C2.6 Acids, Bases And Salts

C2.6 Acids, Bases And Salts

1. C2.6.1 Making Salts

   • The state symbols in equations are (s), ( l ), (g) and (aq).
     • Soluble salts can be made from acids by reacting them with: metals – not all are suitable; insoluble bases – the base is added to the acid until no more will react and the excess solid is filtered off; alkalis – an indicator can be used to show when the acid and alkali have completely reacted to produce a salt solution.
       • Salt solutions can be crystallised to produce solid salts.
         • Insoluble salts can be made by mixing appropriate solutions of ions so that a precipitate is formed, from the reacion of two soluble salts. Precipitation can be used to remove unwanted ions from solutions, for example in treating water for drinking or in treating effluent.

2. C2.6.2 Acids And Bases

   • Substances with a pH of less than 7 are acids. The more strongly acidic the solution, the lower its pH number. Acidic solutions turn blue litmus paper red. They turn universal indicator paper red if they are strongly acidic, and orange or yellow if they are weakly acidic.
     • Substances that can react with acids and neutralise them to make a salt and water are called bases. They are usually metal oxides or metal hydroxides. Bases which dissolve in water are also called alkalis.
       • Alkaline solutions have a pH of more than 7. The stronger the alkali, the higher the pH number. Alkalis turn red litmus paper blue. They turn universal indicator paper dark blue or purple if they are strongly alkaline, and blue-green if they are weakly alkaline.
         • Neutral solutions have a pH of 7. They do not change the colour of litmus paper, but they turn universal indicator paper green. Water is neutral.
           • The particular salt produced in any reaction between an acid and a base or alkali depends on: the acid used and the metal in the base or alkali.
             • Ammonia dissolves in water to produce an alkaline solution. It is used to produce ammonium salts. Ammonium salts are important as fertilisers.
               • Hydrogen ions, H+(aq), make solutions acidic and hydroxide ions, OH–(aq), make solutions alkaline. The pH scale is a measure of the acidity or alkalinity of a solution.
                 • In neutralisation reactions, hydrogen ions react with hydroxide ions to produce water. This reaction can be represented by the equation: H+(aq)+OH−(aq)→H2O(I)