The practical may be conducted in two parts as the fermentation process may take a long time. Some teachers may ask technicians to keep an eye in the fermentation process in between lessons. As long as the temperature is kept within the correct range and the yeast is fresh and ?in date' then there is not too much to go wrong in this practical.
Equipment required (per set)
The mixture can be left in a warm place or in a water bath at round 30°C. Read the recommended temperature for the yeast you use as these temperatures vary with yeast variety.
Yeast powder can be purchased in supermarkets (for break making) or brewing shops (used in the production of beer and wines) Brewer's yeast comes in a variety of types requiring differing temperatures to achieve a good ethanol yield. Fast acting yeast cuts down the fermentation time and is recommended.
Once the mixture has fermented, students may test the gas produced in the conical flask using limewater (see Fig 2) by pouring the gas from the flask into the boiling tube and shaking. The limewater should turn cloudy if there is enough carbon dioxide present.
A teacher may want to demonstrate the distillation of the fermentation product using a Liebig condenser. Firstly the product needs to be filtered and poured into the distillation flask. Ethanol boils at 78°C and so collecting the fraction between 77-82°C should produce some ethanol. This should be burned or disposed of as soon at is produced.
Yeast contains an enzyme called zymase and this catalyses the fermentation process, the formula of which is below:
Glucose zymase → Ethanol + carbon dioxide
C6H12O6 (aq) → 2C2H5OH(aq) + 2CO2(g)
The contents of this page are for information only. Please refer to CLEAPSS or ASE safety advice and/or publications before undertaking any preparation, practical experiment or using any equipment featured on this site or any other.