Reverse osmosis (RO) is a water purification process that uses a partially permeable membrane to filter out impurities from drinking water. In reverse osmosis, an applied pressure is used to overcome osmotic pressure, a colligative property that is driven by chemical potential differences of the solvent, a thermodynamic parameter. The result is that the solute is retained on the pressurized side of the membrane and the pure water is allowed to pass to the other side. To be "selective", this membrane should not allow large molecules or ions through the pores, but should allow filtered clean water to pass freely.