Overview
This lecture covers key methods for identifying chemicals, including tests for purity, chromatography, specific chemical tests, and modern instrumental analysis.
Identifying Pure Substances
- Pure substances have specific melting and boiling points.
- Testing melting/boiling points helps determine if a sample is pure.
Formulations
- A formulation is a specially designed mixture with precise quantities for a specific purpose.
- Examples include paints, fuels, alloys, and fertilizers.
Chromatography
- Chromatography separates substances in a mixture (e.g., pigments, drugs).
- The stationary phase (paper) stays still while the mobile phase (water) moves.
- A pencil line marks the starting point to avoid dissolving in the solvent.
- The RF (retention factor) value is calculated: (distance moved by substance) ÷ (distance moved by solvent).
- RF values are compared to known values to identify substances.
Chemical Tests for Gases
- Hydrogen: burning splint causes a squeaky pop.
- Oxygen: relights a glowing splint.
- Carbon dioxide: turns limewater cloudy.
- Chlorine: bleaches damp blue litmus paper white.
Flame Tests for Metal Ions
- Lithium: crimson flame.
- Sodium: yellow flame.
- Potassium: lilac flame.
- Calcium: orange-red flame.
- Copper: green flame.
Tests for Metal Ions in Solution
- Adding sodium hydroxide: aluminum, calcium, and magnesium produce white precipitates; only aluminum dissolves in excess.
- Copper(II) forms a blue precipitate.
- Iron(II) forms a green precipitate.
- Iron(III) forms a brown precipitate.
- Completing and balancing ionic equations may be required.
Additional Chemical Tests
- Carbonates react with acids to form carbon dioxide (tested with limewater).
- Halide ions (chloride, bromide, iodide) react with silver nitrate and nitric acid to form colored precipitates (white, cream, yellow).
- Sulfate ions form a white precipitate with barium chloride and hydrochloric acid.
Instrumental Methods
- Instrumental tests (e.g., flame emission spectroscopy) are accurate, sensitive, and fast.
- Flame emission spectroscopy detects the unique light spectrum of metal ions to identify them.
Key Terms & Definitions
- Pure Substance — Material with a fixed melting/boiling point and no impurities.
- Formulation — A measured mixture designed for a specific use.
- Chromatography — Technique to separate and identify substances in a mixture.
- Stationary Phase — The phase that does not move in chromatography (e.g., paper).
- Mobile Phase — The phase that moves and carries substances (e.g., water).
- RF Value — Ratio of the distance moved by a substance to the distance moved by the solvent.
- Flame Emission Spectroscopy — Instrumental technique to identify metal ions by their emitted light spectrum.
Action Items / Next Steps
- Practice identifying ions using the described chemical tests.
- Review calculation and interpretation of RF values in chromatography.
- Prepare for lab exercises on chemical and instrumental analysis.