Acid Base & Salt - Class 10th Science

Strenght of Acid and Base

How strong are Acid or Base Solutions?

In distilled water, concentration of hydrogen ions and concentration of hydroxide ions are equal. Thus, distilled water is neither acidic nor basic.

If the hydrogen ions concentration is higher than that of hydroxide ions concentration in a solution, the solution is acidic. And if the hydroxide ions concentration is higher than that of the hydrogen ions, the solution is called basic.

Acid base indicator can only distinguish between acid and base. But an acid base indicator does not tell about the strength of an acid or base, i.e. it does not tell that how much strong or powerful or potential an acid or base.

Since, an acid produces hydrogen ions and a base produces hydroxide ions in their aqueous solution, thus by knowing concentration of hydrogen ions quantitatively the strength of an acid or base can be determined.

Acid which has more hydrogen ion concentration is said to be stronger and those which produce less hydrogen ions is said to be weaker. Similarly, a base which has more hydroxide ions concentration is said to be stronger base and those which produces less hydroxide ions is said to be weaker base.

Example:

Same concentration of hydrochloric acid produces more hydrogen ions compare to acetic acid of same concentration. Thus, hydrochloric acid is stronger acid than that of acetic acid.

pH scale:

The concentration of hydrogen ion in a solution can be determined using pH scale.

In the word pH, p stands for potency or power and H stands for Hydrogen ions. p came from German word ‘potenz’ which means power. In totality it can be said that pH means power of an acid.

pH scale is a tool using which hydrogen ion concentration is measured.

Definition of pH:

pH is defined as the decimal logarithm of the reciprocal of the hydrogen ion activity (a_H⁺) in a solution.

pH scale works on the value from 0 to 14 (zero to fourteen).

0 (zero) value on pH scale indicates highly acidic (strongest acid)

7 (seven) value on pH scale indicates the neutral solution

14 (fourteen) value on pH scale indicates highly basic (strongest base)

Strength of acid, i.e. acidity increases with decreasing of value on pH scale from 7 (seven) to (0) zero

Strength of base, i.e. basicity increases with increasing of value on pH scale from 7 (seven) to 14 (fourteen)

Thus, by knowing the value of on pH scale strength of an acid or a base can be determined.

Universal indicator:

Universal indicator is a mixture of many compounds. It composes generally water, propane-1-ol, phenolphthalein, sodium salt, sodium hydroxide, methyl red, bromothyomol blue monosodium salt and thyomol blue monosodium salt. Universal indicator exhibits colour in colour of solution over a pH value range from 1 to 14. Changing of the range of pH value indicates the acidity or basicity of the solution.

A colour chart is supplied with universal indicator. Different colours in the given chart shows pH values range from 1 to 14.

Universal indicator comes in solution form and paper strips form both.

When universal indicator paper strip is dipped in given solution, colour of paper strip changes. This changed colour is matched with the given chart and value of pH is obtained. According to the value of pH observed acidity or basicity of the given solution is determined.

pH value of some of the common liquid is given here.

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