Why does sharp glass edge become smooth on heating it up to its melting point in a flame? Explain which property of liquids is responsible for this phenomenon.

When a sharp glass edge is heated up to its melting point in a flame, it becomes smooth due to a process called surface tension. Surface tension is the property of liquids that causes their surfaces to behave like a stretched elastic membrane, minimizing surface area and forming spherical shapes. This phenomenon arises from the cohesive forces between molecules in the liquid.

Here's how the process unfolds:

1. Heating the Glass: When the sharp glass edge is heated in a flame, the temperature of the glass increases. As the temperature rises, the kinetic energy of the glass molecules increases, causing them to vibrate more vigorously. Eventually, the temperature reaches the melting point of the glass, causing the glass to transition from a solid to a liquid state.

2. Surface Tension Effect: As the glass melts, the cohesive forces between the glass molecules become dominant. These forces tend to minimize the surface area of the liquid glass. As a result, the liquid glass adopts a spherical shape, pulling inwards at the sharp edges.

3. Smoothing Effect: As the liquid glass retracts and forms a smooth, rounded surface, any sharp edges or protrusions are gradually smoothed out. This process occurs due to the surface tension pulling the liquid glass inward, effectively rounding off any irregularities or sharp features.

In summary, the property of surface tension in liquids is responsible for the phenomenon of a sharp glass edge becoming smooth when heated up to its melting point. Surface tension causes the liquid glass to minimize its surface area, resulting in the rounding off of sharp edges and the formation of a smooth surface.