To Prepare and Show the Production of Alcohol by Yeast

4.1 (a) To conduct a qualitative test for starch

Teacher Notes

Apparatus required per class group of 24 students

Boiling tubes 24

Iodine solution

Starch solution 24 ml

Pipettes/droppers 24

Labels/marker pens 12

Advance chemical preparation (a) 1% w/v Starch solution o Shake 1 g of soluble starch (Analar) on to 100 ml of water and bring to boiling point to obtain a clear solution. Cool. o Make up to100 ml of water and stir.

(b) Iodine solution o Dissolve 2 g potassium iodide crystals in 25 ml of water. o Add 1 g of iodine crystals. Make the solution up to 100 ml.

Safety precautions o Iodine is harmful if in contact with skin and by inhalation. Avoid eye contact. For further information see MSDS.

Expected outcome of experiment o A positive result for starch shows a blue/black colour. o A negative result for starch shows an orange/red colour (the colour of the iodine).

1 Disposal and post-experiment work o Iodine – add a small quantity (50g) in 250 ml of 1 M sodium thiolsulphate (25%). When colourless, flush to foul water drain.

4.1 (a) To conduct a qualitative test for starch

2 Student Notes

Apparatus Required per Group o Starch solution (1%) o Iodine solution o Pipettes/droppers o 2 Boiling tubes o Test-tube racks o Labels/ marker pen Assembled apparatus

Method 1. Label two boiling tubes ‘A’ and ‘B’. 3 2. Place 2 ml of the starch solution into tube A. 3. Place 2 ml of water into tube B. This acts as the control. 4. Add 2–3 drops of iodine solution into each tube. 5. Swirl each tube. 6. Observe any colour change and record result. 7. At the end of the experiment, clean all of the equipment and replace it in its correct place.

Results: Sample Initial colour Final colour A – Starch solution B – Water

Conclusion/Comment