Bouncing custard, and plastic milk, are two informative and entertaining experiments to make chemistry fun for year 9 learners

This session should take four hours and 45 minutes. 

Suggested timings

  • 10.00 Introduction and welcome lecture
  • 10.30 Plastic milk practical session
  • 11.30 Unknown polymers practical session
  • 12.15 Lunch break
  • 13.00 Introduction to afternoon session lecture
  • 13.30 Bouncing custard practical session
  • 14.00 Polymers and smart materials lecture
  • 14.45 Finish


Plastic milk 

  • Hot milk,150 cm3
  • Vinegar, 12 cm3
  • Beaker, 250 cm
  • Beaker, 400 cm
  • Measuring cylinder
  • Square piece of cloth eg sheeting
  • Rubber band
  • Stirring rod
  • Paper towels
  • Small magnet

Bouncing custard

  • 4% polyvinylalcohol (PVA) solution, 15 cm3
  • Measuring cylinder
  • Solid borax (sodium tetraborate Na2B4O7.10H20)
  • 4% borax solution in water
  • Custard powder or cornflour
  • Test tube and rack
  • Beaker, 100 cm3
  • Glass stirring rod
  • Spatula

Health, safety and technical notes

  • Read our standard health and safety guidance here
  • Lab coat or apron and safety glasses to be worn in the laboratory.
  • Students to avoid skin contact with chemicals, if contact occurs, wash off immediately with plenty of water.
  • Students should report any spillages and breakages to a demonstrator immediately


Plastic milk

  1. Take the 250 cm3 beaker and ask the demonstrator to measure out 150 cm3 of hot milk into this.
  2. Measure out 15 cm3 of vinegar using the measuring cylinder provided.
  3. Pour the vinegar into the beaker containing the hot milk.
  4. Immediately start stirring the milk/ vinegar mixture, using a stirring rod.
  5. Continue stirring for 2–3 mins, or until there appear to be no further changes.
  6. Take the 400 cm3 beaker.
  7. Place the piece of cloth over the beaker, and secure in place with the rubber band. Do not stretch the cloth too tight – try and make sure that there is a dip, as you are going to collect the casein here.
  8. Strain the mixture from the other beaker through the cloth.
  9. Squeeze as much liquid as you can from the mixture. You may need to remove the rubber band and then twist the cloth above the mixture, allowing the liquid to fall into the beaker.
  10. Place the solid on a paper towel and pat dry to remove as much of the remaining liquid as possible.
  11. The solid, casein, is an important ingredient in some plastics. Now that you have a sample of homemade plastic, you can use it to make a fridge magnet. Embed a small, powerful magnet (from your demonstrator) into the plastic. Your fridge magnet will be dried in an oven, ready for you to take away at the end of the day

Bouncy custard

  1. Pour 15 cm3 of PVA solution into the beaker.
  2. Add two spatulas of custard powder or cornflour and one spatula of dry borax.
  3. Add 0.5 cm3 of the borax solution and stir vigorously. Keep stirring until the mixture is smooth.
  4. Remove the mixture from the beaker, shape it into a ball and work it between your hands for about two mins. You should feel the ball gradually becoming more elastic.
  5. Test the ball to see how well it bounces.


Different manufactures use different plastics, check carefully prior to event. PVC and PET can both sink in solution six. These can be identified by IR using the ATR or by preparing thin film.