1- DIFFUSION - OSMOSIS - OSMOTIC PRESSURE:

You know what a solution is and how to quantify it.
If you have any problems with this topic go to this tutorial:

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Now, let have l look at three other concepts:

Simple Diffusion

In a solution, solute particles are continually moving about, colliding with one another and moving off in various directions. The random mixing of solute particles in a solution is called simple diffusion. The flow of solute particles moving in any direction equals the flow of particles moving in the opposite direction.

Net Diffusion

Net diffusion is when the flow of solute particle moving in one direction is bigger than the flow of solute particles moving in the opposite direction.

At school you learned that particles move down their gradient of concentration (from a region of high solute concentration to region of low solute concentration) until the same concentration of particles is reached everywhere. This is an incomplete story: solute particles can move also along electrical gradient, temperature gradient, pressure gradient etc... and you have to take all these factors into account.

Thus, It is more accurate to say that Solute particles move from one place to another because of differences in their potential energy.
Solute particles move from a region where their potential energy is greater to a region where their potential energy is lower, regardless of the reason for the potential difference. There is net movement of particles until the same potential energy is reached everywhere.

In the lab exercise, the gradient of potential energy of solute will be caused by their gradient of concentration (everything else: temperature, pressure... is the same in the two regions). As the concentration of solute particles increases, their potential energy increases. Thus, solute particles will move from the region of higher solute particle concentration (where their potential energy is higher) to region of lower solute particle concentration (where their potential energy is lower) until same the concentration (potential energy) is reached in all regions.

Osmosis

Osmosis is the net diffusion of water molecules that is caused by a difference in osmolarity between the two compartments.

Please note: Like solute particles, Water molecules also move from one place to another because of differences in their Potential Energy. A difference in osmolarity between compartments is only one of the many factors that can cause a gradient in Water potential energy.

Water moves from a region where its Potential Energy is greater to a region where its Potential Energy is lower, regardless of the reason for the water Potential Energy difference. Water will move until the same water potential energy is reached in all regions.
As with Solute, the differences in Potential Energy of Water between two regions can be caused by several factors (differences in entropy, osmolarity, pressure, temperature, etc.....)

What is the relationship between osmolarity and water potential energy?
As osmolarity of a solution (i.e. concentration of solute particles) increases, the water potential energy decreases. WHY?

Water molecules move from regions of high water potential energy to regions of lower water potential energy. This means that water moves from regions of low concentration of solute particles (high water potential energy) to regions of high concentration of solute particles (low water potential energy).

In high school, you learned that "as concentration of solute particles increases, the concentration of water molecules per unit volume of solution decreases and vice versa. Thus, water moves from regions of high water concentration to regions of low water concentration"
From now on, relate the movement of water to the gradient of water potential energy between regions. Explaining the movement of water as a function of only the gradient of solute particle concentration or water concentration can be misleading in the long run because you will assume automatically that there is no gradient of temperature and pressure (and other factors which affect water potential energy) and you will forget to take them into consideration when needed.

Osmotic Pressure

Osmotic Pressure of a solution is the force per unit of surface exerted by the flow of water moving by osmosis from a region containing distilled water to a region containing the solution, the two regions being separated by a semi-permeable membrane (membrane permeable to water and impermeable to solutes).

The Osmotic Pressure p of a solution is calculated with the following formula: