Copper sulphate is hygroscopic, meaning it absorbs water from the surrounding air. When left in a dish, it will slowly absorb water and become a liquid. The liquid is a deep blue colour, and it is known as copper sulphate solution. Copper sulphate solution is a corrosive liquid, and it can cause skin irritation and burns. It is important to wear gloves and eye protection when handling copper sulphate solution.

In addition to absorbing water, copper sulphate can also react with other chemicals in the air. For example, it can react with carbon dioxide to form copper carbonate, which is a green solid. Copper carbonate is also a corrosive substance, and it can cause skin irritation and burns.

If copper sulphate is left in a dish for a long period of time, it will eventually evaporate and leave behind a solid residue. The residue is a mixture of copper sulphate and copper carbonate. The solid residue is not as corrosive as the liquid copper sulphate solution, but it can still cause skin irritation and burns.