A test tube made of ordinary glass cracks on plunging into boiling water whereas a red hot test tube made of fused silica can be safely plunged into normal water. Why?

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Solution

Ordinary glass is an insulator i.e, it is a poor conductor of heat.
When a very hot liquid is poured into a glass tumbler, the surface of the glass which is in contact with the hot liquid heats up and expands as per its coefficient of thermal expansion.
Fused silica has very low expansivity.
Hence different layers of fused silica test tubes do not expand or contract differently.
Therefore, it can be safely plunged into the water even when it is red hot.

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