Why do some glasses break when poured with boiling water?
1.Causes of glass cracks
As the subject said, the reason for the broken glass is the pouring of hot water. It is a universal law that objects expand and contract with heat, and glass is no exception. Different materials have different thermal expansion and contraction effects, which are usually expressed by thermal expansion coefficient. The thermal expansion coefficients of common materials are shown in the figure below.
The physical meaning of the thermal expansion coefficient is a measure of the elongation of a material when the temperature is raised by 1K. The higher the value, the more sensitive the material is to temperature. As can be seen from the above figure, the thermal expansion coefficient of ordinary glass is about 7.1e-6 /k. Compared with some metals such as steel and copper, glass is a material that is not easy to expand and contract with heat.
2. The location of the crack
In fact, students who have studied physics in junior high school know that objects will expand with heat and shrink with cold, and cold glass is easy to burst after pouring hot water. However, one should not know the mechanism without systematically studying mechanics. Thermal expansion and contraction are just general reasons. Under this general cause, how does the glass expand, why does the crack appear, and where does the crack appear? Only by learning the mechanics of materials can it be possible to solve the problem.
The breaking of the glass always happens at the moment of pouring the water. After the hot water is poured into the cold glass, the temperature of the inside of the glass rises due to the heating of the hot water, but the outside has not yet had time to rise. This results in the temperature distribution shown above. Red represents high temperature and blue represents low temperature. From the perspective of temperature distribution, its distribution is uniform in both the circumferential and longitudinal directions. However, in the thickness direction of the glass, the temperature distribution is not uniform, and there is a temperature gradient. It is this inhomogeneity in the thickness direction that causes the glass to burst.
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