What is meant by a pure substance?

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.

Smoke and fog both are aerosols. In what way are they different?

On heating,calcium carbonate gets converted into calcium oxide and carbon dioxide.
$(a)$ Is this a physical or a chemical change?
$(b)$ Can you prepare one acidic and one basic solution by using the products formed in the above process? If so,write the chemical equations involved.

Give an example for each of the following:
$(a)$ Solid-liquid homogeneous mixture
$(b)$ Gas-gas homogeneous mixture
$(c)$ Solid-solid homogeneous mixture
$(d)$ Solid-liquid heterogeneous mixture
$(e)$ Liquid-liquid heterogeneous mixture

Crystallisation is a better technique than simple evaporation. Give one reason to justify the statement.