The Ions in Acids & Alkalis (CIE IGCSE Chemistry)

• 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, in other words it is a measure of the amount of ions present in the solution

• 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
• For example, when hydrochloric acid is neutralised a sodium chloride and water are produced:

• The net ionic equation of acid-alkali neutralisations, and what leads to a neutral solution, since water has a pH of 7, is:

H⁺  +  OH^– ⟶ H₂O

The pH scale

• The pH scale is a numerical scale which is used to show how acidic or alkaline a solution is
• It 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 with a pH of 7, such as water, is described as being neutral 

The pH scale showing acidity, neutrality and alkalinity

• 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
• Therefore 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

Universal indicator

• 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