Fermentation is used to make ethanol from carbohydrates such as sugar or glucose. The carbohydrate is dissolved in water to make an aqueous solution and yeast is added. Yeast is a living microorganism, and it provides enzymes that act as catalysts for the reaction.
The equation is C6H12O6 → 2C2H5OH + 2CO2. One molecule of glucose makes two molecules of ethanol and two molecules of carbon dioxide. The carbon dioxide gas bubbles out, and it turns limewater cloudy.
The mixture is kept warm, at about 30 to 40 °C. If it is too cold the enzymes work slowly. If it is too hot the enzymes are denatured, which means their shape changes and they stop working. Air is kept out, so the conditions are anaerobic. Once the ethanol concentration reaches about 15 percent the yeast is killed, so fermentation cannot make a stronger solution than this.
