Chemistry
The Thermodynamic Origin of Osmotic Pressure
Quick fact
At equilibrium, osmotic pressure is equal to the pressure that the solute would exert if it were an ideal gas at the same concentration – this is the van 't Hoff law, π = iMRT.
Why this is interesting
You know that putting a raisin in water makes it puff up, and that this is due to 'osmotic pressure' – but what actually pushes the water in? Surprisingly, it's not the dissolved particles physically shoving the water; it's an invisible push from thermodynamics itself.