightarrow{\text{heat}} \text{Na}_2\text{SO}_4 + 2\text{HCl}\]

Hydrogen chloride

\[2\text{NaNO}_3 + \text{H}_2\text{SO}_4 \xrightarrow{\text{heat}} \text{Na}_2\text{SO}_4 + 2\text{HNO}_3\]

Nitric acid

Teacher's Note

Show students that chemical reactions can create acids from simple components. This demonstrates the practical chemistry happening in laboratories and industries.

What Are Concentrated And Dilute Acids?

An acid is generally used as a solution in the laboratory. For example, a solution of hydrogen chloride (HCl) in water is called hydrochloric acid. If the solution is saturated or contains a large amount of the acid, it is called a concentrated acid. When water absorbs a small amount of the acid, it is said to be dilute.

Diluting A Concentrated Acid

A lot of heat is released when an acid is mixed with water. So the following precautions should be taken while diluting a concentrated acid.

1. The acid should be added to water and not the other way round. Water absorbs the heat produced.

2. A very small quantity of the acid should be added to the water at a time.

3. The solution must be stirred and cooled after each mix.

If you are not careful, there may be an accident.

Teacher's Note

Emphasize the critical safety rule of adding acid to water, not water to acid. This prevents violent boiling and dangerous splashing that could cause burns.

General Characteristics Of Acids

Acids have the following general characteristics.

1. They have a sour taste.

2. They turn blue litmus red. Litmus is a natural dye obtained from certain lichens, which are very small plants.

3. They corrode most metals.

4. They liberate carbon dioxide from carbonates and hydrogencarbonates (i.e., bicarbonates).

5. They lose their acidic property when they react with what are known as bases (i.e., metal oxides and hydroxides and also ammonium hydroxide). These reactions are called neutralisation reactions, which we will study soon.

6. Aqueous solutions of acids allow the passage of electricity through them.

You can easily test for an acid at home too. Take some baking soda (sodium bicarbonate) and put a drop or two of the test liquid on it. Acids will cause release of carbon dioxide with effervescence, i.e., gas bubbles will be produced with a hissing sound. You will find that lemon juice and vinegar will cause that effect, so they are acidic and not neutral.

Teacher's Note

The baking soda test is a safe, hands-on way for students to verify acids. Seeing the fizzing bubbles makes the chemical property immediately observable and memorable.

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