Question

Consider the following:What would happen to the level of liquid in the two arms if the semipermeable membrane separating the two liquids were permeable to a. \(\mathrm{H}_{2} \mathrm{O}\) (the solvent) only? b. \(\mathrm{H}_{2} \mathrm{O}\) and solute?

Short answer

If the semipermeable membrane is permeable to \(\mathrm{H}_{2}\mathrm{O}\) only, water molecules will move from the side of low solute concentration to the side of high solute concentration through osmosis, causing the liquid level to rise on the high solute concentration side and decrease on the other side. If the membrane is permeable to both \(\mathrm{H}_{2}\mathrm{O}\) and solute, the system undergoes diffusion with the solvent and solute molecules moving across the membrane based on concentration gradients. Over time, this process leads to an equilibrium state with equal solute concentrations and liquid levels in each arm.

Step-by-step solution

Semipermeable Membrane Permeable to Water Only

If the semipermeable membrane is permeable to water only (the solvent), then the water molecules will more likely move from the side of low solute concentration to the side of high solute concentration. This process is known as osmosis. It will continue as long as there is a difference in solute concentrations between the two arms. As water molecules move from the side with low solute concentration to the side with high solute concentration, the liquid level on the side with high solute concentration will rise, while the level on the other side will decrease.

Semipermeable Membrane Permeable to Both Solvent and Solute

When the semipermeable membrane is permeable to both water and the solute, the system will go through a diffusion process. In this case, both the solvent and solute molecules will move across the membrane based on concentration gradients. The water molecules will still move from the side of low solute concentration to the side with high solute concentration, and the solute molecules will move from the side of high concentration to the side with low solute concentration. Over time, this will create a new equilibrium state in which the concentrations of solute and the liquid levels in each arm will be the same.

Key concepts

Osmosis

Osmosis is a spontaneous process where water or other solvents move across a semipermeable membrane. This movement occurs from an area of low solute concentration to an area of high solute concentration. The driving force behind osmosis is the concentration gradient of the solute. When you have a semipermeable membrane that only allows solvent molecules (like water) to pass through and blocks solute molecules, osmosis takes center stage. Did you know?: Osmosis is crucial for biological systems. It helps cells maintain their size and shape by controlling the balance of water. In the exercise above, when the membrane is permeable only to water, osmosis will lead to a rise in liquid level on the side with higher solute concentration as water flows in.

Solute Concentration

Solute concentration refers to how much solute is present in a solution compared to the solvent. The concentration of solute matters because it affects the movement of molecules across a membrane. - High solute concentration means that there is a lot of solute in the solution relative to solvents. - Low solute concentration means that there is less solute in relation to solvents. In osmosis, water moves towards the side with a higher solute concentration to balance the differences. Understanding solute concentration helps explain why certain reactions and processes happen. Like why your fingers prune in the sea! The ocean's high salt (solute) concentration draws water out of your skin through osmosis.

Diffusion

Diffusion is the process by which particles spread from areas of high concentration to areas of low concentration. This shifting continues until the particles are evenly dispersed throughout the system. Unlike osmosis, diffusion doesn't specifically involve a semipermeable membrane, although it can happen across one if the membrane allows both solutes and solvents to pass. In the exercise, when both the solute and solvent can cross the membrane, diffusion occurs for both types of molecules. Solute molecules spread from areas of high concentration to low, while water might head in the opposite direction, balancing everything out. Through diffusion, everything naturally aims to reach an equilibrium state where molecule distribution is uniform.

Equilibrium State

An equilibrium state is achieved when there's no net movement of molecules across a membrane. At this point, the concentrations of solute and solvent on both sides of the membrane are equal. Achieving equilibrium doesn't mean molecules stop moving; they just move at equal rates in both directions. For example, in the exercise, when the membrane allows both solute and solvent to pass, diffusion will eventually stop when both sides are balanced. Equilibrium is important in many scientific contexts, ensuring systems remain stable and energy-efficient. Think of it like solving a puzzle—once all pieces are in place, you have equilibrium. Crucially, equilibrium helps living organisms maintain steady internal conditions amidst changing external environments.