4.2.4 The pH Scale and Neutralisation

In aqueous solutions, acids release hydrogen ions (H⁺), while alkalis contain hydroxide ions (OH^-). The pH scale is a numerical scale ranging from 0 to 14 that measures the acidity or alkalinity of a solution. The pH can be determined using indicators such as universal indicator or a pH probe.

On the pH scale, a solution with a pH of 7 is considered neutral. Solutions with a pH less than 7 are acidic, while solutions with a pH greater than 7 are alkaline (basic).

During a neutralisation reaction between an acid and an alkali, hydrogen ions from the acid react with hydroxide ions from the alkali to form water. This can be represented by the following equation:

H⁺ (aq) + OH^- (aq) → H₂O (l)

Universal Indicator

Universal indicator is a mixture of several different indicators that undergo colour changes in response to different pH levels. It can be used to estimate the pH of a solution by observing the colour change that occurs when the indicator is added to the solution. Universal indicator is usually provided as a paper strip or a liquid solution.

When universal indicator is added to a solution, it changes colour depending on the pH of the solution. The colour range spans the entire pH scale, from acidic to alkaline. By comparing the observed colour to a colour chart provided with the universal indicator, one can estimate the approximate pH of the solution. The colour chart typically provides a range of colours corresponding to different pH values.

Wide range indicators function in a similar way, providing a visual indication of the pH of a solution within a broader range. They are designed to display distinct colours at specific pH values, enabling users to estimate the pH based on the observed colour.

Using the pH Scale

The pH scale is a numerical scale that measures the acidity or alkalinity of a solution. It ranges from 0 to 14, with 7 representing neutrality. 

• Solutions with a pH less than 7 are considered acidic. The lower the pH value, the more acidic the solution. For example, a solution with a pH of 1 is highly acidic.
• Solutions with a pH greater than 7 are alkaline (basic). The higher the pH value, the more alkaline the solution. For example, a solution with a pH of 14 is highly alkaline.
• Solutions with a pH of 7 are neutral, meaning they are neither acidic nor alkaline. Pure water, for example, has a pH of 7 and is considered neutral.

Conclusion

By measuring the pH of a solution using pH paper, a pH metre, or indicators such as universal indicator, you can determine whether a solution is acidic, alkaline, or neutral based on its pH value.