Required Practical: Preparing a Salt (Crystallisation)

Purpose of the Practical

This practical is designed to prepare a pure salt using the method of crystallisation. The salt is formed by reacting an acid with a metal carbonate or metal oxide. The key aims are:

• To produce a pure salt by dissolving the reactants and removing any excess solid.
• To remove unreacted metal carbonate or oxide by filtration.
• To obtain dry crystals of the salt by allowing the solution to crystallise.

Note: This practical focuses on the preparation and purification of the salt, not on the chemical reactions involved in making salts (covered in other topics). For details on the chemical reactions, see the topic on Making Salts.

Procedure Steps

The method involves several key steps to ensure a pure salt is obtained:

1. Mix the acid with the metal carbonate or metal oxide: Add the solid slowly to the acid while stirring to ensure the reaction proceeds efficiently.
2. Heat gently: Warm the mixture gently, preferably using a water bath or a Bunsen burner on a low flame, to help the salt dissolve fully in the acid solution, forming a clear solution.
3. Filter the solution: Use filtration to remove any excess unreacted solid metal carbonate or oxide, leaving a clear salt solution.
4. Allow the solution to crystallise: Leave the filtered solution to cool and evaporate slowly so that salt crystals form.
5. Collect and dry the crystals: Once crystals have formed, remove them and dry to obtain pure, dry salt crystals.

This method ensures the salt is pure and free from excess reactants or impurities.

For instance, if you start with a saturated salt solution of known volume, you can calculate the concentration before crystallisation by dividing the mass of salt obtained by the volume of solution in dm³.

Observations and Results

During the practical, you will observe:

• Formation of crystals: As the solution cools and evaporates, salt crystals begin to form on the surface and at the bottom of the container.
• Appearance of salt crystals: The crystals are usually shiny and well-defined, often with a characteristic shape depending on the salt.
• Mass of crystals: After drying, the mass of the salt crystals can be measured to assess the yield.
• Record observations: Note the time taken for crystallisation, the appearance of crystals, and any other changes during the process.

These observations help confirm the success of the crystallisation and the purity of the salt.

Safety and Accuracy

Safety precautions and accuracy tips are essential for a successful and safe practical:

• Handle acids carefully: Acids can cause burns, so always wear safety goggles and gloves, and avoid skin contact.
• Use heat sources safely: Heat gently using a water bath or Bunsen burner on a low flame to avoid overheating or splashing.
• Avoid contamination: Use clean equipment to prevent impurities contaminating the salt crystals.
• Repeat for reliability: Carry out the practical multiple times to check for consistent results and improve accuracy.

Example: Preparing Copper(II) Sulfate Crystals

Suppose you want to prepare copper(II) sulfate crystals by reacting copper(II) oxide with sulfuric acid.

You add copper(II) oxide to sulfuric acid and heat gently until no more solid dissolves. Then you filter the mixture to remove excess copper(II) oxide. After leaving the filtrate to cool, blue crystals of copper(II) sulfate form as the solution evaporates.

This shows how the method produces pure crystals by removing unreacted solid and allowing crystallisation.