A thick glass tumbler cracks when boiling water is poured into it because

Analysis

This question tests your understanding of Thermal Expansion in solids, specifically how heat transfer affects materials with low thermal conductivity, like glass.

Concept Explanation

When you pour boiling water into a thick glass tumbler, you are introducing a high amount of thermal energy to the inner surface of the glass.

1. Thermal Conductivity: Glass is a poor conductor of heat (an insulator). This means heat does not travel quickly from the inner surface to the outer surface.
2. Differential Expansion: Because the inner surface is in direct contact with the boiling water, it heats up and expands almost immediately. However, because glass is a poor conductor, the outer surface remains relatively cool and does not expand at the same rate.
3. Stress: The inner layer of the glass tries to expand, but it is being "held back" by the cooler, rigid outer layer. This creates a significant amount of internal stress (tension) within the glass. When this stress exceeds the strength of the glass, the material fractures, causing the tumbler to crack.

Step-by-Step Solution

• Step 1: Identify the physical process. The heat from the water causes the molecules on the inner wall of the glass to vibrate faster and move further apart (expansion).
• Step 2: Consider the property of the material. Since glass is a poor conductor, the heat is trapped on the inside.
• Step 3: Compare the layers. The inside layer expands rapidly due to the heat, while the outside layer remains at a lower temperature and maintains its original size.
• Step 4: Conclusion. The mismatch in expansion rates between the inner and outer layers creates a strain that leads to the crack.

Identifying the Answer

Comparing our analysis to the options provided:

• Option A correctly describes that the inside expands more rapidly than the outside.
• Options B, C, and D are incorrect because they contradict the physical behavior of glass under rapid heating.

Exam Tip

Whenever you see questions about glass cracking due to temperature changes, remember the keyword: "Poor Thermal Conductivity." This is the reason why the heat doesn't distribute evenly, leading to the uneven expansion that causes the break.