Sophie Duncan explores the tension at the surface
Everyone loves experimenting with water. This week's science corner should give your students a chance to find out more about the properties of water without getting too wet!
Take a large bowl of water and two similar small glasses. Put the glasses into the bowl so that they are full of water. Draw them together so that their openings meet, then move them so that one glass is on top of the other, the rims still together. Now carefully remove the two glasses from the bowl and place them on the table.
Ask your students what would happen if you were to take a coin and slide it between the two glasses.
Once they have had a chance to think about it, try the experiment. You should be able to do it quite easily. The surface tension of the water holds the water in position so that it does not run out of the glass.
Split your students into groups and give each of them a beaker full of water. Ask them to work out how many coins they can add to the beaker before it overflows.
They can then experiment by carefully sliding coins into the water, down the side of the glass.
Make sure that they look at the top of the water in the glass as they add the coins.
They should find they can add quite a few coins, and that the surface of the water bows upwards above the sides of the glass. This, too, is because of the surface tension of the water.
Finally, take a glass of water, and add an ice cube. Make sure the glass is full.
Ask your students to observe the ice cube, and to work out what will happen when it melts.
Observe the glass over time, until the ice cube has melted. You should find that the cup does not overflow.
The ice is less dense than the water - your students may have figured this out, as the ice cube floated.
When it melts, it occupies less volume than when it was ice.