Which substance is a good insulator?

Glass is indeed a good insulator due to its molecular structure, which does not allow for the free movement of electrons. Insulators are materials that resist the flow of electric current or heat, and glass is capable of preventing both electrical conductivity and thermal transfer to a significant degree.

This property makes glass an ideal choice for applications where insulation is crucial, such as in windows, electrical insulators, and heat-resistant containers. The lack of free electrons in its structure means that glass effectively limits the transfer of energy, whether thermal or electrical, making it essential for creating barriers in various environments.

In contrast, water, silver, and aluminum are not suitable choices for good insulation. Water is a moderate conductor, particularly when it contains impurities. Silver and aluminum, on the other hand, are excellent conductors of electricity and heat due to their free-moving electrons. Therefore, glass is the clear choice when identifying a substance that functions well as an insulator.