What causes the reaction when zinc is heated?

It depends on the atmosphere in which zinc is heated. If Zinc metal is heated in an inert atmosphere (e.g. Argon gas), then solid zinc will get hot until it reaches its melting point, then melt, and then slowly evaporate until it reaches its boiling point, then boil and then become zinc vapor. read more

If zinc is heated in an atmosphere of air (or oxygen), it will start to oxidize. It will form Zinc Oxide (ZnO), and with sufficient supply of oxygen, all of the zinc can be converted to zinc oxide. read more

At room temperature, zinc is a solid. As you might guess, heating zinc sufficiently will cause it to change state to a liquid as it melts at 419.53 °C, or 787.15 °F. Heated further, it will boil off, as its boiling point is 907 °C, or 1665 °F. read more