Glucose is prepared in the laboratory by boiling sucrose (cane sugar) with dilute hydrochloric acid or sulphuric acid for about two hours. This hydrolyzes  sucrose to glucose and fructose. In order to separate glucose from fructose, alcohol is added during cooling. Glucose is almost insoluble in alcohol, hence it will crystallize first, whereas fructose is more soluble, it remains in the solution. The solution is filtered and crystals of glucose are obtained. Hence, alcohol is added to separate the product effectively.