The pH Scale & Neutralisation (AQA GCSE Chemistry)

Acids & bases

• When acids are added to water, they form positively charged hydrogen ions (H⁺)

  • The presence of H⁺ ions is what makes a solution acidic

• When alkalis are added to water, they form negative hydroxide ions (OH^–)

  • The presence of the OH^– ions is what makes the aqueous solution an alkali

• The pH scale is a numerical scale which is used to show how acidic or alkaline a solution is

  • It is a measure of the amount of the hydrogen ions present in solution

The pH scale

What is the pH scale?

• The pH scale goes from 1 – 14 

• All acids have pH values of below 7

• All alkalis have pH values of above 7

• The lower the pH then the more acidic the solution is

  • pH 1-3 = strong acid

    • Extremely acidic substances can have values of below 1

  • pH 4-6 = weak acid

• The higher the pH then the more alkaline the solution is

  • pH 8-11 = weak alkali

  • pH 12-14 = strong alkali

• A solution of pH 7 is described as being neutral

The pH scale showing acidity, neutrality and alkalinity

Measuring pH

• pH can be measured using a digital pH meter or an indicator 

• pH meters contain a special electrode with a thin glass membrane that allows hydrogen ions to pass through; the ions alter the voltage detected by the electrode

• An indicator is a substance which changes colour depending on the pH of the solution to which it is added

• There are natural indicators and synthetic indicators which have different uses

• Generally, natural indicators are wide range indicators contain a mixture of different plant extracts and so can operate over a broad range of pH values

• Synthetic indicators mostly have very narrow pH ranges at which they operate

  • They have sharp colour changes meaning they change colour quickly and abruptly as soon as a pH specific to that indicator is reached

• Indicators are intensely coloured and very sensitive so only a few drops are needed

Universal indicator

• Universal indicator is a wide range indicator and can give only an approximate value for pH

• It is made of a mixture of different plant indicators which operate across a broad pH range and is useful for estimating the pH of an unknown solution

• A few drops are added to the solution and the colour is matched with a colour chart which indicates the pH which matches with specific colours

• Universal indicator colours vary slightly between manufacturer so colour charts are usually provided for a specific indicator formulation

pH scale with the universal indicator colours used to determine the pH of a solution

Neutralisation

• A neutralisation reaction occurs when an acid reacts with an alkali

  • Acids are a source of hydrogen ions, H⁺

  • Bases (or alkalis) are a sources of  hydroxide ions, OH^– 

• When they react together in a neutralisation reaction, the H⁺ ions react with the OH^– ions to produce water

• This is the net ionic equation of all acid-base neutralisations and is what leads to a neutral solution, since water has a pH of 7:

H⁺  (aq) + OH^– (aq)⟶ H₂O (l)

• Not all reactions of acids are neutralisations

  • For example when a metal reacts with an acid, although a salt is produced there is no water formed so it does not fit the definition of neutralisation

• Neutralisation is very important in the treatment of soils to raise the pH as some crops cannot tolerate pH levels below 7

  • This is achieved by adding bases to the soil such as limestone and quicklime