Assertion A : For an ideal solution formed by mixing liquids P and...

For an ideal solution formed by mixing two liquids $\text{P}$ and $\text{Q}$, the enthalpy of mixing ($\Delta_{mix}H$) and the volume of mixing ($\Delta_{mix}V$) are both zero. Thus, Assertion $\text{A}$ is correct.

In an ideal solution, the intermolecular attractive forces between the solute and solvent molecules ($\text{P-Q}$ interactions) are of the same magnitude as those between the pure components ($\text{P-P}$ and $\text{Q-Q}$ interactions). It is incorrect to state that no interactions occur between $\text{P}$ and $\text{Q}$. Thus, Reason $\text{R}$ is not correct.

Therefore, $\text{A}$ is correct but $\text{R}$ is not correct.