Question

how would you confirm that a colourless liquid given to you is a pure substance

Answer 1

A very easy solution is to take a sample of colourless liquid and put on stove if it starts boiling exactly at 100 ºC then it is pure water. Any other colourless liquid such as vinegar always have different boiling point.

Answer 2

Pure water is obtained by fractional distillation. It has only water molecules. By this we conclude purity of water.