Identify which of the following are elements,mixtures,or compounds:
$(i)$ Brass $(ii)$ Diamond $(iii)$ Sand.

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

What would you observe when:
$(a)$ a saturated solution of potassium chloride prepared at $60^{\circ} C$ is allowed to cool at room temperature?
$(b)$ an aqueous sugar solution is heated to dryness?
$(c)$ a mixture of iron filings and sulphur powder is heated strongly?

When is a compound said to be pure?

$A$ group of students took an old shoe box and covered it with black paper from all sides. They fixed a source of light (a torch) at one end of the box by making a hole in it and made another hole on the other side to view the light. They placed a milk sample contained in a beaker/tumbler in the box as shown in the figure. They were amazed to see that the milk taken in the tumbler was illuminated. They tried the same activity by taking a salt solution but found that light simply passed through it.
$(a)$ Explain why the milk sample was illuminated. Name the phenomenon involved.
$(b)$ Same results were not observed with a salt solution. Explain.
$(c)$ Can you suggest two more solutions which would show the same effect as shown by the milk solution?

The concentration of a salt solution in terms of mass by mass percentage is $20 \%$ and the mass of the solution is $550 \text{ g}$. Determine the mass of solute present in the solution. (in $\text{ g}$)