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The pH Scale & Neutralisation (AQA GCSE Chemistry: Combined Science)
Revision Note
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
- pH 1-3 = strong 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
Worked example
The following table shows the pH of some substances.
Substance | pH |
lemon juice | 2.2 |
limewater | 10.5 |
saliva | 6.3 |
milk of magnesia | 10.1 |
- Name the strongest alkali
- Name the strongest acid
- Name the substances which is closest to neutral
Answers:
i) Limewater has the highest pH in the table of 10.5 so is the strongest alkali
ii) Lemon juice has the lowest pH in the table of 2.2 so is the strongest acid
iii) Saliva is the closest substance to being neutral as its pH value of 6.3 is closest to pH 7
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
Examiner 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.
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
Worked example
i) Write the balanced symbol for the reaction between copper(II) hydroxide and sulfuric acid to form copper sulfate and water.
ii) Write the simplest ionic equation for the neutralisation reaction
Answer:
i) The balanced symbol equation is:
-
Cu(OH)2 + H2SO4 ⟶ CuSO4 + 2H2O
ii) To write the simplest ionic equation:
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Write the full ionic equation and cancel out the spectator ions
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Cu2+ + 2OH- + 2H+ + SO42- ⟶ Cu2+ + SO42- + 2H2O
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Remove the spectator ions
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2H+ + 2OH- ⟶ 2H2O
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This can be further simplified to
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H+ + OH- ⟶ H2O
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Examiner Tip
Make sure you learn the equation for neutralisation:
H+ (aq) + OH– (aq) → H2O (l)
You should include state symbols, as it is a common question that is asked in exams.
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