A piece of glass is heated to a high temperature and then allowed to cool. If it cracks, a probable reason for this is the following property of glass

Options

A. low thermal conductivity of glass
B. high thermal conductivity of glass
C. high specific heat of glass
D. high melting point ot glass

Answer

To solve the question regarding why a piece of glass cracks when heated and then allowed to cool, we can break down the reasoning step by step.

### Step-by-Step Solution:

1. **Understanding Thermal Conductivity**:
- Glass has low thermal conductivity. This means that when heat is applied to it, the heat does not spread evenly throughout the material.

2. **Heating the Glass**:
- When the glass is heated, the outer surface of the glass heats up more quickly than the inner part because the heat cannot penetrate through the glass efficiently.

3. **Temperature Gradient**:
- Due to the low thermal conductivity, there is a temperature difference between the outer surface (T1) and the inner body (T2) of the glass. The surface temperature (T1) becomes significantly higher than the temperature inside (T2).

4. **Non-Uniform Expansion**:
- The expansion of materials is related to the change in temperature. Since the outer surface is at a higher temperature than the inner part, it expands more than the inner part. This results in non-uniform expansion where the outer surface expands more than the inner body.

5. **Development of Stress**:
- The difference in expansion creates internal stress within the glass. The outer surface tries to expand more, while the inner part does not expand as much, leading to tensile stress at the surface.

6. **Cooling Process**:
- When the glass is allowed to cool, the same non-uniform temperature distribution occurs in reverse. The outer surface cools down more quickly than the inner part, leading to further stress.

7. **Cracking of Glass**:
- Eventually, the accumulated stress due to non-uniform expansion and contraction can exceed the strength of the glass, causing it to crack.

### Conclusion:
The probable reason for the cracking of glass when heated and then cooled is its low thermal conductivity, which leads to non-uniform temperature distribution and resultant stress.