Heat change refers to the transfer of thermal energy between a system and its surroundings. It is often denoted as Q and is measured in units of joules (J) or calories (cal). Heat change occurs due to temperature differences between the system and its surroundings, causing energy to flow from one to the other. When heat is transferred to a system, it increases the internal energy of the system, while heat transferred from a system decreases its internal energy. Heat change is a fundamental concept in thermodynamics and is crucial in understanding various processes such as heating, cooling, and phase transitions.