The Ph Scale & Neutralisation (AQA GCSE Combined Science: Synergy: Physical Sciences): 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 0 – 14 

• All acids have pH values of below 7

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

  • Extremely acidic substances can have values of below 1

• All alkalis have pH values of above 7

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

• A solution of pH 7 is described as being neutral

Measuring pH

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

• pH probes 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 that 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 manufacturers so colour charts are usually provided for a specific indicator formulation

Neutralisation

• A neutralisation reaction occurs when an acid reacts with an alkali to form a salt and water

hydrochloric acid + sodium hydroxide ⟶ sodium chloride + water

HCl + NaOH ⟶ NaCl + H₂O

sulfuric acid + potassium hydroxide ⟶ potassium sulfate + water

H₂SO₄ + 2KOH ⟶ K₂SO₄ + 2H₂O

• Acids are a source of hydrogen ions, H⁺

• Bases (or alkalis) are a source 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

Higher Tier only

• At Higher Tier, you need to be able to use the formulae of common ions to write balanced ionic equations for neutralisation reactions.

• To do this:

  1. Write out all ions separately to give the full ionic equation

  2. Identify the spectator ions

  3. Cancel the spectator ions to leave the net ionic equation