Question

A residue of colour remain in the test tube​

Answer 1

Explanation:

तुम न आए तो क्या सहर न हुई

हाँ मगर चैन से बसर न हुई

मेरा नाला सुना ज़माने ने

एक तुम हो जिसे ख़बर न हुई

Answer 2

Answer:

So it is better to call dehydrated copper sulphate. It's colour changes to original colour of CuSO4CuSO4 that is white. It looks blue due to crystal formation with water molecules inside. When it looses them it looses the crystal hence turns into white.