Question

how can we say that sugar is a pure substance and milk is an impure

Answer 1

Sugar is a pure substance, because it has only one type of molecule. It cannot be separated into two or more components. Milk is a mixture as u can separate components of milk by physical processes.

Answer 2

Bcoz sugar is made up of only it's constituents and it's only crystals but milk is a mixture of many vitamins proteins or more