The viscosity of a fluid is a measure of its resistance to gradual deformation by shear stress or tensile stress. For liquids, it corresponds to the informal notion of thickness.

An isothermal process is a change of a system, in which the temperature remains constant: ΔT = 0. This typically occurs when a system is in contact with an outside thermal reservoir (heat bath), and the change occurs slowly enough to allow the system to continually adjust to the temperature of the reservoir through heat exchange.

A heat pump is a device that transfers heat energy from a heat source to a heat sink against a temperature gradient.

The Claussius-Clayperon equation allows us to estimate the vapour pressure at another temperature, if the vapour pressure is known at some temperature, and if the enthalpy of vaporisation is known.

The third law of thermodynamics is sometimes stated as the entropy of a perfect crystal at absolute zero is exactly equal to zero.

Volume is the quantity of three-dimensional space enclosed by some closed boundary, for example, the space that a substance (solid, liquid, gas, or plasma) or shape occupies or contains.

The general characteristic of binary solution is that when, say, two liquids are mixed together at the same pressure and temperature, the extensive properties (volume, enthalpy, entropy, etc.) of the mixture are not the sum of those of the unmixed components.

Energy is an indirectly observed quantity which comes in many forms, such as kinetic energy, potential energy, radiant energy and many others.

Pressure is the effect of a force applied to a surface. Pressure is the amount of force acting per unit area. The symbol of pressure is p.

Entropy is a measure of the number of specific ways in which a system may be arranged, often taken to be a measure of disorder.

Water is the most abundant compound on earth's surface, covering about 70 percent of the planet. In nature, water exists in liquid, solid and gaseous states.

Hydrogen is a chemical element with chemical symbol H and atomic number 1. With an atomic weight of 1.00794 u , hydrogen is the lightest element and its monatomic form (H1) is the most abundant chemical substance, constituting roughly 75% baryonic mass of the universe .

Heat capacity or thermal capacity, is the measurable physical quantity that specifies the amount of heat required to change the temperature of an object or body by a given amount.

An isenthalpic process is a process that proceeds without any change in enthalpy, H. In a steady-state, steady-flow process, significant changes in pressure and temperature can occur to the fluid and yet the process will be isenthalpic if there is no transfer of heat to or from the surroundings, no work done on or by the surroundings, and no change in the kinetic energy of the fluid.

A heat pump uses some amount of external high-grade energy to accomplish the desired transfer of thermal energy from heat source to heat sink. So temperature is a measure of the thermal energy per particle of matter or radiation.