What happened to your chocolate mixture?

When you first poured your chocolate mixture into the bags, it was a liquid.

After shaking it in the icy salt mixture, it slowly became thicker until it turned into a solid (or nearly solid) chocolate ice cream.

Scientists call this a change of state.

A change of state happens when a material changes between a solid, liquid or gas without becoming a different substance.

Why did it freeze?

Your chocolate mixture did not freeze by itself.

Instead, heat energy moved out of the mixture and into the icy bag around it.

As heat left the chocolate mixture, its tiny particles slowed down and moved closer together.

Eventually they could no longer slide past each other like a liquid, so the mixture became a solid.

This is why your chocolate mixture changed from a liquid into a solid.

What did the salt do?

Ice on its own is very cold.

But when you mix salt with ice, something surprising happens.

The salt causes some of the ice to melt.

To melt, the ice needs energy, so it takes heat from everything around it—including your chocolate mixture.

This makes the salty ice mixture even colder than ordinary ice, allowing it to remove heat much faster.

That is why bags with different amounts of salt may have frozen at different speeds during your investigation.

Why didn't every bag freeze the same?

During this investigation you changed one variable—the amount of salt added to the ice.

Everything else stayed the same.

Scientists call this a fair test.

If one bag froze faster than another, the evidence suggests that changing the amount of salt affected how quickly heat was removed from the chocolate mixture.

Scientists repeat investigations like this many times before deciding whether their evidence supports their prediction.