Step 1: Statement. A solution is called ideal when both of its components follow Raoult's law throughout the whole composition range and at all temperatures, so each component's vapour pressure is proportional to its mole fraction in the liquid.
Step 2: Basis. This happens only when the solute-solvent attraction is essentially identical to the solute-solute and solvent-solvent attractions, so mixing does not change the environment felt by any molecule. Benzene + toluene is a classic pair.
Step 3: Key features (listed). \(\bullet\) Obeys Raoult's law at all concentrations. \(\bullet\) Heat of mixing is nil, \(\Delta H_{mix}=0\), so temperature stays constant on mixing. \(\bullet\) No volume contraction or expansion, \(\Delta V_{mix}=0\). \(\bullet\) The forces \(A\text{-}B \approx A\text{-}A \approx B\text{-}B\).
Step 4: Result. These conditions together define an ideal solution. \[ \boxed{p_A = x_A p_A^{\circ},\ p_B = x_B p_B^{\circ}} \]