The pH Scale & Neutralisation (AQA GCSE Chemistry)
Revision Note
Author
StewartExpertise
Chemistry Lead
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, as it is a measure of the amount of the hydrogen ions present in solution
The pH Scale
- The pH scale goes from 1 – 14 (extremely acidic substances can have values of below 1)
- 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
- The higher the pH then the more alkaline the solution is
- 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 an indicator or a digital pH meter
- 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
Neutralisation
- A neutralisation reaction occurs when an acid reacts with an alkali
- When these substances 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+ + OH– ⟶ H2O
- 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
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
Exam Tip
A common error is to suggest using universal indicator as a suitable indicator for an acid-base titration. This is incorrect as a sharp colour change is required to identify the end-point, which cannot be achieved with Universal Indicator.
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