Given below are two statements. One is labelled as Assertion A and the other is labelled as Reason R.
Assertion A : Sharp glass edge becomes smooth on heating it up to its melting point.
Reason R : The viscosity of glass decreases on melting.
Choose the most appropriate answer from the options given below.

This question tests understanding of the properties of glass and why sharp glass edges become smooth when heated.

Assertion A states that sharp glass edges become smooth on heating up to its melting point. This is true. Glass is an amorphous solid (a supercooled liquid) that does not have a sharp melting point but softens over a range of temperatures. When a sharp glass edge is heated, the surface tension of the softened glass pulls the sharp edge into a rounded, smooth shape to minimise surface area and thus surface energy. This is a well-known phenomenon in glass working.

Reason R states that the viscosity of glass decreases on melting. This is also true. Like all liquids, the viscosity of glass decreases significantly as temperature rises. At higher temperatures, the glass flows more readily.

However, R is NOT the correct explanation of A. The smoothing of sharp glass edges is due to surface tension, not directly due to decreased viscosity. Surface tension causes the molten glass to minimise its surface area by rounding the sharp edges. While decreased viscosity allows the glass to flow (which is necessary for the shape change to occur), the driving force for the smoothing is surface tension — the tendency to minimise surface energy. Decreased viscosity alone does not explain why sharp edges specifically become smooth; it only explains that flow is possible. The direction and nature of the shape change is dictated by surface tension.

Therefore, both A and R are true, but R is not the correct explanation of A. The correct answer is option 2.