Why Do Ice Cubes From the Freezer Crack and Pop in a Drink?
― Ice Can Tear Itself Apart With a Difference of Just a Few Degrees

Drop an ice cube fresh from the freezer into barley tea, and it goes crack, snap, and white fractures race through it. Nobody hit it. So why does the ice break? The answer has two parts: "only the outside warms up and swells suddenly", and "ice is very bad at being pulled". The numbers show that ice can break with a temperature difference of only about 2°C. It is a surprisingly delicate material.

There are only two reasons ice breaks

So the ice is not being broken from outside. Its own outside and inside, at different temperatures, pull against each other until it cracks. Let's take it one step at a time.

Reason 1: Only the surface swells, while the core stays shrunken

A home freezer is usually kept at around −18°C. Ice just out of it is cold to the core, all the way through.

When you put this ice in a drink, heat floods in from the liquid around it. Water is much better at passing on heat than air is. So the surface of the ice quickly warms to nearly 0°C. Heat takes time to travel through the ice, though, so the core stays at −18°C for a while.

Most things swell slightly when they warm up, and ice is no different. The surface layer "wants to grow a little bigger", but the cold core stays small. Outside and inside are joined as one piece, so the mismatch has nowhere to go. In Figure 1 you can see the outside pulling the core outward.

Ice left out in the air is less likely to crack because air passes on heat only slowly. While the surface warms bit by bit, heat reaches the core too, so the gap between outside and inside never opens wide.

Reason 2: Ice is very weak when pulled

Ice is hard. Even the blade of a shaved-ice machine does not make it crumble at once. It has a fair amount of strength against being crushed.

But it is very weak against pulling. The force that breaks ice by pulling is said to be only a fraction of the force that breaks it by crushing, from a few times smaller to about a tenth. Ice also cannot stretch like rubber to spread out the force. Once it goes even slightly past its limit, a crack runs through it at once.

The outer layer that swelled in Reason 1 pulls the core outward in every direction. So the core gets the kind of force ice likes least: a pull. That is why the cracks form inside rather than on the surface. The "crack" sound is the ice vibrating as the fracture spreads in an instant.

So how big a temperature difference does it take? In Figure 2, change the freezer temperature and find out.

The bar in Figure 2 is an upper-limit estimate: the outside is exactly 0°C and the core stays at freezer temperature. In reality, the temperature changes gradually, so the force is smaller than this. Even so, for −18°C ice it exceeds the breaking strength many times over. You can see that ice is a material that "breaks as a matter of course".

Summary

Ice put into a drink has only its surface warm suddenly to 0°C and swell. The cold core stays shrunken, so the outside pulls it in every direction. Ice is very weak when pulled, and the calculation says it can break with a difference of only about 2°C. That is why −18°C ice splits from the inside with a crack and a pop.

It is not the drink that breaks the ice.
It is the tug-of-war between the warmed outside and the still-cold core.

Stones breaking from temperature differences in the same way are covered in "Why do river stones sometimes crack and fly apart when you build a fireplace with them?", and water passing heat faster than air is covered in "A sauna is 90°C, so why can we stand it?". For why cracks look white, "Ice is clear, so why does snow look white?" is a useful read.

*This article is a popular-science explanation for general readers. The figures given are rough guides to help you understand how things work. The physical properties of ice vary greatly with how it was made, its temperature and how many bubbles it contains.