Acids & Alkalis (OCR Gateway GCSE 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 the ions present in solution
• The pH scale 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 of pH 7 is described as being neutral

The pH scale showing acidity, neutrality and alkalinity

• pH can be measured using an indicator or a digital pH meter
• pH meters 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
• 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
• 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
• 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

pH scale with the Universal Indicator colours used to determine the pH of a solution