Chemistry Matters GCE O Level Textbook Study Notes

CHAPTER 1: EXPERIMENTAL CHEMISTRY

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1.1 Measurement of Physical Quantities

In Chemistry, the International System of Units (SI units) is used as a common standard for recording measurements.

Common Physical Quantities

  • Time: Measures duration. SI Unit: second (s). Other units: minute (min), hour (h).

    • 1 min = 60 s

    • $1 \, h = 60 min = 3600 s$

    • Apparatus: Digital stopwatch (accuracy $\pm 0.01 s) or Analogue stopwatch (accuracy $\pm 0.1 s).

  • Temperature: Measures hotness/coldness. SI Unit: kelvin ($K$). Other unit: degree Celsius ($^{\circ}C$).

    • Conversion: K= T(^{\circ}C) + 273

    • Apparatus: Alcohol or mercury analogue thermometers, digital thermometers, or temperature sensors/probes connected to data loggers.

  • Length: Measures distance. SI Unit: metre (m). Other units: cm, dm, mm.

    • Apparatus: Metre rule (accuracy 0.1 cm) or measuring tape (accuracy 0.1 to 0.5 cm).

  • Mass: Measures matter present. SI Unit: kilogram ($kg$). Other units: g, mg, T (tonne), KT (kilotonne).

    • $1 \, t = 1000 \, kg$

    • Apparatus: Electronic balance (accuracy $\pm 0.01 \, g$) or beam balance.

  • Volume: Measures space occupied. SI Unit: cubic metre ($m^3$). Other units: $cm^3, dm^3$.

    • $1 \, dm^3 = 1 \, L = 1000 \, cm^3$

    • $1 \, m^3 = 1000 \, dm^3 = 1,000,000 \, cm^3$

Measuring Volume of Liquids

Apparatus choice depends on volume and required accuracy:

  1. Pipette: Measures accurate fixed volumes (e.g., $25.0 \, cm^3$).

  2. Volumetric Flask: Measures large accurate fixed volumes (e.g., $250 \, cm^3$).

  3. Measuring Cylinder: Measures range of volumes to nearest $0.5 \, cm^3$.

  4. Burette: Measures range of volumes to nearest $0.05 \, cm^3$.

Reading Technique: Position eyes at the meniscus (the curve at the surface) to avoid parallax error. Read at the bottom of a concave meniscus or top of a convex meniscus.

Measuring Volume of Gas

A gas syringe is typically used to measure gas volumes up to $100 \, cm^3$.

1.2 Collection and Drying of Gases

Methods are chosen based on the gas's solubility in water and density compared to air (take air $M_r \approx 30$).

Collection Methods

  • Water Displacement: For gases insoluble to slightly soluble (e.g., $H2, O2, CO_2$).

  • Downward Delivery: For gases denser than air (e.g., $Cl2, HCl, SO2$).

  • Upward Delivery: For gases less dense than air (e.g., $NH_3$).

Drying Agents

  • Concentrated Sulfuric Acid: For most gases ($Cl2, HCl$). Unsuitable for alkaline gases like $NH3$.

  • Quicklime (Calcium Oxide): Specifically for $NH3$. Unsuitable for acidic gases like $CO2$.

  • Fused Calcium Chloride: For $H2, N2, CO2$. Unsuitable for $NH3$.

1.3 Separation and Purification Methods

Techniques depend on differences in physical properties (particle size, solubility, boiling point).

Solid-Solid Separation

  • Magnetic Attraction: Extracts magnetic materials (iron, cobalt, nickel) from non-magnetic ones.

  • Sieving: Separates particles of different sizes.

  • Suitable Solvents: Used if one component is soluble and the other is not (e.g., separating salt and sand using water).

  • Sublimation: Separates solids that go directly from solid to gas (e.g., $I_2$, dry ice, naphthalene).

Solid-Liquid Separation

  • Filtration: Separates insoluble solids (residue) from liquids (filtrate).

  • Evaporation to Dryness: Recovers a solute by heating until all solvent is gone. Solute must be heat-stable.

  • Crystallisation: For solutes that decompose on heating (e.g., sugar). Produces pure crystals from a saturated solution.

Liquid-Liquid Separation

  • Separating Funnel: For immiscible liquids ($oil$ and $water$). They separate into layers based on density.

  • Simple Distillation: Collects a pure solvent from a solution using boiling and condensation ($water$ from sea water).

  • Fractional Distillation: For miscible liquids with different boiling points ($ethanol$ and $water$). Uses a fractionating column with glass beads to provide surface area for condensation cycles.

  • Paper Chromatography: Separates substances based on solubility in a solvent.

    • Retention Factor ($Rf$): Rf = \frac{\text{distance travelled by substance}}{\text{distance travelled by solvent}}

    • Locating Agents: Chemicals used to develop coloured spots for colourless compounds ($amino \, acids$).

1.4 Determination of Purity

Pure substances have specific, sharp melting and boiling points.

  • Impure substances (mixtures): Melt/Boil over a range of temperatures.

  • Impurities lower the melting point and raise the boiling point of a substance.