The pH scale

The pH scale shows how acidic or alkaline a solution is. It runs from 0 to 14, with neutral solutions in the middle at pH 7.

• A pH below 7 is acidic. The lower the pH, the more acidic the solution.
• A pH of 7 is neutral.
• A pH above 7 is alkaline. The higher the pH, the more alkaline the solution.

For example, a solution at pH 3 is acidic, while one at pH 10 is alkaline.

Estimating pH using indicator colours

Universal indicator and other wide-range indicators change colour depending on the pH of a solution. Universal indicator generally shows:

• Red, orange or yellow in acidic solutions.
• Green in a neutral solution.
• Blue or purple in alkaline solutions.

To estimate the pH, add indicator to the solution and compare the resulting colour with the indicator’s colour chart. The chart matches colours to approximate pH values. For example, a colour that matches pH 4 on the chart shows that the solution is acidic.

Measuring pH with a probe

A pH probe measures pH more directly and gives a numerical reading. The reading can be compared with 7 to decide whether the solution is acidic, neutral or alkaline.

Neutralisation

Neutralisation is the reaction between an acid and an alkali. The clearest way to understand this reaction is to look at what happens to the particles in solution.

An acid provides hydrogen ions, $$\ce{H+ (aq)}$$, and an alkali provides hydroxide ions, $$\ce{OH- (aq)}$$. When these ions meet, they react to form water, $$\ce{H2O (l)}$$.

Neutralisation: The particle-level change

This ionic equation is the key change in neutralisation. It shows that the reaction happens because the hydrogen ions from the acid combine with the hydroxide ions from the alkali.

At the particle level, neutralisation can be understood as the removal of the ions responsible for acidity and alkalinity. The $$\ce{H+}$$ ions are responsible for the acidic nature of the solution, and the $$\ce{OH-}$$ ions are responsible for the alkaline nature of the solution. When they react together, they are used up and water is formed.

So, when you recognise a neutralisation reaction, the essential idea to remember is this: neutralisation is the reaction of $$\ce{H+}$$ and $$\ce{OH-}$$ to produce water.

Acids and alkalis in aqueous solution

When an acid or an alkali dissolves in water, the solution contains charged particles called ions. A solution in water is called an aqueous solution. Its acidity or alkalinity depends on the ions present.

An acid produces hydrogen ions, $$\ce{H+}$$, in water. For example, hydrochloric acid contains $$\ce{H+}$$ ions. An alkali produces hydroxide ions, $$\ce{OH-}$$, in water. For example, sodium hydroxide solution contains $$\ce{OH-}$$ ions.

Aqueous solutions can contain both kinds of ion. An acidic solution has a greater concentration of $$\ce{H+}$$ ions than $$\ce{OH-}$$ ions, while an alkaline solution has a greater concentration of $$\ce{OH-}$$ ions than $$\ce{H+}$$ ions.