Osmotic Pressure Experiment

What You Need:

• Cherries (ripe Bing cherries work best)
• Glass bowl, water
• Paper, pencil
• Plastic lid to cover bowl

What You Do:

1. Have your child make a hypothesis of what she thinks might happen to cherries if they are left overnight in a bowl of water. She might think they would shrivel and wrinkle, for example, or she might hypothesize that water will not affect the cherries. Making a hypothesis is an important part of the scientific method.
2. Have your child draw “before” pictures of cherries so she can compare them to what the cherries will look like after being soaked in water.
3. Have your child place 2 or 3 ripe cherries in the glass bowl and fill the bowl with tap water. Leave the cherries in the bowl for about 8 hours (overnight, for example). Place the plastic lid on top of the bowl, which will add more humidity for the cherries.
4. After the time has passed, look at the cherries. What changes does she notice? She’ll probably see some split seams and cracks in the cherries, which have “burst” a bit after soaking in water.
5. Next, have her draw an “after” picture of the cherries. Recording the results of an experiment is also an important part of the scientific method.
6. Explain to your child why the cherries burst: the cherry soaked up water through tiny pores. This is called “osmosis” and is the way that plants absorb water, but water cannot get out through the same tiny pores it came in from. The water builds up inside the cherries, and the pressure it created, called “osmotic pressure,” caused the cherries to burst open. Farmers have observed this effect in cherries even in rain or high humidity!