Acids & Bases

• Testing the pH of a solution means using an indicator
• the pH scale goes from 0 to 14
• The pH scale is a measure of how acidic or alkaline a solution is
• The lower the pH of a solution, the more acidic it is
• The higher the pH of a solution, the more alkaline it is
• A neutral substance (e.g. pure water) has pH 7
• An indicator is a dye that changes colour depending on whether it's above or below a certain pH
• Some indicators contain a mixture of dyes that means they gradually change colour over a broad range of pH. These are called wide range indicators and they're useful for estimating the pH of a solution
• universal indicator gives the colours shown below

  
• A pH probe attacehed to a pH meter can also be used to measure pH electronically. The probe is placed in the solution you are measuring and the pH is given on a digital display as a numerical value, meaning it's more accurate than an indicator

  
• Acids and bases neutralise each other
• An acid is a substance that forms aqueous solutions with a pH of less than 7
• Acids form H+ ions in water
• A base is a substance with a pH greater than 7
• An alkali is a base that dissolves in water to form a solution with a pH greater than 7
• Alkalis form OH- ions in water
• The reaction between acids and bases is called neutralisation:

  acid + base -> salt + water
• Neutralisation between acids and alkalis can be seen in terms of H+ and OH- ions:

  H+(aq) + OH-(aq) -> H2O(l)
• Hydrogen (H+) ions react with hydroxide (OH-) ions to produce water
• When an acid neutralises a base (or vice versa), the products are neutral, i.e. they have a pH of 7
• An indicator can be used to show that a neutralisation reaction is over
• Neutralisation reactions of strong acids and alkalis can be used to calculate the concentration of an acid or alkali by titration