The pH Scale & Neutralisation (AQA GCSE Chemistry: Combined Science)

• 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

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

• 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 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

• A neutralisation reaction occurs when an acid reacts with an alkali
  • Acids are sources of hydrogen ions, H⁺
  • Bases (or alkalis) are 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