CBSE Class 12 Chemistry Notes Chapter 2 Solutions

Solutions are homogeneous mixtures comprising two or more substances, classified as solid, liquid, or gaseous.

• Henry’s Law states that the solubility of a gas is directly proportional to its partial pressure at a given temperature.
• Raoult’s Law describes the lowering of solvent vapor pressure in the presence of a non-volatile solute, with the relative lowering proportional to the solute’s mole fraction.
• Ideal Solutions obey Raoult’s Law across all concentrations and azeotropes occur due to significant deviations from Raoult’s Law.

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Class 12 Chemistry Chapter 2 Notes - Solutions

Components of Solutions

• Solutions exhibit uniform composition and properties throughout the mixture. 
• It consists of solvent and solutes, with the solvent being the predominant component determining the solution's physical state.
• Solvent choice determines the state of the solution.
• The solute is the other component in the solution apart from the solvent, present in lesser quantities.
• Solutions can exist in solid, liquid, or gaseous states depending on the solvent and solute characteristics.

Types of Solutions

The solutions can be classified into three categories which are as follows:

On the basis of water as a solvent

It is divided into two categories which are as follows:

• Aqueous Solution: A solution is said to be an aqueous solution when the amount of solute dissolved in the solute is water.
• Non-aqueous Solution: A solution is said to be an aqueous solution when the amount of solute dissolved in the solute is not water.

On the basis amount of solute

It is divided into three categories which are as follows:

• Saturated Solution: A solution is said to be saturated when it cannot dissolve any more solute.
• Unsaturated Solution: A solution is said to be saturated when it can dissolve any more solute.
• Supersaturated Solution: A solution is said to be saturated when it dissolve maximum amount of solute.

On the basis of amount of solvent

It is divided into two categories which are as follows:

• Concentrated Solution: A solution is said to be concentrated when the amount of solute is more than the amount of solvent.
• Diluted Solution: A solution is said to be concentrated when the amount of solute is less than the amount of solvent.

Types of Solutions

Expressing Concentration of Solutions 

• Mass Percentage = Mass of solute/Mass of solution × 100
• Volume Percentage = Volume of solute/Volume of solution × 100
• Mass by Volume Percentage = Mass of solute/Volume of solution × 100
• Parts per Million (1 ppm) = Mass of solute/Total mass of solution ×106
• Mole Fraction = Moles of solute/Total moles of solute and solvent
• Molarity (M) = Moles of solute/volume of solution in liters
• Molality (m) = Moles of solute/Mass of solvent in kilograms

Solubility

• Solubility refers to the maximum amount of solute that can dissolve in a solvent at a given temperature and pressure to form a saturated solution.
• It is usually expressed in terms of grams of solute per 100 grams of solvent, or in moles per liter of solution.

Solution

Solubility of a Solid in a Liquid

• Describes the maximum amount of solid solute that can dissolve in a liquid solvent at a given temperature.
• Usually increases with temperature for most solid solutes.
• Expressed as grams of solute per 100 grams of solvent, or in moles per liter of solution.

Solubility of a Gas in a Liquid

• Refers to the maximum amount of gas solute that can dissolve in a liquid solvent at a specific temperature and pressure.
• Generally decreases with increasing temperature for most gases.
• Often expressed as the volume of gas dissolved per unit volume of solvent, or in moles per liter of solution.

Vapour Pressure of Liquid Solutions

• Vapour pressure is the pressure exerted by a vapor in thermodynamic equilibrium with its condensed phases.
• In liquid solutions, the presence of a non-volatile solute lowers the vapor pressure of the solvent.
• The decrease in vapor pressure is proportional to the mole fraction of the solute in the solution.

Vapour Pressure of Liquid Solutions

Raoult’s Law as a Special Case of Henry’s Law

• Raoult’s Law applies to ideal solutions and states that the vapor pressure of each component in the solution is directly proportional to its mole fraction.
• It's a special case of Henry’s Law, which relates the concentration of a gas in a solution to its partial pressure above the solution.
• Raoult's Law applies primarily to liquid-liquid solutions where the components are volatile.

Vapour Pressure of Solutions of Solids in Liquids

• When a non-volatile solute is dissolved in a liquid solvent, the vapor pressure of the solution decreases.
• This decrease is directly proportional to the mole fraction of the solute and is described by Raoult’s Law.
• Solids in liquids do not exert significant vapor pressure unless they undergo a chemical reaction with the solvent.

Ideal Solutions

• Ideal solutions obey Raoult’s Law over the entire range of concentrations.
• In ideal solutions, the interactions between solute-solute, solvent-solvent, and solute-solvent particles are similar.
• The vapor pressure of each component in an ideal solution follows Raoult’s Law.
• It states that the partial vapor pressure of each component is directly proportional to its mole fraction in the solution.

Vapour Pressure diagram showing positive deviation

Nonideal Solutions

• Non-ideal solutions deviate from Raoult’s Law at certain concentrations or conditions.
• Deviations from ideal behavior occur due to differences in the nature and strength of intermolecular forces between solute and solvent molecules.
• Nonideal behavior can result in positive or negative deviations from Raoult’s Law, depending on the specific interactions between solute and solvent molecules.
• Positive deviations occur when the vapor pressure of the solution is higher than predicted by Raoult’s Law, while negative deviations occur when the vapor pressure is lower than predicted.

Colligative Properties and Determination of Molar Mass

• Colligative properties are those properties of a solution that depend only on the number of solute particles and not on their chemical nature.
• Examples include lowering of vapor pressure, elevation of boiling point, depression of freezing point, and osmotic pressure.
• Colligative properties are used to determine the molar mass of solutes.
• By measuring the extent of colligative property changes, such as freezing point depression or boiling point elevation, the molar mass of the solute can be calculated using colligative property equations.

Relative Lowering of Vapour Pressure

It is the decrease in the vapor pressure of a solvent in a solution compared to the pure solvent due to the presence of solute particles.

Elevation of Boiling Point

The boiling point of a solvent increases when a non-volatile solute is added to it, as it requires higher temperature to overcome the increased vapor pressure.

Depression of Freezing Point

Adding a solute to a solvent lowers its freezing point, meaning the solution freezes at a temperature lower than the pure solvent due to reduced vapor pressure.

Osmosis and Osmotic Pressure

• Osmosis is the movement of solvent molecules across a semi-permeable membrane from a region of lower solute concentration to a higher solute concentration.
• Osmotic pressure is the pressure required to prevent the osmotic movement of solvent molecules across the membrane.

Reverse Osmosis and Water Purification

• Reverse osmosis is a water purification process that uses a semi-permeable membrane to remove ions, molecules, and larger particles from drinking water.
• Pressure is applied to the solution side, forcing water through the membrane while leaving contaminants behind.

Reverse Osmosis

Abnormal Molar Masses

• Some solutes in solution exhibit molar masses that are different from their actual molecular weights due to association or dissociation.
• Dissociation leads to lower apparent molar masses, while association results in higher apparent molar masses.

Van't Hoff Factor (i)

• The Van't Hoff factor (i) is the ratio of the observed colligative property to the expected colligative property for a given solute concentration.
• It helps quantify the extent of association or dissociation of solute particles in a solution.
• The Van't Hoff factor accounts for the number of particles into which a solute dissociates or associates in solution.

Van't Hoff Factor (i)

There are Some important List Of Top Solutions Questions Asked In CBSE CLASS XII

Solutions are mixtures containing multiple pure substances whose properties depend on their composition. Liquid solutions play a crucial role in chemistry, influencing reactions, solubility, and various other phenomena. Class 12 Chemistry Chapter 2 comprehensively covers solid, liquid, and gaseous solutions. It includes topics and subtopics related to the formation, properties, and behavior of solutions. Chapter 2 is a scoring topic with a weightage of 7 marks in the Class 12 Chemistry exam 2024.