Which of the following substances is NOT considered a heat insulator?

Metals are good conductors of heat, which means they do not act as heat insulators. Heat insulators are materials that hinder the flow of thermal energy, keeping heat in or out of a space. Cotton wool, felt, and woollen clothes are all materials known for their insulating properties, as they trap air within their fibers, reducing heat transfer. This is why they are commonly used in applications to maintain temperature by preventing heat loss. In contrast, metals allow heat to flow through them readily, which makes them ineffective as heat insulators.