Heat - Effects of Heat

Heat is a form of energy that flows from a body at a higher temperature to a body at a lower temperature. Its SI unit is the Joule ($J$), but it is often measured in calories ($cal$).

Change in Temperature: When a body absorbs heat, its average kinetic energy increases, leading to a rise in temperature ($T$). Conversely, losing heat leads to a fall in temperature.

Change in State: Heat can cause a substance to change from solid to liquid (melting), liquid to gas (vaporization), or solid directly to gas (sublimation). During these changes, the temperature remains constant.

Thermal Expansion: Most substances expand when heated and contract when cooled. Expansion is greatest in gases, followed by liquids, and least in solids.

Anomalous Expansion of Water: Water behaves uniquely; it contracts when heated from $0^{\circ}C$ to $4^{\circ}C$ and expands when cooled from $4^{\circ}C$ to $0^{\circ}C$. The density of water is maximum at $4^{\circ}C$.

Chemical Change: Heat energy can initiate chemical reactions. For example, heating Potassium Chlorate ($KClO_3$) in the presence of Manganese Dioxide ($MnO_2$) causes it to decompose into Potassium Chloride ($KCl$) and Oxygen ($O_2$).