Solutions | Unit 2 | Chemistry

Chapter 2 – Solutions

Solutions and their Concentration

A solution is a uniform combination of two or more components.

The mixture’s composition and qualities are constant.

The solute is the component that is present in lesser amounts, while the solvent is the component that is present in higher amounts.

Binary Solution:

 A binary solution is made up of the two elements solvent and solute.

Various Types of  Solutions

Both solvent and solute might be in solid, liquid, or gaseous forms.

There are nine different forms of binary solutions, depending on the solute or solvent state.

Solution’s Composition

We must first comprehend the components of solutions in order to comprehend them.

We need to comprehend both qualitative and quantitative methods of expressing the makeup of solutions.

A concentrated solution is one that has a lot more dissolved solute than solvent in comparison to a diluted solution.

When compared to a solvent, a dissolved solute must make up a small portion of a solution to be considered diluted.

60 g of salt are dissolved in 100 g of water to make a solution that is 60% salt by mass. 

Volume Percentage

90 mL of vinegar is included in 100 mL of water to make up the 90% vinegar solution in water.

  •  Mass by Volume  :- 40% The term “sugar solution in water mass by volume” refers to the process of dissolving 40 g of sugar in 100 ml of water.
  • Molality:-


  • Mole fraction:-

  • Molality(M):-


The term “solubility” refers to a substance’s maximal capacity to dissolve in a given amount of solvent at a certain temperature.

The nature of the solvent and the solute affects how they are soluble. Pressure and temperature are further factors.

A solid’s solubility in a liquid

If there are equivalent intermolecular interactions between the two, a solute will dissolve in the solvent. In polar solvents, polar solutes dissolve, while non-polar solutes dissolve in non-polar solvents.

For instance, naphthalene does not dissolve in water as easily as sodium chloride does. On the other hand, sodium chloride does not readily dissolve in benzene but naphthalene does.

A state of dynamic equilibrium exists when the amount of solute particles entering a solution equals the number of solute particles separating out.

  • Solute + Solvent ⇌ Solution
Saturated solution

At the same temperature and pressure, no more solute can be dissolved in a saturated solution.

Unsaturated solution

A solution that allows for the dissolution of more solute at the same temperature is an unsaturated solution.

Temperature impact:

Temperature variations have a big impact on how soluble a solid is in a liquid. Le Chatelier’s Principle must be followed for the dynamic equilibrium. In general, the dissolution process is endothermic if in a nearly saturated solution.

Effect of pressure:

Because solids and liquids are highly incompressible, the solubility of solids in liquids is not much impacted by pressure.

Gas Solubility in a Liquid

Pressure, temperature, and solubility of gases in liquids all have significant effects.

With an increase in pressure, gases become more soluble.

Hénry rule

William Henry established a quantifiable relationship between pressure and a gas’s solubility in a solvent in 1803.

The solubility of a gas in a liquid is directly proportional to the partial pressure of the gas, according to Henry’s equation, when the temperature is constant.

The partial pressure of a gas determines how soluble it is in a liquid solution.

The mole fraction of the gas (x) in the solution determines the partial pressure of the gas in the vapour phase (p).

  • p = KH x
Applications of Henry’s law: 

The bottle is sealed under high pressure to increase the solubility of CO2 in soft drinks and soda water.

The solubility of ambient gases in blood rises with pressure. The pressure steadily drops as the divers approach the surface. This causes the dissolved gases to be released and causes nitrogen bubbles to develop in the blood. By obstructing capillaries, this leads to bends, a medical ailment. Scuba divers’ tanks are filled with air that has been diluted with helium to prevent bends.

The partial pressure of oxygen is lower at high altitudes than it is at sea level. As a result, climbers or those who live at high altitudes have low oxygen levels in their blood and tissues. 

High altitude climbers who experience anoxia, or low blood oxygen levels, become weak and lose their mental clarity.

A gas’s solubility in a liquid and temperature

The breakdown of a gas in a liquid releases heat as it happens.

Consequently, a decrease in solubility is caused by a rise in temperature.

Liquid Remedies

When the solvent is a liquid, liquid solutions are created. A solid, liquid, or gas might be the solute.

One or more volatile components could be present in a liquid solution. The volatile substances will eventually evaporate when ingested in a closed container.

Liquid-liquid solution vapour pressure.

Each component of a solution of volatile liquids has a partial vapour pressure that is directly proportional to the mole fraction of that component that is present in the solution.

Let p total stand for the total vapour pressure and p 1 and p 2 represent the corresponding partial vapour pressures of the two components. The mole fractions x 1 and x2 of the two components 1 and 2, respectively, are connected to these partial pressures.

The first component:-

  • p 1 =p 1oox 1p2

where, p total is the pure component 2’s vapour pressure at the same temperature.

This quantitative relationship, often known as Raoult’s law, was discovered by the French scientist Francois Marte Raoult in 1886.

The law of partial pressures of Dalton:-

The partial pressures of the solution’s components added together will result in the total pressure (p total), which is represented as follows:

  • p total=p 1 +p 2=p 1o (1−x 2 )+p 2o x 2=pp1. +(p 2o −p 1o )x 2.

One can link the total vapour pressure over the solution to the mole fraction of any one component, according to the equation.

The mole fraction of component 2 affects the total vapour pressure over the solution linearly.

The total vapour pressure over the solution falls or increases with an increase in the mole fraction of component, depending on the vapour pressures of the pure components 1 and 2. 1. The partial pressures of the constituents dictate the composition of the vapour phase in equilibrium with the solution.

Using the partial pressures rule of Dalton:-

  • P 1 equals Y 1 P ooverall ,P 2 = Y 2 P overall.
Henry’s Law with a twist: Raoult’s Law

The partial pressure of the volatile component or gas is precisely proportional to its mole fraction in solution, according to the formulae for Raoult’s law and Henry’s law.

Only K H, the proportionality constant, deviates from p 1 o. As a result, Raoult’s rule turns into a specific instance of Henry’s law in which K H equals p 1 o.

Vapour Pressure of Solids in Liquid Solutions

Liquids vaporise at a specific temperature, and under equilibrium circumstances, the pressure that the liquid’s vapours exert over the liquid phase is referred to as vapour pressure.

Examples of solids that have been dissolved in liquids include sodium chloride, glucose, urea, and cane sugar in water, as well as iodine and sulphur.

It is discovered that the vapour pressure of the solution is lower than the vapour pressure of the pure solvent at the same temperature.

Because there are solvent and solute molecules on the surface of the solution, the amount of surface covered by solvent molecules decreases.

Regardless of the type of non-volatile solute present in the solution, the amount will determine how much the solvent’s vapour pressure will fall.

In other words, according to Raoult’s law, the partial vapour pressure of any volatile component in a solution is inversely proportional to the mole fraction of that component.

Only the solvent molecules are present in the vapour phase and contribute to vapour pressure when the solute is non-volatile.

Let p 1 represent the .

According to Roult Law;

p 1∝x 1p 1 =p 1o x1


Alternatives: Ideal and Non-Ideal

On the basis of Raoult’s Law, the liquid-liquid solutions can be divided into ideal and non-ideal solutions.

Ideal answers

Ideal solutions 

 Those that follow Raoult’s law throughout the whole concentration range.

The qualities of the optimum solution are as follows:

The pure components’ complete mixing to create the solution has no enthalpy. It denotes that when the components are combined, neither heat is absorbed nor released.

Specifically, combine H=0

The mixing volume is also zero. The solution’s volume would match the combined volumes of the two components. Specifically, blend V=0

Non-Ideal solution 

A solution is referred to as a non-ideal solution when it does not follow Raoult’s Law throughout the whole concentration range.

The attractive forces between solute and solvent molecules are not equivalent to those between solute and solvent molecules, i.e., the interactions between solute and solvent molecules A-A and B-B are not equivalent to those between solute and solvent molecules A-B.

Additional categories for the non-ideal solution include:-

  • Result demonstrating a positive deviation
  • Solution displaying a deviation in the negative
  • Positive departure from the optimum solution
  • A solution’s vapour pressure is more than what Raoult’s Law predicts.

In this situation, the intermolecular attractive forces between the solute-solvent molecules are weaker than those between the solute-solute and solvent-solvent molecules because A-B interactions are weaker than those between A-A or B-A.

Examples:-  ethanol and acetone, carbon disulphide and acetone


Azeotropes are binary mixes that boil at a constant temperature and have the same composition in their liquid and vapour phases.

The terms lowest boiling azeotrope and maximum boiling azeotrope refer to two different forms of azeotropes.

The solutions that deviate significantly from Raoult’s law at a given composition create a minimal boiling azeotrope. For instance, fractional distillation of an ethanol-water mixture produced by the fermentation of sugars results in a solution that contains about 95% by volume of ethanol.

The solutions that exhibit a significant departure from Raoult’s law at a given composition form the maximum boiling azeotrope. This type of azeotrope can be seen, for instance, in nitric acid and water. This azeotrope has a boiling point of 393.5 K and an estimated composition of 68% nitric acid and 32% water by mass.

Coligative Characteristics

Collaborative qualities are those of a solution that depend just on the quantity of solute particles, not on the type of solute.

Four colligative qualities exist:-

  • Lowering of the vapour pressure relative
  • a rise in the boiling point
  • Lowering of the freezing point

READ MORE :- Chemistry Class 12 Chapter 1 – The Solid State

  1. Osmotic force

Vapour pressure Relatively Declining First

Raoult demonstrated that the reduction in vapour pressure is solely a function of the solute particle concentration.

The link between mole fraction, solvent vapour pressure, and solution vapour pressure is shown by the equation below:-

  • P \s1\s​\s =X \s1\s​\s P \s1\s0

​The decrease in the solvent’s vapour pressure (P 1) is expressed as:-

  • P 1 =P 1 0 P 1 =P 1 0 P 1 0 X 1.

  1. Boiling Point Elevation

A liquid’s vapour pressure rises as its temperature climbs. When the vapour pressure of an object equals the air pressure, it boils.

At 373.15 K, the vapour pressure of a solution of sucrose in water is less than 1.013 bar. The temperature of this solution must be raised above the boiling point of the pure solvent in order to for its vapour pressure to rise to 1.013 bar (water).

A solution’s boiling point is almost usually higher than the boiling point of the pure solvent used to create it.

  1. Freezing Point Depression

In comparison to the pure solvent, a solution’s freezing point decreases as its vapour pressure decreases.

The temperature at which a substance’s vapour pressure in its liquid phase equals that substance’s vapour pressure in its solid phase is known as the freezing point of that substance.

According to Raoult’s law, the vapour pressure of the solvent lowers when a non-volatile solid is introduced. The solvent’s freezing point drops as a result.

  •  4.Osmosis and Osmotic Pressure

Semipermeable membranes are those that let smaller solvent molecules, such as water, pass through them but block the passage of larger molecules (SPM).

Osmosis is the process by which solvent molecules move from a pure solvent to a solution across a semi-permeable membrane. The flow won’t stop until balance is reached.

Osmotic pressure is the additional pressure that must be supplied to a solution in order to block the flow of solvent into the solution across a semipermeable membrane.

​Applications of osmosis: 

The transfer of water from the soil into the roots of plants and then into their higher parts.

A bacterium on salted meat or sour fruit loses water by osmosis, shrivels, and eventually dies.

renal reabsorption of water.

Water purification and Reverse Osmosis

If a pressure greater than the osmotic pressure is applied to the solution side, the direction of osmosis can be reversed.

Reverse osmosis is the process of moving a solvent from a solution to a pure solvent through a semipermeable membrane by exerting too much pressure on the solution side.

Abnormal Molar Masses

Sometimes it is discovered that the theoretical estimates of molecular mass that are derived from the collative properties of solutions don’t match the estimates that were acquired empirically. Abnormal molar masses are the common term used to describe these readings.

The total number of particles after the dissociation or affiliation of the solute molecules in the solvent or solution determines abnormal molar masses.

The number of particles rises as a result of the breakdown of solute molecules into many ions. As a result, the solution’s colligative qualities improve.

For instance: One mole of sodium chloride (NaCl) dissociates into two moles of sodium ions (Na +) and one mole of chloride ions (Cl ) when it is dissolved in water. As a result, the solution contains two moles of particles.

  • NaCl→Na+Cl=2

The number of particles in the solution drops as a result of solute particles interacting with one another, which lowers the colligative characteristics. The molar mass values found here are higher than anticipated.

As an illustration, when acetic acid (also known as ethanol) is dissolved in benzene, its molecules form hydrogen bonds with one another, therefore lowering the number of particles. This typically occurs in liquids with low dielectric constants.

After conducting numerous studies, the Dutch scientist Van’t Hoff developed a factor known as the Van’t Hoff factor, which accounts for molar masses that are higher or lower than anticipated as a result of solute dissociation or association.

Elevation of Boiling point, ΔT 
  • b=iK
Depression of Freezing point,ΔT 
  • f=iK f m
Osmotic pressure of solution,
  • Π=iCRT

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