Nappy Soak

Dobbers might wear nappies but scientists can use them to experiment with polymers!

Australian Curriculum links

  • Science > Chemical Sciences > Year 4 > ACSSU074
  • Science > Science Inquiry Skills > Year 4 > ACSIS064
  • Science > Chemical Sciences > Year 7 > ACSSU113

You'll need

  • Scissors
  • 3 disposable nappies
  • 3 x 500mL beakers or plastic containers
  • 3 x 1 litre containers
  • Salt
  • Water

Try this

  1. Using the scissors, cut open the inner lining of each of the three nappies exposing the fibrous, absorbent pad.
  2. Remove the entire absorbent pad from each nappy and place one pad in each beaker. Label the beakers ‘A’, ‘B’ and ‘Fresh Water’.
  3. In a 1 litre container, dissolve salt in 1 litre of water until no more salt will dissolve. Label this ‘Solution A’.
  4. Pour 400mL of the concentrated salt solution into another 1 litre container. Add 600mL of water to dilute this solution. Label this ‘Solution B’.
  5. Fill the last 1 litre container with 1 litre of fresh water. Label this ‘Fresh Water’.
  6. Slowly pour small amounts of ‘Solution A’ into beaker ‘A’, waiting until each addition has been absorbed before adding more of the solution.
  7. Continue until no more of the solution will be absorbed. Observe how much of the solution could be absorbed.
  8. Repeat Step 6 for ‘Solution B’.
  9. Repeat Step 6 for ‘Fresh Water’.
  10. Determine whether ‘Solution A’, ‘Solution B’ or ‘Fresh Water’ was best absorbed by the absorbent pads.

Further investigation

Design an experiment to test how well plants grow with and without the help of water storage crystals in the soil. How could you test how much you need to water a plant to keep it alive, with and without water storage crystals in the soil? You will need a number of plants of the same size and type, water storage crystals and a way of measuring the amount of water you give each plant.

What's happening?

The main substance responsible for the super absorbent properties of disposable nappies is a chemical called sodium polyacrylate.
Sodium polyacrylate is a polymer (meaning that it is a long-chain molecule made up of many smaller molecules joined together) and it’s really good at absorbing water. It can absorb up to 800 times its weight in distilled water!

This amazing polymer absorbs water through the process of osmosis. Water molecules are drawn into the sodium polyacrylate until the concentrations of sodium ions inside and outside the polymer are equal.

The nappies in this activity absorbed a lot more water from the ‘Fresh Water’ container than from either of the two salt water solutions. Because of the high initial concentrations of sodium ions in solutions A and B, the sodium polyacrylate could only absorb a limited amount of water before the concentrations of sodium ions inside and outside the polymer became equal.

The ability of sodium polyacrylate to absorb water depends in the concentration of dissolved salts in water. This is important in determining how well a nappy can absorb urine because urine contains salts.

Real world links

Super absorbent gels, similar to those used in nappies, can be used to protect property during bush fires. If the water-soaked gel is sprayed over a building, the gel provides a protective layer that prevents the heat of the bushfire from damaging the building. Thick layers of the gel can be built up, providing a great deal of protection due to the fact that it takes a lot of energy to increase the temperature of the water in the gel.

Sodium polyacrylate in the form of water storage crystals can also be used to protect plants against drought by storing water in the soil.