Making a soluble salt: the practical method
Chemistry · Key Stage 3 · The School
Starting with excess solid
To make a salt from an acid and an insoluble base or metal, you add the solid to the acid a little at a time, in EXCESS — more than is needed to react with all the acid. This guarantees that every bit of acid reacts. Adding too little solid would leave acid behind in the final product, contaminating it.
Filtering off the leftover
Once the reaction is complete, some solid remains unreacted, sitting at the bottom, because there was deliberately more than needed. FILTER the mixture. The leftover solid stays in the filter paper. The liquid that passes through — the FILTRATE — is a pure solution of just the salt dissolved in water.
Evaporating slowly to grow crystals
Heat the filtrate gently to evaporate some of the water, then leave the rest to evaporate slowly at room temperature. SLOW evaporation is essential: it gives the salt time to form large, well-shaped crystals. Evaporating too fast, all at once, gives a fine powder instead of proper crystals.
Add copper oxide, a little at a time, to warm dilute sulfuric acid, stirring until no more dissolves — that excess proves all the acid has reacted. Filter the mixture: black copper oxide powder stays in the paper, and a clear blue filtrate of copper sulfate solution passes through. Warm the filtrate gently to reduce its volume, then leave it undisturbed for several days. Large blue crystals of copper sulfate slowly appear as the remaining water evaporates. Three steps, in that exact order: excess to guarantee no leftover acid, filtering to remove the excess solid, and slow evaporation to grow real crystals rather than a powder.