Fig.5) on the can of concentrated sulphuric acid and on the bottle of sodium hydroxide pellets.

Mixing an acid or base with water result in decrease in the concentration of ions (H3O +/OH– ) per unit volume. Such a process is called dilution and the acid or the base is said to be diluted.

Can you decide the strength of an acid or base solutions?

Let us find.

Strength of acid or base:

Activity 10

    A test to know whether the Acid is strong or weak.

- Take two beakers A and B.

- Fill the beaker A with dil. CH3 COOH (acetic acid) and beaker B with dil. HCl (Hydrochloric Acid)

- Arrange the apparatus as used in activity-7, and pass electric current through the solutions in separate beakers.

• What do you observe?

• Can you guess the reason for the changes you observed?

You notice that the bulb glows brightly in HCl solution while the intensity of the bulb is low in acetic acid solution. This indicates that there are more ions in HCl solution and fewer ions are present inacetic acid solution. More ions in HCl solution mean more H3O+ ions.Therfore it is a strong acid. Whereas acetic acid has fewer H3O+ ions and hence it is weak acid.

Carry out the above experiment by taking bases like dil. NaOH (sodium hydroxide) and dil. NH4OH (ammonium hydroxide) instead of acids.

• What do you observe? Explain your observations.

The universal indicator indicator can also be used to know the strengthof acid or base. Universal indicator is a mixture of several indicators. The universal indicator shows different colours at different concentrations of hydrogen ions in a solution.



page no:43

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