Question

State whether true or false:
Test tubes are made of borosil glass as it does not crack on heating.

Answer 1

When a glass is subjected to heating it cracks due to the thermal stress, which is developed because of the thermal differential between the inner surface and outer surface of glass.
Borosilicate glasses have a very low coefficient of thermal expansion which means a very little thermal stress develops when it is subjected to heat.

Complete step by step answer:
A glass is made from liquid sand (silicon dioxide). Normal glass is an insulator i.e. it is a poor conductor of heat.
When a normal glass tube is exposed to heating, the surface of the glass which is in contact with the hot flames heats up and expands as per its coefficient of thermal expansion. But the inner layer remains cold and does not expand since glass is a poor conductor and does not transfer the heat quickly. This causes the differential thermal expansion which eventually leads to cracking due thermal stress.
Borosilicate glass or Borosil glass is made up of boron trioxide and silicon dioxide as the main glass-forming constituents. Borosilicate glasses are known for having very low coefficients of thermal expansion $\left( { \approx 3\; \times \;{{10}^{ - 6}}\;{K^{ - 1}}\;at{\text{ }}20\;^\circ C} \right)$ which makes them resistant to thermal shock, more than any other common glass. Such glass is subjected to less thermal stress and can withstand temperature differentials without fracturing of about $165\;^\circ C{\text{ }}\left( {329\;^\circ F} \right)$. It is commonly used for the construction of reagent bottles and flasks.
Borosil glass has a melting point of $820^\circ C$ and the glass can withstand higher temperature gradients and also sudden temperature changes.
So, Borosil glass does not crack when subjected to heating while normal glass cracks.
Hence, the answer to the question is true.

Note:
Borosilicate glass wares are also called Pyrex glass, but now they are two different things. Pyrex is a brand which makes heat resistant glass wares using borosilicate only. But now they use a mixture of borosilicate, soda-lime and some heat resistant material for their glassware.