When is a compound said to be pure?

Write your observations when the following processes take place:
$(a)$ An aqueous solution of sugar is heated to dryness.
$(b)$ $A$ saturated solution of potassium chloride prepared at $60^\circ C$ is allowed to cool at room temperature.
$(c)$ $A$ mixture of iron filings and sulphur powder is heated strongly.
$(d)$ $A$ beam of light is passed through a colloidal solution.
$(e)$ $Dil. HCl$ is added to the mixture of iron and sulphur.

State whether the following statement is true or false:
Non-metals are lustrous and good conductors of heat and electricity.

What is meant by crystallisation? How is impure copper sulphate purified by the process?

Classify each of the following as an element,compound,or mixture:
Gold,Air,Marble,Milk,Sugar.

Give an example each of a mixture that shows the following characteristics:
$(i)$ $A$ mixture of two coloured components.
$(ii)$ $A$ mixture of two volatile components with a boiling point difference of less than $25\ K$.
$(iii)$ $A$ mixture of a volatile and a non-volatile solid component.