How to Pick Up an Ice Cube with a String (and a Dash of Science)

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Have you ever seen those big trucks that dump salt onto frozen roads to melt the ice and make it safer to drive and walk? If so, I’ve got a fun STEM experiment today that helps you learn the science behind it.

When you rest a piece of string on a block of ice, nothing will happen. However, if you add salt to the ice block, it will lower the freezing point of the ice long enough to allow the ice to melt slightly and refreeze over a piece of string, allowing you to lift the ice from the water.

How to make the Pick Up Ice with a String STEM experiment

Supplies you will need

For this experiment, you will need the following:

• Water
• Ice cubes
• Salt
• String
• Empty glass

Before you start

Be sure not to let the ice water sit for too long, so the ice cubes stay large enough for the experiment.

Instructions

Here is how to do this experiment with your child:

Step 1: Add ice and water to an empty glass

Add ice to your empty glass so that it creates a layer of ice cubes. If you add too few, the ice will melt before the experiment begins!

Fill the glass about half full of water.

Step 2: Touch the string to the ice

While the glass has only water and ice in it, try to get the string to stick to the ice. Leave the string sitting on top of the ice for about 30 seconds and then pull the string out. What happens? Does it stick to the ice or just come out?

Step 3: Add salt to the glass

Now, add some salt to the water. Some of the salt will sink down into the water and some will sit on the ice.

We didn’t add much salt, just a few dashes of it.

Step 4: Rest the string on the ice

Now, try resting the string on the ice again. Leave the string touching the ice (and don’t touch the string) for about 30 seconds to a minute.

Once your time is up, lift the string out of the water. Does the ice stick to it?

How cool!

When we put a string on an ice cube and try to lift it, nothing happens—the string just slips off.

But if we sprinkle salt on the ice, the magic begins!

Salt makes the ice melt a tiny bit under the string. After a minute, the melted water freezes again because the ice is still very cold. This time, the ice freezes around the string, like it’s glued in place. Now the string can lift the ice cube!

The STEM behind the Pick Up Ice with a String Experiment

This experiment teaches:

• Salt lowers the freezing temperature
• States of matter
• Environmental science applications

How it works

In this experiment, we are testing whether a piece of string can lift ice out of a glass.

At first, when we simply place the string on the ice and try to pick it up, nothing happens. That’s because the ice is frozen and there’s nothing to hold the string to the ice. When we sprinkle salt on the ice, the string is able to stick to the ice and we can pull the ice out of the glass!

This happens because salt actually lowers the freezing point of water, which is called freezing point depression. The salt makes the ice melt a little where it touches, forming a very thin layer of liquid water right below where the string touches.

Since ice is still very cold overall, that melted water refreezes after a short period. But this time, since the string is resting on the melted ice layer, that melted ice layer refreezes around the string.

When you lift the string out of the water, the ice comes with it!

There are some fun real-world applications to this experiment: it’s how we make homemade ice cream and why adding salt to frozen streets helps the ice melt so it’s safe to drive.

Salt lowers the freezing temperature

Water freezes at or near 32 degrees Fahrenheit, or 0 degrees Celsius. However, we can play around with the freezing point of water by adding our key ingredient of this experiment: salt.

When we add salt to the ice water, the freezing point is now slightly lower than it was before. This is called freezing point depression.

How does it work?

When water freezes into ice, those loose water molecules start to turn a more ordered and rigid structure.

By adding salt to the water, we’re introducing solute particles that make it harder for the water molecules to bond together into the rigid structure of ice.

So when we add even a little bit of salt to the water, it makes the ice melt ever so slightly. After about 30 seconds to a minute, the surrounding colder ice pulls heat out of the melted layer. The water then refreezes around the string, making it stuck to the string and you can pull it out of the glass!

States of matter

In this experiment, we also get to see a pretty quick phase change happening inside of the glass. A phase change is when something changes from one state of matter to another, like from solid to a liquid or liquid to a gas.

Since we’re looking specifically at water in this experiment, let’s talk about what that means with water.

Water changes between a solid (ice) and a liquid (water) depending on the temperature and pressure of other substances. Water turns into a solid around 32 degrees Fahrenheit (or 0 degrees Celsius), except when you add a substance to get in the way of that.

That’s the salt!

The salt in our experiment gets in the way of the water turning into ice by interfering with the water molecules becoming more structured and rigid. That means the water has to be at a lower temperature (below 32F or 0C) in order to freeze into ice.

So, in this experiment, we are going from a solid (ice) to a liquid (melting water slightly) to a solid again (refreezing around the string).

Environmental science applications

Now, onto how this is used in the real world!

Have you ever seen big trucks pouring salt onto the streets and highways in the wintertime, especially when there is ice on roads?

When temperatures drop below freezing, water on roads turns into ice. Ice makes roads slippery and dangerous for both cars and people walking. To solve this, road crews spread salt. The salt mixes with the thin layer of water on the ice’s surface and lowers its freezing point.

Normally, water freezes at 32 degrees Fahrenheit (0 degrees Celsius). But with salt, water might not freeze until it’s 15 to 20 degrees Fahrenheit, depending on how much salt is added. This means that ice can melt even when the outside temperature is still below freezing!

More experiments about freezing point depression to try out with your child

• The Science Scoop: How to make ice cream in a bag (and learn while you do!)