What indicates a phase change in a substance?

A phase change in a substance is typically indicated by a constant temperature during the transition from one phase to another, such as from solid to liquid (melting) or from liquid to gas (boiling). During these phase changes, the heat energy being added or removed from the system is used to break or form intermolecular forces rather than to change the temperature.

For instance, when ice is melting, the temperature remains constant at 0°C until all the ice has converted to liquid water, despite the continuous addition of heat energy. This plateau in temperature signifies that energy is being effectively utilized to change the state of the substance rather than increasing its temperature.

The other options do not directly indicate a phase change. A rapid temperature drop may signify cooling but does not confirm a phase change, as it might merely reflect a cooling process. An increase in pressure can influence the phase of a substance (for example, causing gases to liquefy) but doesn't inherently signify a phase change on its own. A change in chemical structure is indicative of a chemical reaction rather than a physical change, which is what phase changes represent.