This question already has an answer here:
I have read the following definition of enthalpy in my textbook:
A substance has to occupy some space in its surroundings depending upon its volume ($V$). It does against the compressing influence of the atmospheric pressure ($p$). Due to this,the substance possesses an additional energy called $pV$ energy which is given by the product of pressure ($p$) and volume ($V$) of the system.
The sum of internal energy and $pV$ energy of any system, under given set of conditions, is called enthalpy. It is denoted by $H$ and is also called heat content of the system. Mathematically, it may be put as $$H = U + pV$$
Let's suppose we have some water in a test tube at room temperature open to atmosphere. It will have internal energy which consist of kinetic energy of molecules, chemical energy, rotational energy and some other forms of energy. In order to occupy some space in its surroundings, the molecules of the water should vibrate in such a way that they cancel out the compressing influence of the air molecules present in the atmosphere.
So if we try to measure the internal energy of the system, isn't the energy required to make space for the surroundings already included in the internal energy of the system. Why do we add an extra term $pV$ along with internal energy $U$ to measure the heat content of the system?