2.2 Acids, bases and salts

• Acids

  • Acidic solutions release hydrogen ions, H+, in solution
  • Strong acid = completely dissociates to release H+ ions in aqueous solution
  • Hydrochloric, nitric, sulfuric acids
  • Weak acid = partially dissociates to release H+ ions in aqueous solution
  • Ethanoic, citric and carbonic acids
• Alkalis

  • Alkali solutions contain hydroxide ions, OH-
  • Strong bases fully dissociate to release OH- ions in aqueous solution
  • Sodium hydroxide, potassium hydroxide
  • Weak bases partially dissociate to release OH- ions in aqueous solution
  • Ammonia, ammonium hydroxide
• Describing acids and bases

  • Acids and bases can be referred to as strong/weak and dilute/concentrated.
  • Dilute/concentrated refers to the amount of substance present - to the number of moles of that acid/base in solution
  • Weak/strong refers to the degree of ionisation of the acid or base - how readily the acid releases H+ ions or how readily the base releases OH- ions
• Reaction of dilute acid with metals


  • Acid + metal --> salt + hydrogen
  • The reaction depends on the reactivity of the metal. The reactivity series shows the relative reactivities of different metals
• Neutralisation of dilute acids

  • Acid + alkali --> salt + water
  • Acid + base --> salt + water
  • Acid + metal carbonate --> salt + water + carbonate
• Preparing soluble salts

  Soluble salts can be made from acids reacting them with solid insoluble substances, such as metals, metal oxides, hydroxides or carbonates:

  1. Measure a set volume of your acid
  2. Heat the acid gently
  3. Add the chosen base in excess (until no more will dissolve). You know 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 (to make the solution more concentrated)
  6. Leave the rest to evaporate slowly so crystals of the salt form
• How to carry out a titration:

  1. Add acid to the burette using a funnel, record the start volume of the burette
  2. Add a known volume of alkali to a conical flask and add a few drops of indicator
  3. Place the conical flask on a white tile so you can see the colour change clearly
  4. Turn the tap of the burette to slowly add acid to alkali until you reach the neutralisation point when the indicator changes colour
  5. Calculate the volume of acid. This is called the titre
  6. Repeat until you get consistent titres - titres within cm³ of each other
• How to calculate the concentration of the alkali:

  • Calculate the number of moles using moles = concentration x volume
  • Calculate the mole ratio of acid to alkali using the balanced chemical equation for the reaction
  • Work out how many moles of alkali you have using the mole ratio and moles of acid
  • Calculate the concentration of the alkali using concentration = moles/volume
  • The units of concentration when calculated this way are given in mol/dm-3 
• Test for carbon dioxide gas

  Bubble gas through the limewater (calcium hydroxide solution) and it will turn milky (cloudy) if carbon dioxide is present
• Test for carbonates

  • Carbonates react with dilute acids to create carbon dioxide
  • This gas can be bubbled through limewater to see if carbon dioxide is present
• Test for sulfate ions

  • First add dilute hydrochloric acid, followed by barium chloride solution
  • A white precipitate will form if sulfate ions are present