This practical involves separating milk into curds and whey using vinegar and then neutralising the curds with a variety of bases to make several different glues.
The glue takes time to dry and so this practical may take place over several lessons.
Technicians may need to wash up any beakers and stirring rods used as soon as the lesson has finished because the glue can be surprisingly sticky and may prove difficult to remove once dried.
Equipment required (per set):
Students mix around 100-150ml of milk with 20-25ml of vinegar in a beaker and heat it gently. The mixture should be stirred constantly until lumps form. Vinegar separates the curds from the whey. This mixture should be filtered and the solid part (curds) kept (see Fig 1).
- bunsen burner
- heatproof mat
- 250ml beakers (2)
- 100ml measuring cylinders (2)
- filter funnel
- filter paper
- UI paper strips
- stirring rod
- milk (approx 150ml)
- colourless vinegar (small amount)
- ice cream sticks or similar to stick together
- bases (milk of magnesia, magnesium carbonate, calcium carbonate, sodium hydrogen carbonate - all low hazard)
The curds are transferred to another beaker and 15-20ml of water is added. It is mixed until smooth. The base should now be added. The mixture should be pH neutral so more base may need to be added to get this correct.
Different bases can be used each time to create a new type of glue, some work better than others.
Students may test how good the glues are by sticking two ice cream sticks together, leaving to dry and then separating.
Students could experiment with different types of milk also. Skimmed milk works particularly well, the more fat in the milk, the less effective the glue tends to be.