Question

Why does a hot glass crack when placed under cold water?

Answer 1

When allowed to cool on its own, the glass cools down evenly on all sides;inside and out. When cooled fast, e.g. by running cold water on it, the glass cools down unevenly, and therefor cause the glass to crack because the inside contracts while the outside remains expanded. ... Glass expands when hot, contracts when cold.

Answer 2

Glass is a nonconductive material.

A nonconductive material cannot conduct heat evenly throughout it's body.

So when a hot glass is placed in cold water it expands due to conduction of heat.

But the heat doesn't flow evenly in the glass.

Hence, the expansion happens unevenly.

So due to unven expansion, the glass cracks.