The Ions in Acids & Alkalis (CIE IGCSE Chemistry: Co-ordinated Sciences (Double Award))

What is a neutralisation reaction?

• 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

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 0-2 = strong acid
    • Extremely acidic substances can have values of below 1
  • pH 3-6 = 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

The pH scale showing acidity, neutrality and alkalinity

The pH scale and hydrogen ions

• We have already seen that acids are substances that contain hydrogen ions in solution
• The more hydrogen ions the stronger the acid, but the lower the pH
• The higher the concentration of hydroxide ions in a solution the higher the pH
• So pH is a measure of the concentration of H⁺ ions in solution, but they have an inverse relationship
• The pH scale is logarithmic, meaning that each change of 1 on the scale represents a change in concentration by a factor of 10

Concentration of hydrogen ions and pH

An acid with a pH of 3 has ten times the concentration of H⁺ ions than an acid of pH 4. An acid with a pH of 2 has 10 x 10 = 100 times the concentration of H⁺ ions than an acid with a pH of 4

How is univesral indicator used?

• Universal indicator is a mixture of different indicators which is used to measure the pH
• A drop is added to the solution and the colour is matched with a colour chart which indicates the pH which matches specific colours

The pH scale with the Universal Indicator colours which can be used to determine the pH of a solution