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Questions on Expansion and Contraction

Q1. If you pour boiling water straight into an ordinary glass it can shatter. The only way to do it is to pour the water in very slowly onto a spoon resting within the glass - and even then shattering is very possible!

Use the idea of expansion and contraction to explain why this occurs.

(6 marks)

The heat energy from the boiling water is transferred to the inner surface of the glass on contact. The heat energy then has to be transferred by conduction across the glass inner surface and through the body of the glass to the outer surface. This is a slow process with 'ordinary' glass.

The rise in temperature causes the glass to expand. The inner part of the glass expands to a greater degree than the outer surface becasue it is hotter, this causes stress and strain within the glass structure and makes it shatter.

 

If you pour the water in slowly, and use a good conductor such as a metal spoon, at the point of application of the hot liquid, the heat is conducted throughout the spoon and less heat is applied to a point on the inner surface. That heat energy is then gradually distributed across the inner surface and through the glass body before the next load of heat energy is delivered- this reduces the sharp temperature rise at the point of application. The gradual increase in temperature gives the outer shell of the glass time to expand along with the inner one and fracture is avoided.

(Max - 6 marks)