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An obvious question is what makes water move at all? Imagine a beaker with a semipermeable membrane separating the two sides or halves. On both sides of the membrane the water level is the same, but there are different concentrations of a dissolved substance, or solute, that cannot cross the membrane otherwise the concentrations on each side would be balanced by the solute crossing the membrane.
If the volume of the solution on both sides of the membrane is the same but the concentrations of solute are different, then there are different amounts of water, the solvent, on either side of the membrane.
If there is more solute in one area, then there is less water; if there is less solute in one area, then there must be more water. To illustrate this, imagine two full glasses of water. One has a single teaspoon of sugar in it, whereas the second one contains one-quarter cup of sugar. If the total volume of the solutions in both cups is the same, which cup contains more water? Because the large amount of sugar in the second cup takes up much more space than the teaspoon of sugar in the first cup, the first cup has more water in it.
Osmosis : In osmosis, water always moves from an area of higher water concentration to one of lower concentration. In the diagram shown, the solute cannot pass through the selectively permeable membrane, but the water can.
Returning to the beaker example, recall that it has a mixture of solutes on either side of the membrane. A principle of diffusion is that the molecules move around and will spread evenly throughout the medium if they can. However, only the material capable of passing through the membrane will diffuse through it. In this example, the solute cannot diffuse through the membrane, but the water can. Water has a concentration gradient in this system.
Thus, water will diffuse down its concentration gradient, crossing the membrane to the side where it is less concentrated. This diffusion of water through the membrane—osmosis—will continue until the concentration gradient of water goes to zero or until the hydrostatic pressure of the water balances the osmotic pressure. In the beaker example, this means that the level of fluid in the side with a higher solute concentration will go up. Privacy Policy. Skip to main content.
Search for:. Diffusion and Effusion Gas Diffusion and Effusion Due to their constant, random motion, gas molecules diffuse into areas of lower concentration, and effuse through tiny openings. Learning Objectives Explain the concepts of diffusion and effusion. Key Takeaways Key Points Gaseous particles are in constant random motion.
Gaseous particles tend to undergo diffusion because they have kinetic energy. Diffusion is faster at higher temperatures because the gas molecules have greater kinetic energy.
Effusion refers to the movement of gas particles through a small hole. Key Terms diffusion : movement of particles from an area of high concentration to one of low concentration mean free path : the average distance traveled by a particle between collisions with other particles Effusion : movement of gas molecules through a tiny hole. Example What is the ratio of the rate of effusion of ammonia, NH 3 , to that of hydrogen chloride, HCl?
Osmosis Osmosis is the movement of water across a membrane from an area of low solute concentration to an area of high solute concentration. Learning Objectives Describe the process of osmosis and explain how concentration gradient affects osmosis.
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