Question

how can we say that sugar is a pure substance where areas milk is not

Answer 1

Sugar is a pure substance because it has only one type of molecule and it cannot be separated into two or more components. milk is made up of more than one type of molecules like fat, vitamin etc. so we can separate components of milk by physical process.

Answer 2

Sugar is pure substance because it is made only with sugar particles but milk has fats protein water ECT. Therefore sugar is a homogeneous and milk is heterogeneous