Another route to ΔH is to measure the internal energy change by using a
bomb calorimeter, and then to convert ΔU to ΔH. Because solids and liquids have
small molar volumes, for them pVm is so small that the molar enthalpy and molar
internal energy are almost identical (Hm = Um + pVm ≈ Um). Consequently, if a process
involves only solids or liquids, the values of ΔH and ΔU are almost identical.
Physically, such processes are accompanied by a very small change in volume; the
system does negligible work on the surroundings when the process occurs, so the
energy supplied as heat stays entirely within the system.