Making soluble and insoluble salts, hydrated salts

Last time

Before we start: last time, in Neutralisation and oxides, you learned these points. Here they are again on one card. Keep them in mind, because today builds on them.

Blue crystals

Beautiful blue crystals of copper sulfate can be grown on a school bench, from black copper oxide powder and clear acid. A salt is what you get when an acid has its hydrogen replaced by a metal. The method you use to make it depends on one question: is the salt soluble or insoluble?

Solubility rules

All sodium, potassium and ammonium salts are soluble. All nitrates are soluble. Chlorides are soluble, except silver chloride and lead chloride. Sulfates are soluble, except barium sulfate, calcium sulfate and lead sulfate. Carbonates and hydroxides are insoluble, apart from those of sodium, potassium and ammonium.

Soluble salt, insoluble base

To make copper sulfate: warm dilute sulfuric acid in a beaker. Add copper oxide a little at a time, stirring. Stop when some black solid stays at the bottom. The solid is in excess, so all the acid has reacted. Filter off the excess copper oxide. The blue filtrate is copper sulfate solution. Heat the filtrate to evaporate some of the water, until crystals start to form at the edge of a glass rod. Then leave it to cool. Filter off the crystals, wash with a little cold distilled water, and dry them between filter paper.

Why excess?

The solid base is added in excess so that all the acid reacts. You can see when it is in excess. The excess is insoluble, so it is easy to remove by filtration. The filtrate has only the salt and water. The same method works with a metal, such as magnesium, or with a carbonate, such as zinc carbonate.