Structure 1.1: Elements, Compounds, and Mixtures

Matter Classification

  • Matter can be broadly classified into pure substances and mixtures.

  • Pure substances consist of elements and compounds.

  • Mixtures can be homogeneous or heterogeneous.

Elements

  • An element is a substance that cannot be broken down into simpler substances by chemical means.

  • Some elements exist as diatomic molecules, which are electrically neutral groups of two or more atoms bonded together.

    • Examples:

      • Hydrogen: H_2

      • Chlorine: Cl_2

      • Oxygen: O_2

      • Nitrogen: N_2

Compounds

  • A compound is formed from two or more different elements chemically joined in a fixed ratio.

    • Examples:

      • Water: H_2O

      • Ethanol: C2H5OH

      • Sodium chloride: NaCl

  • Compounds have different properties from the elements they are made from.

    • Example:

      • Formation of Sodium Chloride: 2Na(s) + Cl_2(g) \rightarrow 2NaCl(s)

Mixtures

  • Mixtures are classified into homogeneous and heterogeneous mixtures.

  • Homogeneous mixtures have a constant composition throughout.

  • Heterogeneous mixtures have visibly different substances or phases.

Homogeneous Mixtures

  • Have a uniform composition throughout.

  • Have no visible boundaries; components are mixed as individual atoms, ions, or molecules.

  • Also called solutions or aqueous solutions (if water is part of the mixture).

    • Examples:

      • Air: 78% Nitrogen, 20.9% Oxygen, 0.90% Argon, 0.03% Carbon Dioxide, Other Gases ->0.17%

Heterogeneous Mixtures

  • Do not have a uniform composition.

  • Have one or more visible boundaries between the components.

Separation Techniques

  • Filtration: Separates a mixture with a solid that is not dissolved in the solvent.

  • Evaporation: Separates a mixture with a solute dissolved in the solvent (an aqueous solution).

  • Distillation: Separates a mixture of liquids with different boiling points.

  • Chromatography: Separates a mixture of substances with different solubilities in the mixture.

Technique Descriptions

  • Filtration: Used to separate a heterogeneous mixture, such as sand in water.

  • Recrystallization: Used to purify a solid.

  • Evaporation: Used to separate a homogeneous mixture, such as a salt solution.

  • Distillation: Used to separate two miscible liquids with different boiling points.

  • Chromatography: Used to separate a mixture of solutes in a solvent.

Solvation

  • Solvation involves the formation of ion-dipole forces between a solute and a solvent.

  • When a salt dissolves, ions are pulled apart from the lattice structure and surrounded by water molecules.

  • The forces of attraction between solute particles and water molecules are ion-dipole forces.

States of Matter

  • Solid: Has a fixed shape and volume.

  • Liquid: Has a fixed volume but no fixed shape.

  • Gas: Has neither a fixed volume nor a fixed shape.

Changes of State

  • Heat is absorbed (endothermic):

    • Sublimation: Solid to Gas

    • Melting: Solid to Liquid

    • Evaporation/Boiling: Liquid to Gas

  • Heat is released (exothermic):

    • Freezing: Liquid to Solid

    • Condensation: Gas to Liquid

    • Deposition: Gas to Solid

State Symbols

  • State symbols show the physical state of a substance.

    • (s) – solid

    • (l) – liquid

    • (g) – gas

    • (aq) – aqueous

Temperature Scales

  • Celsius scale: Based on 0^\circ C for the freezing point of water and 100^\circ C for the boiling point of water.

  • Kelvin scale: Directly proportional to the average kinetic energy of particles in a substance; absolute temperature scale with the lowest possible value being zero (absolute zero, 0 K).

  • One degree on the Kelvin scale is equal to one degree on the Celsius scale.

  • Absolute zero: 0 K = -273 ^\circ C

  • Water freezes: 273 K = 0 ^\circ C

  • Water boils: 373 K = 100 ^\circ C

  • Conversion between Celsius and Kelvin: Add or subtract 273 (e.g., 25 ^\circ C = 298 K).

  • Absolute temperature in Kelvin (K) is directly proportional to the average kinetic energy of particles in a substance.

Heating and Cooling Curves

  • As heat is added, temperature increases unless there is a phase change when the temperature remains constant.

  • During phase changes, the energy added is used to overcome intermolecular forces between molecules.