Spooky Science – Shrunken Potato Heads

This cool science experiment for children helps them learn about osmosis. Great all year round, but especially as a Halloween themed science lesson!

Learning about osmosis is important for the study of biological systems, but can be a difficult concept to grasp. This fun science experiment adds a creative twist to a classic science experiment for children.

What is osmosis?

Osmosis is the passive movement of water molecules from an area of high water potential to an area of low water potential. Osmosis is very similar to diffusion, as it is a passive process, which does not require any energy input. Both diffusion and osmosis occur through a fairly random movement of molecules, but with a net (overall) movement from high to low concentration. In effect, the molecules ‘want’ to be evenly spread out until they reach an equal distribution, or equilibrium.

Using Potatoes to Demonstrate Osmosis

A classic experiment to demonstrate the osmosis can be carried out using cubes of cut raw potato:

  • Prepare two beakers - one containing water, and the other containing salt solution. The concentration of the salt solution doesn’t really matter, but the stronger the better – this can be used to reinforce the idea of saturated solutions if children are preparing the beakers themselves.
  • Cut a potato into 1cm cubes using a ruler and a knife. Make one cube for each beaker.
  • Place the 1cm cubes of potato into the beakers, and leave for about 30 minutes. During this time students can be writing up their method, and preparing a results table.
  • After 30 minutes remove the cubes of potato and measure them.

Students should find that the cube of potato which was immersed in water has increased in size, and that the cube that was immersed in salt solution has decreased in size. For a more accurate measurement the cubes could be weighed before and after, and the increase or decrease in mass calculated.

Osmosis Explained

The cells within a potato are made up mainly of water (60 to 80 % depending on the variety), but not entirely. This is the key to the experiment.

Let’s assume that our potato is 70% water. When we place it into pure (100%) water, there is an imbalance. The water potential outside the potato cells is higher than inside the cells. This means that the water molecules will move into the potato to try and balance things out – to achieve equilibrium. This makes the cube of potato swell up, making it bigger (or heavier) when measured.

The opposite is true when we place the potato into a high concentration salt solution. Lots of the water molecules in the salt solution are tied up with dissolving the salt, so the water potential outside the potato cells is lower than on the inside. This means that water will move from the potato into the solution to try to balance its concentration. This causes the cube of potato to shrink and lose mass.

Getting Creative and Turning Science Spooky!

Cubes of potato are all well and good, but they aren’t as much fun as this spooky variation of the experiment:

  • Provide each group of students with a potato and a selection of clay sculpting implements.
  • Cut the potato in half, and remove the skin.
  • Carve a face into each potato half.
  • Weigh the carved potatoes, and record their mass.
  • Immerse in water and salt solution (as above).
  • After 30 minutes re-weigh the potato faces.

The potato face which was in water should have increased in mass and swelled. The potato face which was in salt solution should have decreased in mass and shrivelled up to produce a spooky shrunken head effect.

Differentiation and Adaptability

This activity can be used as a fun project for younger children by using only salt solution to produce shrunken Halloween heads at home, and observing the effect.

For more able students calculations such as percentage change in mass can be incorporated into the lesson.