Yesterday I gave the girls a challenge. I gave them a bowl of water (with blue food coloring for better visibility), an ice cube, a piece of string and some salt and challenged them to pick up the ice cube with the string.
Rules are you can’t pick the ice cube up with your hands and you can’t tie a knot in the string. It’s possible, but you have to know a bit of science to figure it out.
The girls tried dangling the string in the water, scooping up the ice with it and twirling it around. Charlie got close by laying the string over the ice, but she wasn’t quite sure how to proceed. It was fun watching the look of concentration on the girls faces.
If you want, get your supplies out and try it yourself. See if you can do it before you read on. We’ll wait, I promise.
We finally showed them the trick to picking up the ice cube and I’ll share it with you too.
First, lay the string over the ice cube.
Next sprinkle a little salt over the string onto the ice.
Wait 30 seconds to a minute. Waiting is the hardest part.
Finally, pick up your ice cube with a single piece of string!
Ta da! Challenge completed successfully.
So, how does this work? Why does the string stick to the ice when salt is added?
Salt lowers the melting point of the ice and lowers the freezing point of the water. This means the salt melted the ice a bit and then as the water cooled it refroze over the string allowing it to stick to the ice cube.
Were you able to pick up the ice cube without reading the trick? Share your experience in the comments!