Required Practical: Making a Salt

Purpose of Practical

This practical is designed to prepare a pure sample of a salt and to help you understand how salts are formed through acid-base reactions. It involves reacting an acid with a base to produce a salt and water, demonstrating neutralisation in action. The focus is on producing a solid salt that can be isolated and purified by crystallisation.

Materials and Setup

• Acid (e.g. hydrochloric acid, sulfuric acid, or nitric acid)
• Base (usually an insoluble base like copper oxide or a carbonate such as sodium carbonate)
• Heat source (Bunsen burner or electric heater)
• Conical flask or beaker for reaction
• Glass stirring rod
• Filter funnel and filter paper
• Crystallisation dish (evaporating basin)
• Tripod and gauze (if using a Bunsen burner)
• Thermometer (optional, to monitor temperature)

Procedure Steps

1. Add the acid to a beaker or conical flask.
2. Gradually add the base (e.g. copper oxide) to the acid while stirring continuously.
3. Continue adding the base until no more reacts, indicated by excess solid remaining or the solution stops fizzing (if carbonate is used).
4. Filter the mixture to remove any unreacted solid base, leaving a clear salt solution.
5. Pour the filtrate into a crystallisation dish.
6. Heat the solution gently to evaporate some water, concentrating the salt solution but not boiling it dry.
7. Allow the concentrated solution to cool slowly at room temperature to form crystals.
8. Once crystals have formed, filter them out and leave to dry.

This method produces pure crystals of the salt formed from the acid-base reaction.

Observations and Results

• During the reaction, the acid reacts with the base to form a clear solution if the base dissolves or a suspension if it is insoluble.
• Excess base remains as a solid if the acid is completely neutralised.
• After filtration, the solution is clear and free from solid impurities.
• On heating, water evaporates and the solution becomes more concentrated.
• As the solution cools, salt crystals begin to form and grow in the crystallisation dish.
• The mass of the dry crystals can be measured to determine the yield of salt.

Safety and Accuracy

• Handle acids carefully to avoid skin and eye irritation; wear safety goggles and gloves.
• Use heat sources safely; never leave a flame unattended and use tongs or heatproof mats for hot equipment.
• Ensure the acid is completely neutralised by the base to avoid contamination of the salt.
• Filter carefully to avoid losing solution or crystals.
• Avoid contamination by using clean apparatus and avoiding mixing different chemicals accidentally.

Understanding the Process

The salt forms when the hydrogen ions (H⁺) from the acid react with the hydroxide ions (OH⁻) from the base, producing water and the salt. The salt remains dissolved in the solution until water is evaporated, allowing the salt to crystallise out as solid crystals.

For example, reacting hydrochloric acid with copper oxide produces copper chloride salt and water:

CuO + 2HCl → CuCl₂ + H₂O

Copper oxide is insoluble, so excess solid remains when the acid is neutralised. Filtering removes this excess solid, leaving a pure salt solution.

Heating evaporates water, concentrating the solution until crystals form on cooling.

For instance, if 50 cm³ of hydrochloric acid reacts with copper oxide until neutralised, and after crystallisation 5.0 g of copper chloride crystals are obtained, this shows successful salt preparation.

Example calculation: If 0.1 mol of acid reacts completely, the theoretical mass of salt can be calculated using the molar mass of copper chloride. This helps estimate the expected yield.