We think of certain things as sticky – chewing gum, some kids of candy. And when you get something sticky on your skin, you often use water to get the sticky stuff off. But here’s a weird fact about water – it’s sticky, too! And if you think that’s weird, listen to this! You may think of water as see-through, but it really has a “skin.” Get ready to see why these things are true.
What You Need:
What To Do
1. Fill one glass with water and place the second glass on a flat, stable surface. Pour water from the first glass into the second until the second glass is filled to the very top.
2. Add one coin at a time to the glass of water by sliding the coin over the edge of the glass. Do not drop the coin into the water (okay I found this confusing at first I thought we were meant to balance the coin on the edge of the glass but what it meant was to slide the coin slowly into the water not just drop it in so the water splashed everywhere). How many coins do you think you can put into the glass before the water spills over the edge?
3. Keep adding coins one at a time. Each time you add a coin, look at the glass from the side. What does the water’s surface look like when you look at it from this angle?
4. When water begins to run down the outside of the glass, stop adding coins. Dump out all the water and count the coins. How many are in the glass.
We got 5 coins!
Don’t eat those coins Annie!!!
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| From March 2010 |
5. Repeat step 1, only this time add a tablespoon of salt to the water in the first glass before you pour it into the second. How many coins did you add before the water began to drip?
We got 3 coins this time.
What Happened?
When you added the first few coins, the water did not slop over the edge of the glass. Instead, the water formed a shallow dome shape above the rim of the glass. This is because the molecules that water is made up of are attracted to each other more than they are attracted to the molecules that make up the air or the glass. The parts that make up the top surface of the water form a film or “skin.” This thin film, or skin, lifts up to form a dome shape as each additional coin is added and the coins “stick” to the water. When enough coins are added, the film cannot hold all the water beneath, so it breaks and the water begins to drip down the side of the glass. When salt is added to the water, it makes the water less sticky. This means that fewer coins can be added before the water drips over.
What did we get out of this?
Annie realised she had to follow the instructions and be patient, she thought it was too slow and tedious sliding the coins in over the edge of the glass. But when she dropped a handful of coins in it made a big splash and we had to start again.
I found that I need to test out experiments by myself without children around first to make sure I’ve got a handle on how things work so that I can explain it properly to Annie. Although I think learning together also has its benefits but Annie was far too impatient to do the experiment and didn’t want to wait for mummy to figure things out.
Annie also found out that not all science experiments are full of big bang excitement (like when you mix vinegar with bicarb soda) but you can still find out some pretty neat stuff without the big reactions.
Follow up ideas
Draw a picture of the dome shape water above the top of the glass.
Experiment more with salt – using two glasses compare how something floats in regular water and how it floats in salty water.
Try colouring the water with food colouring to see if we can take some photos showing how the experiment works.
I was thinking of doing some Paper Mache because we could make paper mache paste using water and flour. But Annie loathes paper mache – I think it is a texture thing – and I’m not ready just yet to work through that issue, we’ve got enough on our plates as it is.
Water Glue Experiment From “Bathtub Science” by Shar Levine & Leslie Johnstone – we got this book at our local library book sale, $5 for a bag full of books, I was very excited 
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