🧂 Topic 4: Preparation of a Soluble Salt (RP8)

What's the goal of this practical?

Make pure, dry crystals of a soluble salt (e.g., copper sulfate) from an insoluble base (copper oxide) and acid (sulfuric acid). Must be uncontaminated.

Why add EXCESS copper oxide?

Ensures all acid reacts completely. No acid left in final product means salt is pure. You can tell when excess remains because it stops dissolving.

What are the full steps in order?

1) Add excess copper oxide to warm sulfuric acid until no more dissolves. 2) Filter to remove excess solid. 3) Evaporate filtrate using water bath until crystals form. 4) Dry crystals with filter paper.

Why filter the mixture?

Removes unreacted copper oxide. Only want dissolved copper sulfate in solution. Filtrate = pure salt solution, residue = excess base.

Why use a water bath for evaporation, not direct heat?

Prevents spitting and violent boiling. Allows slow, controlled evaporation so larger, purer crystals form. Direct heat causes small, impure crystals and may decompose the salt.

What are the variables?

Independent = mass/volume of acid used. Dependent = mass of dry crystals produced. Control = type of acid, type of base, temperature, concentration.

How to improve accuracy and purity?

Ensure complete drying (weigh repeatedly until mass constant). Filter carefully to remove all solid. Allow slow crystallization. Calculate percentage yield to check efficiency.

Safety precautions for making salts?

Wear goggles (acid is corrosive). Use tongs for hot equipment. Add solid to acid slowly (exothermic reaction). Work in ventilated area.

What's the word equation?

Copper oxide + Sulfuric acid → Copper sulfate + Water. This is a neutralization reaction between a base and acid.