Chemistry Notes on Mixtures and Compounds

Mixtures

  • Definition: An impure substance made up of two or more elements or compounds mixed together mechanically in any proportion.
  • Examples:
    • Sugar in water forms a sugar syrup (mixture).
    • Sodium chloride in water forms a salt solution (mixture).
    • Air is a mixture of several gases.

Types of Mixtures

  1. Homogeneous Mixtures

    • Have the same composition and properties throughout.
    • Examples: Salt and water, sugar and water, alcohol and water.
    • Form miscible mixtures.

  2. Heterogeneous Mixtures

    • Have different compositions and properties throughout.
    • Examples: Oil and water, sand and water.
    • Form immiscible mixtures.

Characteristics of a Mixture

  • Proportions: Components are present in indefinite proportions.
  • Chemical Combination: Do not combine chemically.
  • Properties Retention: Constituents retain their properties.
    • Example: Iron and sulfur retain their individual properties in the mixture.
  • Separation: Can be separated by physical means (e.g., magnetic method).

Activity to Demonstrate Differences between Mixture and Compound

  • Mixture of Iron and Sulfur:
    • Observe under magnifying glass: Yellow sulfur and grey iron particles are separate.
    • Magnet Test: Iron filings attract to a magnet, leaving sulfur behind.
  • Heating: When heated, iron and sulfur combine chemically to form iron sulfide, losing individual properties (they can no longer be separated with a magnet).
  • Conclusion: Iron and sulfur mixture is distinct from the compound formed when they react chemically.

Comparative Study: Compounds vs Mixtures

ParameterCompoundsMixtures
CompositionFixed proportionVariable proportions
PropertiesDo not retain original propertiesRetain original properties
SeparationCannot be separated by physical methodsCan be separated by physical methods
Energy ChangeEnergy is absorbed or evolved during formationEnergy is neither evolved nor absorbed

Separation of Mixtures

  • Reasons for Separation:
    • To remove unwanted substances (e.g., separating stones from rice).
    • To obtain useful components (e.g., petrol from crude oil).
    • To obtain pure substances (e.g., salt from seawater).
Methods of Separation

  1. Solid-Solid Mixtures:

    • Hand Picking: Removes undesirable components based on size, shape, or color.
      • Example: Picking stones from rice.
    • Sieving: Separates larger particles from smaller ones.
      • Example: Separating bran from flour.
    • Magnetic Separation: Utilizes the magnetic properties of materials.
      • Example: Separating iron from non-magnetic materials.

  2. Solid-Liquid Mixtures:

    • Sedimentation: Settling down of insoluble particles.
    • Filtration: Using filter paper to separate solid particles from liquid.
    • Evaporation: Separating soluble solids by converting liquid to vapor.

  3. Special Techniques:

    • Loading: Adding a substance (like alum) to accelerate sedimentation.
    • Sublimation: Separation based on the ability of certain solids to transition directly from solid to vapor.

  4. Filtration: Most effective for solid-liquid mixtures.

    • Process: Pass mixture through filter paper to retain solid (residue) while liquid passes through (filtrate).
    • Example: Separating sand from water.
Conclusion
  • The separation methods chosen depend on the mixture's characteristics and the desired outcome. Filtration is often preferred due to its sensitivity and effectiveness in removing small particulates.

Check Your Progress

  • Practice Questions on Compounds and Mixtures:
    1. Identify if given examples are compounds or mixtures.
    2. Describe methods of separation for specific mixtures.