Which factor primarily determines the direction of osmosis across a semi-permeable membrane?

The direction of osmosis across a semi-permeable membrane is predominantly determined by the concentration gradient of solute. Osmosis is the movement of water molecules from an area of lower solute concentration to an area of higher solute concentration. This process continues until the solute concentrations on both sides of the membrane reach equilibrium.

When considering the concentration gradient, it is important to remember that water moves in response to differences in solute concentration. Therefore, the greater the difference in solute concentration, the more significant the osmotic pressure, driving water to move toward the area with higher solute concentration. This movement helps to equalize solute concentrations on both sides of the membrane.

While other factors such as temperature and pressure can influence the rate of osmosis or the physical behavior of the solutions involved, the fundamental driving force for osmosis itself is the concentration gradient of solute. This is why the concentration gradient is the primary factor determining the direction of osmosis.