Which of the following statements are true for pure substances?
$(i)$ Pure substances contain only one kind of particles.
$(ii)$ Pure substances may be compounds or mixtures.
$(iii)$ Pure substances have the same composition throughout.
$(iv)$ Pure substances can be exemplified by all elements other than nickel.

Calculate the mass of sodium sulphate required to prepare its $20\%$ (mass percent) solution in $100\,g$ of water. (in $,g$)

Differentiate between an element and a compound (any two points). Give one example each.

Which of the following are not compounds?
$(a)$ Chlorine gas
$(b)$ Potassium chloride
$(c)$ Iron
$(d)$ Iron sulphide
$(e)$ Aluminium
$(f)$ Iodine
$(g)$ Carbon
$(h)$ Carbon monoxide
$(i)$ Sulphur powder

When is a compound said to be pure?

Give an example each for the mixture having the following characteristics. Suggest a suitable method to separate the components of these mixtures:
$(a)$ $A$ volatile and a non-volatile component.
$(b)$ Two volatile components with appreciable difference in boiling points.
$(c)$ Two immiscible liquids.
$(d)$ One of the components changes directly from solid to gaseous state.
$(e)$ Two or more coloured constituents soluble in some solvent.