OCR Gateway GCSE Chemistry

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3.3.3 Acids & Alkalis

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Acids & Alkalis

  • 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

  • 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, IGCSE & GCSE Chemistry revision notes

The pH scale showing acidity, neutrality and alkalinity

Measuring pH

  • 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

Universal-indicator-and-the-pH-scale2, IGCSE & GCSE Chemistry revision notes

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

Exam Tip

A common error is to suggest using Universal Indicator as a suitable indicator for an acid-base titration. This is incorrect as a sharp colour change is required to identify the end-point, which cannot be achieved with Universal Indicator

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