Acids & Bases (Edexcel GCSE Chemistry: Combined Science)

• 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 the ions present in solution

• The pH scale goes from 0 – 14 (extremely acidic substances can have values of below 0)
• 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 of pH 7 is described as being neutral

The pH scale showing acidity, neutrality and alkalinity

• Two colour indicators are used to distinguish between acids and alkalis
• Many plants contain substances that can act as indicators and the most common one is litmus which is extracted from lichens
• Synthetic indicators are organic compounds that are sensitive to changes in acidity and appear different colours in acids and alkalis
• Phenolphthalein and methyl orange are synthetic indicators frequently used in acid-alkali titrations

Two Colour Indicators Table

• Synthetic indicators are used to show the endpoint in titrations as they have a very sharp change of colour when an acid has been neutralised by an alkali and vice-versa
• Litmus is not suitable for titrations as the colour change is not sharp and it goes through a purple transition colour in neutral solutions making it difficult to determine an endpoint
• Litmus is very useful as an an indicator paper and comes in red and blue versions, for dipping into solutions or testing gases