Question

how can we say suger is pure but milk is not

Answer 1

sugar is pure because it is made of only sugar particles whereeas milk is a mixture of different substances

Answer 2

sugar is pure substance but not milk because sugar is made up of a single molecule. it can not be divides into two types of molecules by any physical process. and milk is not a pure substance because it is made up of more than two molecules like water molecules, fat molecules, protein molecules and etc.