A solid substance melts at -21°C. If the object is known to change phase at 81°C, will the object be a solid, liquid, or gas at 90°C?

At a temperature of 90°C, the object will be in a gaseous state. Initially, the substance is a solid, as it only melts at -21°C. However, if it changes phase at 81°C, that indicates it melts into a liquid at this temperature. Since the given temperature of 90°C is well above the melting point of 81°C, the substance will transition from solid to liquid as it reaches 81°C and continue to heat up to 90°C.

In addition, the temperature of 90°C is significantly above the boiling point for most substances that melt at lower temperatures. Therefore, it is reasonable to conclude that the substance will vaporize and exist as a gas at 90°C, since it is well above typical melting and boiling points for substances in the context of this question.