What it lacks in explosive power, copper sulphate more than makes up for in its looks – creating brilliant blue crystals when its hydrated form is dissolved in hot water.
Copper sulphate is a type of salt, and is most commonly encountered as a powder – copper sulphate pentahydrate (CuSO4*5H2O). This is a way of expressing five water molecules are attached to the copper sulphate molecule; it is hydrated.
For blue crystals to form, copper sulphate pentahydrate is added to hot water up until the point where no more can dissolve. This is referred to as a saturated solution, and a hotter solution can dissolve more copper sulphate than a colder one.
When the solution starts to cool, some of the copper sulphate can no longer exist in a dissolved state, so the molecules gather in an organised repeating pattern, forming crystals. This is an example of a physical change since the material is altering its structure rather than its makeup.
Suspending a nylon wire in the solution creates a surface for the crystals to latch on to, encouraging growth. Eventually the water evaporates, but copper sulphate can’t so it’s forced into an ever-smaller space. The molecules of copper sulphate continue crystallising until no water is left.