Two beakers are placed in a sealed container surrounded with air. In one beaker is pure H2O (Beaker A) and the other beaker is a 10% Sugar solution (Beaker B). As the system approaches equilibrium what change occurs to the volume of water in beaker A? What happens to the concentration of the sugar solution in Beaker B. Explain why these changes occur.
The osmotic pressure of the sugar solution is higher (the vapor pressure is lower) and water molecules from the beaker of pure water will evaporate and condense in the beaker with the sugar solution. It does this in an effort to, in effect, dilute the sugar solution to pure water. Of course that never happens.
To understand what happens to the volume of water in beaker A and the concentration of the sugar solution in beaker B as the system approaches equilibrium, we need to consider the process of osmosis and the movement of water molecules.
Osmosis is the movement of solvent molecules (in this case, water) from an area of lower solute concentration to an area of higher solute concentration through a semi-permeable membrane. In this case, the semi-permeable membrane is the boundary between the two beakers.
Since beaker A contains pure water and beaker B contains a 10% sugar solution, the concentration of solute (sugar) in beaker B is higher than in beaker A. As a result, water molecules will tend to move from beaker A to beaker B in an attempt to equalize the concentration of solute on both sides of the membrane.
As the system approaches equilibrium, the following changes occur:
1. Volume of water in beaker A: The net movement of water molecules is from beaker A to beaker B. This means that water is leaving beaker A, resulting in a decrease in volume in beaker A.
2. Concentration of the sugar solution in beaker B: The net movement of water molecules from beaker A to beaker B dilutes the sugar solution in beaker B. As water enters beaker B, it decreases the concentration of the sugar, resulting in a lower sugar concentration in beaker B.
These changes occur because of the natural tendency of water molecules to move from an area of lower solute concentration (pure water) to an area of higher solute concentration (sugar solution) in an attempt to equalize the concentration on both sides of the membrane. This process continues until the concentration becomes equal or until equilibrium is reached.