The Ultimate Face-off: Hydrostatic Pressure Vs. Osmotic Pressure

Examples of hydrostatic pressure and osmotic pressure
The concepts of hydrostatic pressure and osmotic pressure can be quite confusing, and at times difficult to imagine and grasp. This piece illustrates and explains them in a simple manner, and provides a direct comparison between the two, for better understanding.
Did You Know?
Hydrostatic and osmotic pressure play a vital role in our blood capillaries. Hydrostatic pressure ensures proper supply of blood to all parts of the body, whereas osmotic pressure is responsible for the exchange of all necessary fluids.
It would not be an exaggeration to say that fluids are prominently responsible for the genesis and survival of life on earth. They form all of our environment and the elements in it. And like all matter, fluids have certain composition and physical properties. These properties make them function in certain ways in particular situations. Hydrostatic and osmotic pressures are phenomena responsible for the interactions and actions of fluids that we use to get a lot of our work done. They are prominent in areas like biology, chemistry, plant sciences, hydrostatics, besides many other fields.

Both phenomena work on the simple principle of pressure, but under very different circumstances, conditions, and ways of working. Pressure is defined as force applied per unit area.
Hydrostatic Pressure Vs. Osmotic Pressure
Hydrostatic Pressure Definition
This is the pressure exerted by a certain amount of fluid in static condition. It is applied due to the weight of the fluid. We can also say that it is the pressure experienced by a certain point in the fluid. The pressure at a certain position increases with the increase in the height of the fluid column. The opposite is also true, i.e., for a fluid of a given column height, the pressure increases with increasing depth.
Osmotic Pressure Definition
It is the minimum pressure required to prevent the flow of a fluid through a semi-permeable membrane. Whenever a solution (solvent + solute) is separated by a semi-permeable membrane (which allows the passage of solvent only) with different proportions of solute in either cells, the solution is in an unstable state. The solvent tends to attain equilibrium by moving on the higher density side, thus obtaining an equal density solution throughout. This process of attaining equilibrium is called osmosis.
Understanding Hydrostatic Pressure
A simple way of understanding hydrostatic pressure is illustrated in the image.
Bucket with holes and water flows
A drum is pierced with 3 holes of equal size at different heights. Water is filled in the drum up to a level above all 3 holes. As you can see, the reach of the water coming out from each of these holes is different. This happens due to different pressures at each position. The hole with the most height has the least reach, i.e., the pressure at this point is the least. Whereas, the hole with the least height has the highest reach. So, the pressure at this point is the highest. This is due to the force of water weight acting on that level. The amount of water in the drum above the highest level point is the least. And evidently, the force due to its weight is also less, as compared to the other two hole levels.
Understanding Osmotic Pressure
As seen in the image, two states of a solution are shown. A shows the initial condition, where the solution is in an unstable state. B shows the equilibrium state.
Osmosis concept
The U-tube contains a solution in both compartments separated by a semi-permeable membrane at the center. The green spheres indicate molecules of the solute, whereas, the red ones indicate those of the solvent. Consider A, where the amount of solute on both sides is different, whereas, the amount of solvent is the same. Osmosis needs force to work. In this case, the force is supplied by the interaction between the solute and the membrane. The solute particles move randomly in the solution. When they come near the membrane, they are repelled. This repulsion gives them a momentum which is directed away from the membrane. The momentum is transferred to the other solvent molecules, resulting in them moving away from the membrane as well. The repulsive force is greater in the compartment with greater number of solute molecules, as the number of molecules randomly coming near the membrane are greater. So, the solvent from the left compartment (low density) effectively moves to the right compartment (high density), creating a solution of equal concentration throughout. Conclusively, the final equilibrium state is B, as shown.
Calculation of Hydrostatic Pressure
The following formula can be used for the calculation of the hydrostatic pressure of a column of liquid (in SI units):

Hydrostatic Pressure (N/m2) = Height (m) × Density (kg/m3) × Gravity (m/s2)


Pa = h × ρ × g

Density changes with change in temperature. So it is preferred to mention the temperature at which the pressure is being measured.
Calculation of Osmotic Pressure
The osmotic pressure can be considered to be acting for gases. So, ideal gas laws can be applied to the fluid. The osmotic pressure equation, therefore, can be given as

P = nRT/V

P is the osmotic pressure
V is the volume of the solution
n is the amount of solute or number of moles of molecules present in the volume
T is the average temperature of the solution
R is the gas constant value which is 8.314 joules per degree Kelvin.
Examples of Hydrostatic Pressure
The principle of hydrostatic pressure is used to encounter various real-life problems.

For example, a water cooler with an inverted water container works on this very principle. The pressure at the tap is due to the hydrostatic pressure created by the fluid.
Electric water cooler
Another classic example is hydroelectric power generation. Water is collected in a dam, and after reaching a sufficient amount, it is released on turbines causing them to rotate at high speeds. These rotating turbines are coupled with generators that produce electricity at the output.

On a domestic level, water storage tanks are located at a height, so water is supplied to all pipes because of the hydrostatic pressure created.
Examples of Osmotic Pressure
Osmosis and the use of osmotic pressure can be observed for various applications in everyday life. A lot of significant biological processes take place because of it.

It is responsible for water transport in plants. Water enters the roots via osmosis, and is supplied to all the parts due to this pressure difference.
Watering to plant
Food preservation techniques like using sugar to store jams and jellies, and using salt to preserve meat, use osmosis to keep away organisms that spoil the food. When such organisms come in contact with these foods, the water is drawn out by osmosis, and they die of dehydration.
Instruments to Measure Hydrostatic Pressure
  • Manometers
  • Piston-type gauge
  • Bourdon pressure gauge
  • McLeod gauge
  • Aneroid gauge
  • Diaphragm gauge
Closeup of manometer
Instruments to Measure Osmotic Pressure
An osmometer is an instrument that is used to measure the osmotic strength of a solution, which is nothing but the solution's concentration. You get he value of 'n' in the above mentioned formula. As the rest of the values are either known or deducible, the value of osmotic pressure 'P' can be calculated.
This was a brief overview and some subtle differences between osmotic pressure and hydrostatic pressure. Both these concepts have a vital role in the functioning of various daily life processes. Feel free to share other instances/examples, in the comments section below!