explain how and why is glass a viscous liquid?
When a liquid is cooled its viscosity normally increases, but viscosity also has a tendency to prevent crystallisation. Usually when a liquid is cooled to below its melting point, crystals form and it solidifies; but sometimes it can become supercooled and remain liquid below its melting point because there are no nucleation sites to initiate the crystallisation. If the viscosity rises enough as it is cooled further, it may never crystallise. The viscosity rises rapidly and continuously, forming thick syrup and eventually an amorphous solid. The molecules then have a disordered arrangement, but sufficient cohesion to maintain some rigidity. In this state it is often called an amorphous solid or glass.