Thermodynamics: a few definitions system: the portion of the universe under study surroundings: the rest of the universe boundary: separates the system from the surroundings closed system: matter cannot enter or leave (impermeable boundary) heat may enter or leave; work may be done open system: matter may enter or leave (permeable boundary) isolated system: boundary prevents any interaction with the system no heat or matter may enter or leave no work may be done by the system on the surroundings or by the surroundings on the system rigid, impermeable, adiabatic wall adiabatic system: no heat exchange between the system and the surroundings the boundary is an adiabatic wall an adiabatic system is a perfectly insulated system state: the state of a system is defined by specifying its properties properties with numerical values can be divided into two categories: intensive properties: magnitude does not depend upon the amount of substance temperature, pressure, concentration, density, molar volume extensive properties: magnitude depends on the amount of substance total value equals the sum of its values for all parts of the system volume, mass, change in enthalpy homogeneous system: intensive properties constant throughout (opposite: heterogeneous) a phase is a homogeneous part of a system A state variable (also called a state function, thermodynamic variable, thermodynamic function) depends only on the state of the system. state of equilibrium: (a) system shows no further tendency to change its properties with time (b) removal from contact with surroundings causes no further change in properties if (a) but not (b), system is in a steady state) can specify an equilibrium state by its state functions; in fact, only a few are needed (not as easy to specify a nonequilibrium state) one can further subdivide the concept of equilibrium: 1. mechanical equilibrium: no unbalanced forces acting on or within the system (a system in mechanical equilibrium has a well-defined pressure) 2. material equilibrium: no net chemical reaction no net transfer of matter from one part of the system to another 3. thermal equilibrium: a system and its surroundings are in thermal equilibrium if there are no changes in the properties of the system when the system and surroundings are separated by a thermally conducting wall .