Difference Between Osmolarity and Tonicity

Main Difference – Osmolarity vs Tonicity

The terms Osmolarity and Tonicity are closely related to each other. They refer to the concept of concentrations of solutions and how they affect the osmotic pressure. However, the way they express their effects is different from each other. Both terms relate to situations where solutions are separated by permeable membranes where osmosis takes place. Osmolarity takes into consideration all the molecules present in the solution for its collective effect whereas, tonicity only considers the concentration of the non-permeable solute molecules within the solution. Therefore, it can be said that tonicity is a measure of the external environment, whereas osmolarity concerns the given solution itself. This is the main difference between osmolarity and tonicity.

What is Osmolarity

Osmolarity is a way of expressing the concentration of a solution by means of the number of osmoles present in the solution rather than the well-known way of considering the number of moles in a solution which is referred to as ’molarity’. Therefore instead of the unit mol/L like in the case of molarity, osmolarity uses the units osmoles of solute per litre or osmol/L. In many cases, the value for osmolarity is similar to the value of molarity, however, in the case of water-soluble compounds, the osmolarity becomes greater than the solution’s molarity. This is because, osmolarity involves the total amount of solutes present whereas the molarity involves the concentration of the compound as a whole. For instance; NaCl once dissolved in water separates into its ions as Na+ and Cl. If  NaCl had a concentration of 2 mol/L at the start, upon the dissolution in water, its molarity remains 2 mol/L but its osmolarity increases to 4 osmol/L, due to the consideration of the total number of solutes in solution now, which are the separated ions. (2 osmol/L of Na+ and 2 osmol/L of Cl).

Depending on the characteristics of the surrounding environment, the osmolarity of a given solution can take three different forms. Isosmotic, hyperosmotic and hypoosmotic. When the osmotic pressure of the given solution is equal to its surrounding, it is called an isosmotic solution. A solution is hyperosmotic if its osmotic pressure is higher than its surrounding. Similarly, a solution is called hypoosmotic if its osmotic pressure is lower than its surroundings.

Difference Between Osmolarity and Tonicity

What is Tonicity

Unlike osmolarity, tonicity is influenced by the solutes that do not cross the membrane. Therefore, it is a measure of the osmotic pressure gradient and not the osmotic pressure itself. The concept of tonicity is helpful in describing the reactions and behaviour of a cell immersed in different strengths of concentrations. If a cell is placed in an isotonic solution, the concentration of the solution is similar to the concentration of the cellular contents. Therefore, there will not be an influx or efflux of the solvent molecules through the semi-permeable membrane. However, if the cell is placed in a solution which is hypertonic, the concentration of the solution is higher than the concentration of the cellular contents. Therefore the solvent molecules (in this case its water), will flow out of the cell till the concentrations are equal on either side of the membrane, causing it to shrink. Similarly, if the cell was placed in a hypotonic solution, as the concentration of the solution is lower than that of the cellular contents, there will be an influx of water into the cell, causing the cell to swell and burst off eventually.

Main Difference - Osmolarity vs Tonicity

Effect of different solutions on blood cells

Difference Between Osmolarity and Tonicity


Osmolarity is a measure of the osmotic pressure of a given solution.

Tonicity is a measure of the osmotic pressure gradient between two solutions separated by a permeable membrane.

Units of Measurement

Osmolarity is measured in osmol/L.

Tonicity is related to concentration measurements and is expressed in terms of mol/L.

Defining Concentration

Osmolarity takes into consideration the total number of solutes that penetrates and do not penetrate the membrane.

Tonicity takes into account the concentration relating to the non-permeable solutes.

Effect of Surrounding

Osmolarity often represents the analysis of a given solution.

Tonicity is used as a measure of the external environment.Difference Between Osmolarity and Tonicity- infographic

Image Courtesy:

“Osmotic pressure on blood cells diagram” by LadyofHats. (Public Domain) via

“Osmosis diagram” by KDS4444 – Own work. (CC0) via

About the Author: admin

Related pages

definition of a plateau landformcast iron vs wrought ironwhat is the difference between anaerobic respiration and aerobic respirationeluded meansdefinition of alkali earth metalare corduroy pants business casualmacro and micronutrients for humansblank verse refers todifference between an atheist and agnosticabstract diction examplestypes of cocciwhy india called bharatthe superego definitionart nouveau movement artistssoldering brazingomniscient examplesdifferent between frog and toadtypes of seeds monocot and dicotparallelism example in literaturespell aestheticsdifference between jaguar and leoparddefinition totipotentdifferences between reducing and nonreducing sugarhomophonic puns examplesuracil definition biologyhow does a concrete noun differ from an abstract nouncell plate formation in plantsdifference between cast iron and pig irontetrad definitionwhat is the meaning of paralanguagefet vs bipolar transistordifference between a dolphin and a porpoisevibration oscillationbigamy and polygamydifference between static and kinetic frictionde fakto i de jurethe meaning of didactickinds of nounswhat is orthopnoeadifference between declarative and imperativegravity and electromagnetismwhat is difference between curriculum and syllabusmultinational firm definitionthermosetting plastic and thermoplasticpimple vs zitwhat is the difference between race and ethnicity and nationalitysmooth endoplasmic recticulumbacteriophage definition biologygroan and moanthe difference between evaporation and condensationdifference between solenoid and inductorsimilarities and differences between glucose and fructoseadenoma definemam sir spellingsigns of german measlesautotroph and heterotroph definitionendosperm of gymnospermis filtration active or passiveeasy ways to memorize a speechmeaning of multinational corporationpreface prologuedifference between thrombosis and embolismwhat does smooching meanwhat is difference between malt and shakecomedy of manners characteristicsdefinition of stanza in literaturekinetic vs static frictionmultinational organisation definitionwhat does the word enunciate meandifference between federation and republichow to tell difference between yam and sweet potatobaking powder bicarbonate of soda differenceoncotic pressure definitiondifference between emigrants and immigrantsdifferent kinds of adjectivescilia and flagella movementliterary definition of monologuethe definition of static characterdifference between cold blooded animals and warm blooded animals