Acid & Insoluble Reactant

Specification Point 3.15:
  • Explain why, if soluble salts are prepared from an acid and an insoluble reactant:
    a) excess of the reactant is added
    b) the excess reactant is removed
    c) the solution remaining is only salt and water
  • During the preparation of soluble salts, the insoluble reactant is added in excess to ensure that all of the acid has reacted.
  • If this step is not completed, any unreacted acid would become dangerously concentrated during evaporation and crystallisation.
  • The excess reactant is then removed by filtration to ensure that only the salt and water remain.
  • Since all of the acid has reacted and the excess solid base has been removed then the solution left can only be salt and water.
  • If a carbonate was used as the solid base instead of an oxide or hydroxide, then any carbon dioxide gas produced would have been released into the atmosphere.

Acid & Soluble Reactant

Specification Point 3.16:
  • Explain why, if soluble salts are prepared from an acid and a soluble reactant:
    a) titration must be used
    b) the acid and the soluble reactants are then mixed in the correct proportions
    c) the solution remaining, after reaction, is only salt and water
  • During the preparation of soluble salts, the titration technique must be employed as both reactants are in the liquid state.
  • This means that if one of them is added in excess then it cannot be easily separated from the product, hence the volumes of reactants have to be added together in the correct proportions.
  • This is most easily done using a titration apparatus.
  • If done correctly then the exact amount of acid has been added to the alkali, thus the solution contains only salt and water.

Titration Apparatus, Edexcel GCSE Chemistry

Diagram showing the set-up for a titration

Core Practical: Preparing Copper Sulfate Crystals

Specification Point 3.17:
  • Investigate the preparation of pure, dry hydrated copper sulfate crystals starting from copper oxide including the use of a water bath.

Aim: 

  • To investigate the preparation of pure, dry hydrated copper sulfate crystals starting from copper oxide including the use of a water bath

Materials:

  • Concentrated sulfuric acid, copper oxide powder, goggles and lab coat
  • Measuring cylinder, beaker, glass rod, conical flask, spatula
  • Funnel and filter paper, Bunsen burner, Universal Indicator paper
  • Water bath

Hydrated crystals of CuSO4 are regular shaped and blue in colour

Method:

  • Add H2SO4 (100cm3) to a conical flask and warm in the water bath.
  • Add 1 spatula of CuO to the acid and stir with the glass rod.
  • When the reaction stops, add another spatula of CuO.
  • Repeat until the solid oxide is in excess.
  • Use U.I. paper to check that all of the acid has been neutralised.
  • Filter the mixture to remove the excess CuO.
  • Add the filtrate (CuSO4 solution) to an evaporating basin and heat gently to remove some of the water.
  • When the volume has been reduced to roughly half of its size, pour the solution into an evaporating dish.
  • Leave in a dry place so the remaining water evaporates, allowing crystallisation to occur.
  • This may take a few days depending on ambient conditions.

Analysis of results:

  • After a few days record the appearance of the CuSO4crystals to include a diagram and/or photograph.
  • In particular comment on their colour and shape.

Conclusion:

  • The excess solid is added to make sure all of the acid has reacted.
  • Filtering removes the unreacted solid, leaving only salt and water.
  • Evaporation of the water leaves the desired pure salt.

Edexcel GCSE Chemistry Notes

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Morgan Curtin Chemistry

Author: Morgan

Morgan’s passion for the Periodic Table begun on his 10th birthday when he received his first Chemistry set. After studying the subject at university he went on to become a fully fledged Chemistry teacher, and now works in an international school in Madrid! In his spare time he helps create our fantastic resources to help you ace your exams.