Class 10 Properties of Acid-Base

You have learnt that when an acid reacts with a metal, hydrogen gas is evolved. This shows that hydrogen gas is present in all acids. But this does not mean that all those chemicals which contain hydrogen are acidic. For example; hydrochloric acid (HCl), sulphuric acid (H₂SO₄), nitric acid (HNO₃), etc. are acids and all of them contain hydrogen. But glucose (C₆H₁₂O₆) does not show acidic property in spite of containing hydrogen atom. The following activity shows this.

Activity:

Take a beaker and keep a cork at its base.

Fix two nails in the cork; with some gap between them. Connect the nails with a battery, bulb and a switch; as shown in the figure.

Take some dilute hydrochloric acid in the beaker. You will notice that the bulb starts glowing.
Repeat this step with sulphuric acid and nitric acid. The bulb glows in each condition.
Now, repeat this step with glucose solution. The bulb does not glow in this case.

When ions are available in the solution, it results in glowing of the bulb. But if ions are not available in the solution, the bulb does not glow. This means that an acid dissociates into hydrogen ions when the acid is in aqueous solution. Thus, it can be said that H⁺ ions in aqueous solution are responsible for the acidic property of the solution. This also means that a material shall not show acidic property unless it is in aqueous solution. This can be further confirmed by the following activity.

Important Note: Ionic compounds dissociate into ions in aqueous solution. Hence, ionic compounds conduct electricity in aqueous solution. On the other hand, covalent compounds (like glucose) do not dissociate into ions in aqueous solution and hence fail to conduct electricity in aqueous solution.

Activity:

Take a test tube and put 1 g NaCl in it.

Now, pour a small amount of concentrated sulphuric acid in the test tube.
Close the mouth of the test tube with a cork and insert a delivery tube in the cork.
Bring a dry litmus paper (blue) near the delivery tube and observe any change in its colour.
Now, bring a wet litmus paper (blue) near the delivery tube and observe any change in its colour.

You will notice that no colour change is noticed when dry litmus paper is used. But in a case of wet litmus paper being used, the colour changes from blue to red. This shows the acidic property of an evolved gas. This also shows that a substance shows acidic property only in the presence of water.

This reaction can be given by the following equation:

The hydrochloric gas (evolved in this reaction) behaves like hydrochloric acid in presence of water. This happens because of a liberation of hydrogen ions when an acid is in aqueous solution. This dissociation can be shown by the following equation.

Hydrogen ions cannot exist alone. They can only exist when they combine with a water molecule to form hydronium ion (H₃O⁺).

So far, you have learnt that when an acid is dissolved in water, it produces hydrogen ions (H⁺). Let us now see what happens when a base is dissolved in water. When a base is dissolved in water, it produces hydroxide (OH^–) ions. Following an example of NaOH shows this.

Important Note:

All bases do not dissolve in water. A base which can dissolve in water is called an alkali.

It is now clear that all acids give hydrogen ion in aqueous solution. Similarly, all bases give hydroxide ion in aqueous solution. From this perspective, neutralisation reaction can be seen as follows:

Acid + Base → Salt + Water

HX + MOH → MX + HOH

Let us now see what happens when water is mixed with an acid or a base.

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Chemical Reactions and Equations:

Types of Chemical Reaction:

Acids, Bases and Salts:

Common Properties of Acid and Base:

Carbon and its Compounds:

Periodic Classification of Elements: