Thermodynamics is a science in which the storage, transformation, and transfer of energy are studied. Energy is stored as internal energy (associated with temperature), kinetic energy (due to motion), potential energy (due to elevation) and chemical energy (due to chemical composition); it is transformed from one of these forms to another; and it is transferred across a boundary as either heat or work. In thermodynamics, we will derive equations that relate the transformations and transfers of energy to properties such as temperature, pressure, and density. Substances and their properties, thus, become very important in thermodynamics. Many of our equations will be based on experimental observations that have been organized into mathematical statements or laws; the first and second laws of thermodynamics are the most widely used.