* Chemical change: Involves the formation of new substances with different chemical properties. This often involves breaking and reforming chemical bonds.

* Physical change: Alters the form or appearance of a substance but doesn't change its chemical composition.

When glass melts, the molecules within the glass structure become more mobile and move further apart. This changes the state of matter from solid to liquid, but the chemical composition of the glass remains the same. The silicon dioxide (SiO2) and other components that make up the glass are still present, just in a different state of matter.

Think of it this way: Melting ice is a physical change. The water molecules (H2O) remain the same, just in a different state (solid vs. liquid). The same principle applies to glass.