THE osmosis studies the spontaneous passage of solvent from a more dilute to a more concentrated solution through a semipermeable membrane. The word osmosis comes from Greek osmóswhich means boost.

THE osmometry studies the osmotic pressure measurement of solutions.

The solutions must be of the same solute in order to equalize the concentration.

To prevent the solvent from passing to the more concentrated solution, pressure is added to the concentrated solution.

For osmosis, there are three types of membranes:

- permeable
- waterproof
- semipermeable

At permeable membranes allow solvent and solute to pass through. For example, a thin cotton cloth is used.

At waterproof membranes they do not let solvent or solute pass through. At semipermeable membranes has selective action as to the type of substance that can pass through it.

They let themselves go through some substances, but not others. Allow the passage of the solvent and prevent the passage of the solute. Examples of semipermeable maneuvers:

- vegetal paper
- parchment paper
- animal gut (those involving sausage and sausage)
- animal bladder
- cellulose acetate film - cellophane paper
- cell membrane
- porcelain membrane containing cupric ferrocyanide Cu2Fe (CN)6

Osmotic pressure It is the pressure that must be applied to the solution so as not to let the solvent cross the semipermeable membrane.

It is the pressure that must be exerted on a system to prevent osmosis from occurring naturally. For the calculation of osmotic pressure, the following expression is used:

For ionic solutions:


Solutions can be rated for their osmotic pressures. Being two solutions A and B with the same temperature:

Hypertonic, isotonic, and hypotonic refer to solution A relative to solution B.

Osmotic pressure calculation example:

Calculate the osmotic pressure of a sucrose solution (C12H22O11) 34.2 g of this solute were dissolved in 0.5 L of solvent at 27 ° C. Given: MM = 342g / mol.

Firstly, the number of moles in 34.2g of sucrose should be calculated. Then find the value of 27 ° C in Kelvin, totaling 273 or 273.15.

And lastly, apply the formula without Van't Hoff's correction factor, because sucrose is a molecular and nonionic compound.

The ionic colligative effect is greater than the molecular colligative effect.

Osmotic pressure is very important and explains a number of phenomena that occur.

The fact that vegetables wither after being seasoned with salt happens because salt removes water from the cells of the vegetables. Dried fruits, such as black plum, swell when placed in water.

It also helps in the preservation of food, such as salted meat and fruits cooked in very sweet syrup, preventing them from spoiling easily.

Fish have different metabolism according to the type of water they live in. The fish's body is made up of lots of water and other substances dissolved in it.

Because saltwater has more salts than freshwater, a saltwater fish could not live in freshwater and vice versa. It would cause an imbalance between the internal osmotic pressure of the fish organism and the external osmotic pressure of the water.

In cases of dehydration, where there is a lot of water loss, it is necessary to replace the body with saline, composed of boiled water, a tablespoon of table salt and two spoons of sugar, to rebalance the osmotic pressure of the body.

Summary of Colligative Properties




Decreased solvent vapor pressure


Boiling temperature increase


Freezing temperature decrease


Increased Osmotic Pressure