Freezing point: An icy pick-me-up

Here’s a challenge: Can you pick up an ice cube floating in a glass of water without touching it, using just a piece of string? To help you out, we’ll also let you use some salt.

Maybe you tried slipping the string underneath the ice. Or maybe you tied a lasso and threw it at the cube, all the time wondering what the salt was for. Tricky, isn’t it? 

Here’s how to do it: Lay one end of the string across top of the cube. Then sprinkle two or three pinches of salt on top of the string. Then wait a couple of minutes.

If all goes well, the string will sink into the ice cube, which will then refreeze on top of the string. Now you can easily pick up the ice!

Here’s how it works: Salt, when dissolved in water, actually lowers water’s freezing point below 32 degrees Fahrenheit, and this causes the ice to melt faster than it otherwise would. The ice around the string then refreezes, trapping the string inside.

In the winter, cities will often put salt on the roads to make them less slippery. More importantly, salt is also an important ingredient in ice cream and its nondairy equivalents. Cream freezes at a lower temperature than ice does, which means that ice can’t be used to freeze cream. But if you add salt to the mixture, then it can get cold enough to create a semisolid creamy confection.

To explore this further, fill two cups with water, pour four tablespoons of salt into one of them, and place the cups in the freezer. Which cup will freeze? The water or the salt water? Write down what you think will happen and wait three hours. Were you correct? 

You can experiment with other common ingredients, such as sugar, flour, or pepper. What substance is best for lowering the freezing point of water? 

This experiment is part of the Monitor’s occasional Science at Home series.