Chapter 2: Solutions



Introduction:

  • A solution is a homogeneous mixture composed of two or more substances.
  • The substance present in a larger amount is called the solvent, and the substance present in a smaller amount is called the solute.
  • Solutions play a crucial role in various fields, including chemistry, biology, medicine, and industry.

Types of Solutions:

  1. Solid Solutions:

    • These solutions have a solid solvent and a solid solute.
    • The solute particles are dispersed throughout the solvent.
    • Examples include alloys like brass (copper-zinc) and steel (iron-carbon).

  2. Liquid Solutions:

    • These solutions have a liquid solvent and a solute that can be solid, liquid, or gas.
    • The solute particles are uniformly distributed in the solvent.
    • Common examples include sugar dissolved in water, alcohol mixed with water, etc.

  3. Gaseous Solutions:

    • These solutions have a gas as the solvent and another gas, liquid, or solid as the solute.
    • Examples include air (a mixture of gases), oxygen dissolved in water, etc.

Concentration of Solutions:

  • Concentration refers to the amount of solute present in a given amount of solvent or solution.
  • Different methods are used to express the concentration of solutions, such as mass percentage, mole fraction, molarity, molality, etc.

  1. Mass Percentage:

    • It is the ratio of the mass of solute to the mass of the solution, multiplied by 100.
    • Mass Percentage = (Mass of Solute / Mass of Solution) x 100

  2. Mole Fraction:

    • It is the ratio of the moles of one component (solute or solvent) to the total moles of all components in the solution.
    • Mole Fraction of Component A = (Number of Moles of A) / (Total Number of Moles)

  3. Molarity:

    • It is the number of moles of solute present in one liter of the solution.
    • Molarity (M) = (Number of Moles of Solute) / (Volume of Solution in Liters)

  4. Molality:

    • It is the number of moles of solute per kilogram of solvent.
    • Molality (m) = (Number of Moles of Solute) / (Mass of Solvent in kg)

Solubility:

  • Solubility is the maximum amount of solute that can be dissolved in a given amount of solvent at a particular temperature.
  • It is often expressed in terms of grams of solute dissolved per 100 grams of solvent.
  • Substances can be classified as soluble, insoluble, or partially soluble based on their solubility.

Factors Affecting Solubility:

  1. Nature of Solvent and Solute:

    • Like dissolves like: Polar solvents dissolve polar solutes, and nonpolar solvents dissolve nonpolar solutes.
    • The intermolecular forces between solvent and solute molecules play a crucial role.

  2. Temperature:

    • In general, solubility increases with an increase in temperature for most solid solutes in liquid solvents.
    • However, for some solutes, such as certain salts, solubility decreases with an increase in temperature.

  3. Pressure:

    • Pressure has a significant effect on the solubility of gases in liquids.
    • Henry's Law states that the solubility of a gas in a liquid is directly proportional to the partial pressure of the gas above the liquid.

Preparation of Solutions:

  • Solutions can be prepared by dissolving the solute in the solvent using various methods like stirring, heating, or grinding.
  • The solute is added gradually to the solvent while stirring to ensure uniform mixing.

Colligative Properties of Solutions:

  • Colligative properties depend on the number of solute particles rather than their nature.
  • Four important colligative properties are:

  1. Vapor Pressure Lowering:

    • The presence of a nonvolatile solute lowers the vapor pressure of the solvent.
    • Raoult's law relates the vapor pressure of the solvent in the solution to its mole fraction.

  2. Boiling Point Elevation:

    • The presence of a nonvolatile solute raises the boiling point of the solvent.
    • The change in boiling point is proportional to the molal concentration of the solution.

  3. Freezing Point Depression:

    • The presence of a solute lowers the freezing point of the solvent.
    • The change in freezing point is proportional to the molal concentration of the solution.

  4. Osmotic Pressure:

    • It is the pressure applied to stop the flow of solvent molecules across a semipermeable membrane.
    • It depends on the concentration of solute particles in the solution.