The pH Scale & Neutralisation (Oxford AQA IGCSE Combined Science Double Award)

The pH Scale

• The pH scale measures how acidic or alkaline a solution is

• All acids have pH values of below 7, all alkalis have pH values of above 7

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

  • pH 0-2 = strong acid

    • Extremely acidic substances can have values of below 1

  • pH 3-6 = weak acid

• The higher the pH, 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

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

The pH Scale

Hydrogen ions

• Acids contain hydrogen ions (H⁺)

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

Example: hydrochloric acid

HCl (aq)   →    H⁺ (aq)    +    Cl^– (aq)

Hydroxide ions

• Alkalis contain hydroxide ions (OH^–)

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

Example: sodium hydroxide

NaOH (s)   →    Na⁺ (aq)    +    OH^– (aq)

What Ions React Together in Neutralisation Reactions?

• 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

• The production of water means that a neutral solution, pH 7, is formed

• The net ionic equation of all acid-base neutralisations is:

H⁺  (aq) + OH^– (aq)⟶ H₂O (l)

• Not all reactions of acids are neutralisation reactions

  • 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, such as limestone and quicklime, to the soil