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## Solutions

Homogeneous mixtures

The component present in larger amount and determines the physical state of the solution is called solvent

The component which is present in lesser amount

Composition and properties are uniform throughout the mixture

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## Solubility

Solubility: Maximum amount of substance that can be dissolved in a specified amount of solvent at a specified temperature is called its solubility.

Saturated solution: A solution in which no more solute can be dissolved at a given temperature is said to be a saturated solution.

Unsaturated solution: A solution in which the solute can be dissolved at a given temperature is said to be a non-saturated solution or unsaturated solution

### Factors affecting solubility of a solid in a liquid:

1. Nature of solute and solvent: Polar solutes dissolve in polar solvents and non-polar solutes in non-polar solvents.
2. Effect of temperature:

a)      If the dissolution process is endothermic (∆sol H > 0), the solubility increases with rise in temperature.

b)      If dissolution process is exothermic (∆sol H < 0), the solubility decreases with rise in temperature.

### Factors affecting solubility of a gas in a liquid:

1. Effect of temperature: As dissolution is an exothermic process, then according to Le Chatelier’s principle, the solubility should decrease with increase of temperature.

### Effect of pressure:

#### Henry’s law Pgas = KH x Xgas

“The partial pressure of the gas (p) is directly proportional to the mole fraction of the gas (x) in the solution”

KH is the Henry’s law constant and is different for different gases at a particular temperature.

Higher the value of KH at a given pressure, the lower is the solubility of the gas in the liquid.

#### Limitation of Henry’s Law

(i) This law is applicable to the gaseous solutes which show only ideal behaviour at high temperature and low pressure.

(ii) This law is applicable to solute which does not undergo association or dissociation when the solute is dissolved in solvent.

(iii) This law is applicable to gaseous solute which does not form any product by the chemical reaction, when gaseous solute is dissolved in liquid solvent.

#### Raoult’s law

It states that for a solution of volatile liquids, the partial vapour pressure of each component of the solution is directly proportional to its mole fraction present in

If Y1 and Y2 are the mole fractions of the components 1 and 2 respectively in the vapour phase then, P1 = Y1 Ptotal and P2 = Y2 Ptotal

## Ideal Solutions

1. The Solutions which obey Raoult’s Law at all Compositions or Concentrations
2. The enthalpy of mixing of pure components to form the solution is zero. ∆Hmix = 0
3. The volume change during mixing is also zero. ∆Hmix = 0

Examples:

1. Toluene and Benzene
2. Ethyl Iodide and Ethyl Bromide
3. Bromobenzene and Chlorobenzene
4. n-Heptane and n-Hexane

Non – Ideal Solutions

The solutions which do not obey Raoult’s law at all compositions or concentrations

Types of non-ideal solution

Non – Ideal Solution with Positive Deviation

Non – Ideal Solution with Negative Deviation

## Azeotropes

The mixtures of liquids which boil at constant temperature like a pure liquid and possess same composition of components in liquid as well as vapour phase are called constant boiling mixtures or azeotropic mixtures

### Minimum Boiling Azeotropes

Show large positive deviations from Raoult’s law

The mixture will have a minimum boiling point which is less than the boiling point of two components.

95% ethanol + 5% water

97% chloroform + 3% water

### Maximum Boiling Azeotropes

Show large negative deviations from Raoult’s law

The mixture will boil at a higher temperature than that of the pure components.

1.Sulfuric acid (98.3%) / water, boils at 338 °C

2.Nitric acid (68%) / water, boils at 120.2 °C
3.Perchloric acid (71.6%) / water, boils at 203 °C
4.Hydrofluoric acid (35.6%) / water, boils at 111.35 °c

### Colligative properties

These are the properties of dilute solutions of non-volatile solute in a volatile solvent which depend only on the number of solute particles but not on its nature.

The types of colligative properties

1. Relative lowering of vapour pressure
2. Elevation in boiling point
3. Depression in freezing point
4. Osmotic pressure

#### Relative lowering of vapour pressure

1. When a non- volatile solute is dissolved in a solvent, vapour pressure of the solution is lower than that of the pure solvent which is known as lowering of vapour pressure.
2. Relative lowering of vapour pressure is equal to the mole fraction of the solute in the solution. P10= Vapour pressure of pure solvent

P1= Vapour pressure of pure solution

n2= number of moles of solute

n1= number of moles of solvent

Hence the equation can be written as

For a dilute solution, n1 >>> n2

Where w1, w2 are the masses of solvent and solute and M1 and M2 are the molecular masses of solvent and solute respectively #### Elevation in boiling point

The boiling point of a solution containing a non-volatile solute is always higher than the boiling point of the pure solvent. This increase in boiling point is termed as elevation in boiling point.

The difference in the Boiling Points of Solution and that of Pure Solvent is known as Elevation in Boiling Point.

Where Tb0 and Tb are boiling points of pure solvent and solution respectively

It is observed that the elevation in boiling point is directly proportional to molal concentration of the solute. ### Sanjay Bhandari

Hello Friends, My name is Sanjay Bhandari. I am a chemistry Teacher.