Second law of thermodynamics: Second law of thermodynamics states that there is a definite limit to the amount of mechanical energy, which can be obtained from a given quantity of heat energy.

According to Claussius, this lay may be stated as ” It is impossible for a self acting machine working in a cyclic process, to transfer heat from a body at a lower temperature to a body at a higher temperature without the aid of an external agency”.

The second law of thermodynamics has also been stated by Kelvin Planck as ” It is impossible to construct an engine working on a cyclic process, whose sole purpose is to convert heat energy into work.” According to this statement, the second law of thermodynamics is sometimes called as law of degradation of energy.

Avagadro's Law states, "Equal volumes of all gases, at the same temperature and pressure, contain equal number of molecules."

Thus, according to Avogadro's law, 1 m³ of oxygen (O₂) will contain the same number of molecules as 1 m³ of hydrogen (H₂) when the temperature and pressure is the same.