Question

When cold water dropped on a hot chimney, it breaks because
A. Water change into vapour
B. The chimney contracts
C. The chimney expands
D. Any unknown reason

Answer 1

Hint: Glass is a very brittle material. As the chimney is also made up of glass, it is very brittle. Brittles materials like glass have no tendency to resist stress concentration beyond its limit. When cold water falls on hot chimneys, the temperature of the surface of the chimney changes. This phenomenon can be used to find why the hot chimneys break when cold water is dropped on it.

Complete answer:
The chimneys are made up of glass. When cold water falls on the hot chimney, there is a sudden change in the temperature at the point where water falls. The outer surface of the chimney cools down while the inner surface still remains hot. Glass has moderately low thermal conductivity. As the cold water falls, it tries to shrink. But the hot inner surface prevents the glass from shrinking. Because of the unequal contraction, stress is developed and it exceeds its limits of resisting stress. This causes the chimney to break.
Hence, when cold water is dropped on a hot chimney, the chimney contracts and it breaks.

So, the correct answer is “Option C”.

Note:
The principles that make a hot chimney made up of glass break, the same principles allow us to make tempered glass extremely stronger. Under controlled situations, glass is much stronger as compared to metals. But in reality, the conditions aren’t controlled, so glass is brittle. Students must remember that the glass is exceptionally strong in compression. If we put glass under great internal stress, then it becomes more resistant to external strength. Rapid cooling destroys regular hot glass but it strengthens a really hot glass.