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

Compare physical and chemical changes.

Two substances,$A$ and $B$,were made to react to form a third substance,$A_2B$,according to the following reaction: $2A + B \to A_2B$. Which of the following statements concerning this reaction are incorrect?
$(i)$ The product $A_2B$ shows the properties of substances $A$ and $B$.
$(ii)$ The product will always have a fixed composition.
$(iii)$ The product so formed cannot be classified as a compound.
$(iv)$ The product so formed is an element.

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

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

How can we obtain pure copper sulphate from an impure sample?