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1 Osmosis Article by: Francis J. Johnston, Department of Chemistry, University of Georgia, Athens, Georgia Access to this content is available to Ward's World readers for free from McGraw Hill's AccessScience, an award-winning, digital STEM resource that provides immediate, authoritative answers to students' thirst for scientific knowledge on topics such as climate change, virology, pollution, and more. Ward's World and McGraw Hill have partnered to offer educators a no-obligation, free trial subscription to this product. Request your free trial today and discover how valuable AccessScience can be for you and your students. The selective passage of solvent molecules through a semipermeable membrane from a dilute solution to a more concentrated one. A semipermeable membrane allows the passage of solvent molecules but blocks the passage of sol- ute molecules. The movement of water by osmosis through cell membranes is essential for all physiologi- cal processes. Osmosis also has diverse experimental and industrial applications. Mechanism of Osmosis The phenomenon of osmosis may be observed by dividing a container into two equal volumes with a semipermeable membrane. As shown in Fig. 1, the left side contains pure water, the right side contains a dilute glucose solution, and the water levels are equal. Over time, the level on the right rises as water flows from the left to the right side. As the solution level on the right increases, pressure forces water back through the membrane. Eventually, the system equilibrates, such that the rates of the forward and reverse water flow through the membrane are equal and the level on the right stops rising. The osmotic pressure is the pressure required to reach equilibrium (that is, to stop osmosis) and is a characteristic of the solution (Fig. 2). + ward ' s science Key Concepts • Osmosis is the transport of a solvent through a semipermeable membrane that separates two solutions of differing solute concentration. • During osmosis, the solvent moves from the solution that is lower in solute concentration to the solution that is higher in solute concentration. • Osmotic pressure describes the minimum pressure that, when applied to the solution phase, prevents the solvent from passing through a semipermeable membrane into the solution. • The decrease in Gibbs free energy accompanying the dilution of the solution is important in driving osmosis. • Reverse osmosis involves movement of solvent from a higher solute concentration to a lower solute concentration. This happens when hydrostatic pressure is greater than osmotic pressure. Fig. 1: Osmosis process. (Credit: McGraw Hill) Osmotic Pressure Diffusion is the process by which particles or molecules spread out and move from an area of high concentration to an area of low concentra- tion. Because the solute cannot cross the semipermeable membrane, but the solvent can, the solvent will tend to cross the membrane to equalize the concentration of the solute on the two sides of the mem- brane in the process of osmosis. If the final result of a reaction is a more stable solution, then the molar free energy is positive, and the reaction is likely to happen. The molar free energy of a solvent in a solution is

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