Chemistry Experimental Techniques and Chemical Analysis

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Chapter 20: Experimental Techniques & Chemical Analysis

This guide covers the essential theory, apparatus, and chemical tests required for IGCSE Chemistry analysis.

20.1 Experimental Design (Measurement)

1. Units of Measurement

  • Time: S.I. Unit is the Second (s). Measured with stopwatches (accuracy ± 0.01s).
  • Temperature: Kelvin (K) or Celsius (°C).
  • Mass: Kilogram (kg) or gram (g). Measured with electronic balances.
  • Volume: Cubic meter (m3), dm3, or cm3.
Temp (K) = Temp (°C) + 273
1 kg = 1000 g
1 dm3 = 1000 cm3

2. Measuring Liquid Volume

Apparatus Accuracy Use
Beaker Low Approximate volumes only.
Measuring Cylinder Medium General measurements (to nearest 0.5 cm3).
Pipette High (Fixed) Accurately measures one fixed volume (e.g., 25.0 cm3).
Burette Very High Accurately measures variable volumes (to 0.05 cm3).

Note: Always read the bottom of the meniscus at eye level.

3. Methods of Collecting Gases

Downward Delivery Denser than Air Upward Delivery Less Dense than Air Water Displacement Insoluble gases
Methods: 1. Downward, 2. Upward, 3. Water Displacement
  • Displacement of Water: For insoluble gases (H2, O2).
  • Downward Delivery: For gases denser than air (CO2, Cl2, HCl).
  • Upward Delivery: For gases less dense than air (NH3).

20.3 Acid-Base Titrations

Titration is used to determine the exact volume of acid needed to neutralize an alkali (or vice versa).

Indicators

Indicator Acid Color Alkali Color Use
Methyl Orange Red Yellow Strong Acid + Weak Alkali
Thymolphthalein Colorless Blue Weak Acid + Strong Alkali

20.4 Chromatography

Used to separate substances based on their solubility in a solvent.

Rf = Distance travelled by substance / Distance travelled by solvent
  • Pure Substance: Produces only 1 spot.
  • Impure Substance: Produces 2 or more spots.
  • Locating Agent: Used to make colorless substances (like amino acids) visible (e.g., Ninhydrin).

20.5 Separation Techniques

1. Filtration

Separates an insoluble solid from a liquid (e.g., Sand from water).

  • Residue: The solid trapped in the filter paper.
  • Filtrate: The liquid that passes through.

2. Crystallisation

Separates a soluble solid from a solution. Used when the solid might decompose on strong heating (e.g., Copper(II) sulfate).

3. Simple Distillation

Separates a pure solvent (liquid) from a solution (e.g., Water from salt water).

4. Fractional Distillation

Separates miscible liquids with different boiling points (e.g., Ethanol and Water). Ethanol boils at 78°C and distils first.

Purity Analysis

  • Pure substances have sharp, fixed melting and boiling points.
  • Impurities lower the melting point and raise the boiling point.

20.6 Identification of Ions (Qualitative Analysis)

1. Tests for Aqueous Cations

Using Aqueous Sodium Hydroxide (NaOH) and Aqueous Ammonia (NH3).

Cation Effect of NaOH (aq) Effect of Ammonia (aq)
Aluminum (Al3+) White ppt, soluble in excess White ppt, insoluble in excess
Zinc (Zn2+) White ppt, soluble in excess White ppt, soluble in excess
Calcium (Ca2+) White ppt, insoluble No ppt / slight ppt
Copper (Cu2+) Light Blue ppt, insoluble Light Blue ppt, soluble in excess (Dark Blue solution)
Iron(II) (Fe2+) Green ppt Green ppt
Iron(III) (Fe3+) Red-Brown ppt Red-Brown ppt
Ammonium (NH4+) Ammonia gas produced on warming -

2. Flame Tests (Cations)

  • Lithium (Li+): Red flame
  • Sodium (Na+): Yellow flame
  • Potassium (K+): Lilac flame
  • Copper (Cu2+): Blue-Green flame

3. Tests for Anions

Anion Test Observation
Carbonate (CO32-) Add dilute acid Effervescence, CO2 produced
Chloride (Cl-) Acidify with nitric acid, add Silver Nitrate White precipitate
Bromide (Br-) Acidify with nitric acid, add Silver Nitrate Cream precipitate
Iodide (I-) Acidify with nitric acid, add Silver Nitrate Yellow precipitate
Sulfate (SO42-) Acidify, add Barium Nitrate White precipitate
Nitrate (NO3-) Add NaOH, Al foil, warm Ammonia gas produced

4. Tests for Gases

  • Hydrogen (H2): "Pops" with a lighted splint.
  • Oxygen (O2): Relights a glowing splint.
  • Carbon Dioxide (CO2): Turns limewater milky/cloudy.
  • Chlorine (Cl2): Bleaches damp blue litmus paper.
  • Ammonia (NH3): Turns damp red litmus paper blue.
Handout generated for IGCSE Chemistry Revision