Question

a. Why does the level of the more-concentrated solution rise when two solutions of different concentrations are separated by a semipermeable membrane? b. When does the level of the solution stop rising? c. When the level stops rising, what is the net movement of water molecules across the membrane?

Short answer

The level rises due to osmosis. It stops when hydrostatic pressure equals osmotic pressure. Net movement of water is zero when it stops.

Step-by-step solution

Understanding Osmosis

Osmosis is the movement of water molecules from a region of lower solute concentration to a region of higher solute concentration through a semipermeable membrane.

Why Does the Level Rise?

Water molecules move from the less-concentrated solution to the more-concentrated solution through the semipermeable membrane to balance the solute concentrations. This causes the level of the more-concentrated solution to rise.

When Does the Level Stop Rising?

The level of the more-concentrated solution stops rising when the pressure exerted by the height of the liquid column, known as hydrostatic pressure, equals the osmotic pressure driving water molecules across the membrane.

Net Movement of Water Molecules

When the level stops rising, the net movement of water molecules across the membrane is zero because the osmosis-driven flow of water into the more-concentrated solution is balanced by the hydrostatic pressure pushing water back.

Key concepts

semipermeable membrane

A semipermeable membrane is a barrier that allows certain substances to pass through while blocking others. In the context of osmosis, it permits water molecules to move but not solute particles. This selective permeability is key in biological systems and many scientific applications. For example, in cells, the cell membrane acts as a semipermeable membrane, enabling essential processes like nutrient uptake and waste removal. By allowing only specific molecules to cross, these membranes help maintain the internal environment of cells.
In experiments dealing with solutions of different concentrations, the semipermeable membrane allows water to move from a less-concentrated solution to a more-concentrated one. This movement aims to balance the concentrations on both sides of the membrane.

hydrostatic pressure

Hydrostatic pressure is the pressure exerted by a fluid due to its weight. In the context of osmosis, water moving into a more-concentrated solution increases the volume on that side, making the liquid column higher. The weight of this column generates hydrostatic pressure, which opposes the osmotic flow of water.
This pressure plays a crucial role in determining when osmosis stops. As water continues to move through the semipermeable membrane, the column rises until the hydrostatic pressure equals the osmotic pressure. At this point, osmosis stops because the forces driving water into the solution are balanced by the downward pressure from the liquid column above.

osmotic pressure

Osmotic pressure is the force that drives water molecules across a semipermeable membrane from a region of lower solute concentration to a higher one. It occurs due to the difference in solute concentrations on either side of the membrane.
In simpler terms, osmotic pressure can be thought of as the 'pull' that a more-concentrated solution exerts on water molecules. The bigger the difference in concentration, the stronger the osmotic pressure. Eventually, the hydrostatic pressure created by the rising liquid column will balance this osmotic pressure, bringing osmosis to a halt.
Understanding osmotic pressure helps in many fields, including medicine, where it is essential for intravenous therapy and dialysis.

concentration gradient

A concentration gradient exists when there is a difference in solute concentrations between two regions. In osmosis, this gradient drives the movement of water from a less-concentrated area to a more-concentrated one.
Think of it like rolling a ball downhill; the water 'moves' down the gradient. This process continues until equilibrium is reached, meaning the concentrations are equal on both sides of the membrane. The existence of a concentration gradient is vital for many biological processes, as it helps in nutrient absorption and waste elimination in cells.
In simple scenarios like our exercise, understanding the concentration gradient lets us predict which way the water will move and what the eventual outcome will be.