Formation of a solution from two components can be considered as :
(i) Pure solvent $\rightarrow$ separated solvent molecules, $\Delta H_1$
(ii) Pure solute $\rightarrow$ separated solute molecules, $\Delta H_2$
(iii) Separated solvent and solute molecules $\rightarrow$ solution, $\Delta H_3$
Solution so formed will be ideal if : -

By Hess's law, the enthalpy of solution is the sum of the enthalpy changes of the steps:
Step (i): separating solvent molecules needs energy against solvent–solvent attractions, $\Delta H_1$ (positive).
Step (ii): separating solute molecules needs energy against solute–solute attractions, $\Delta H_2$ (positive).
Step (iii): mixing the separated molecules releases energy from solvent–solute attractions, $\Delta H_3$ (negative).
$\Delta H_{soln} = \Delta H_1 + \Delta H_2 + \Delta H_3$
For an ideal solution, the solute–solvent attractions are the same as the solute–solute and solvent–solvent attractions, so $\Delta H_3 = -(\Delta H_1 + \Delta H_2)$ and $\Delta H_{soln} = \Delta H_1 + \Delta H_2 + \Delta H_3 = 0$ (along with $\Delta V_{mix} = 0$).
So the ideal solution condition is expressed by $\Delta H_{soln} = \Delta H_1 + \Delta H_2 + \Delta H_3$ (which equals zero).