Acids, Bases and Salts

• Acidic solution


  A solution with pH less than 7
• Alkaline solution


  • A solution with pH more than 7
  • An alkali is a base dissolved in water
• The lower the pH, the stronger the acid
• The higher the pH, the stronger the alkali
• The pH of a neutral solution is 7
• Acids


  Contain hydrogen ions, H+
• Alkalis


  Contain hydroxide ions, OH-
• Strong acid


  An acid which completely dissociates to release H+ ions in an aqueous solution
• Weak acid


  An acid which partially dissociates to release H+ ions in an aqueous solution
• Strong base


  A base which completely dissociates to release OH- ions in an aqueous solution
• Weak base


  A base which partially dissociates to release OH- ions in an aqueous solution
• Product formed when a metal is added to dilute acid

  Acid + Metal -> Salt + Hydrogen
• The more reactive the metal in the reactivity series, the faster the reaction
• Neutralisation reactions of dilute acids

  • Acid + Alkali -> Salt + Water
  • Acid + Base -> Salt + Water
  • Acid + Metal carbonate -> Salt + Water + Carbon dioxide
• The composition of salt formed depends on the acid used and the positive ions in the base, alkali or carbonate
• Neutralisation


  The reaction of hydrogen ions, H+, with hydroxide ions, OH-, to form water, H2O
• Test for the presence of carbonates

  1. Carbonates react with dilute acids to form carbon acid

  2. The gas formed can be bubbled through limewater; if limewater turns cloudy, the gas is CO2
• Preparation of crystals of soluble salts

  1. Measure a set volume of acid

  2. Heat the acid gently

  3. Add the chosen base in excess, the acid has been neutralised when excess solid sinks to the bottom

  4. Filter the excess base using filter paper and a funnel

  5. Heat the salt solution to evaporate the water

  6. Leave the rest to evaporate slowly so crystals of the salt form
• Soluble salts


  Can be made from acids by reacting them with insoluble substances, such as metals, metal oxides, hydroxides or carbonates
• Salts formed from different acids

  • Hydrochloric acid produces chlorides
  • Nitric acid produces nitrates
  • Sulfuric acid produces sulfates
• Test for sulfate ions

  1. First add dilute hydrochloric acid, followed by barium chloride solution

  2. A white precipitate will form if sulfate ions are present in the solution
• Titration


  • A method to prepare solutions of soluble salts
  • To determine the relative and actual concentrations of solutions of acids/alkalis
• Concentration of a solution

  • Concentration = mol ÷ volume
  • The units of concentration are mol dm-3
• Concentration of an acid/base

  The amount of substance present - the number of moles of that acid/base in solution
• Strength of an acid/base

  The degree of ionisation of the acid or base - how readily the acid releases H+ ions or how readily the base releases OH- ions
• Preparation of insoluble salts

  1. Insoluble salts can be formed in a precipitation reaction

  2. React 2 solutions which contain the ions of the desired salt

  3. A solid precipitate of your salt is produced

  4. Filter the salt out using a filter paper and funnel

  5. Wash the salt with distilled water and leave it to dry on the filter paper