Introduction: Burning your hand on a hot stove or making your hand colder after touching a glass of lemonade means that you've experienced changes in enthalpy, or a quantity that relates to the total heat content of a system. Burning one's hand on a stove is associated with a positive change in enthalpy, since burning one's hand involves a transfer of heat out from an object towards one's hand. This positive change in enthalpy, with heat flowing out of a system to the surroundings, is known as an exothermic reaction. Making one's hand colder, on the other hand, is associated with a negative change in enthalpy, since making one's hand colder involves a transfer of heat into an object, away from one's hand. This negative change in enthalpy, with heat flow into a system from the surroundings, is known as an endothermic reaction.

Another important quantity in thermodynamics is entropy, or a quantity representing the degree of disorder or randomness that exists in a system. Typically, entropy is associated with phase changes, cooling and heating, changes in the number of gas particles, and physical processes that include dissolving. With a phase change like melting, entropy increases because the distance between the particles increases. With dissolving, entropy increases because particles are being dissolved, leading to more resulting disorder. 

Changes in enthalpy and entropy will be important as you learn more about thermodynamics later in your exploration of chemistry. For now, it is important to consider how enthalpy changes in individual processes and how entropy changes in individual processes, so that you will later be able to incorporate knowledge of both of these processes into a deeper understanding of thermodynamics.