Magic Tea Party

What you need

  • One packet of sodium polyacrylate - you can buy it in garden centres as 'Water Retaining Gel'.
  • Two cups (see-through if you'd like to see how this works, opaque if you want the trick version). Make sure you can dispose of the cups afterwards.
  • Some water.
  • A tablespoon.

What you do

  1. Put a tablespoon of the powder in the bottom of one cup.
  2. Pour some water into each of the cups.
  3. Swish the water around in each cup and turn them upside down (you might want to do this over a sink because at least one of your cups will spill).
  4. Poke and prod the gel in the bottom of the cup with a spoon.

What's going on?

The water retaining gel is sodium polyacrylate. It is a organic chemical, a polymer made up of a long chain of carbon, hydrogen, oxygen and sodium atoms.

The polymer looks like this &

CH2--CH(CO2Na)--

... over and over again. Have a look at this link to see it drawn out.

On its own this polymer is wound up tightly. When water is added the sodium breaks off, dissolving in the same way that salt (sodium chloride) dissolves in water. The sodium is a positively charged ion, and what remains, the carboxyl ion (attached to the rest of the polymer), is negatively charged. Because all the negatively charged carboxyl ions on the polymer repel each other, the whole polymer starts to unwind.

In a water molecule (H20) the oxygen atom attracts the two hydrogen atoms' electrons, leaving the hydrogen atoms slightly positive and the oxygen atom slightly negative: this is the water molecule's polarity. The oxygen side of the water molecules is attracted to the positive sodium ions from the sodium polyacrylate polymer. At the same time, the hydrogen atoms in the water molecules are attracted to the carboxyl groups of the polymer. The polymer has some cross links - so instead of a perfectly long line, the polymer forms a spread out net with water molecules held within it.

Adding other ions to the mix, such as salt, disrupts how the water is held in the gel. Adding salt to the gel turns it back into a watery liquid.

Special safety advice

Do not try and drink the liquid / gel, and don't use the cups after performing the experiment. Do not dispose of the gel down a sink as it may cause a blockage.