Identifying substances and testing for purity using chemical and instrumental methods.
In chemistry, a pure substance contains only one element or one compound. Pure substances melt and boil at specific, fixed temperatures.
Impure substances (mixtures) melt over a range of temperatures and usually have a lower melting point than the pure substance.
A mixture that has been designed as a useful product (e.g., medicines, paints, fuels). Each component is present in a measured quantity for a specific purpose.
Chromatography separates substances based on their distribution between a mobile phase (solvent) and a stationary phase (paper). A pure substance produces only one spot on a chromatogram.
$$ R_f = \frac{\text{distance moved by substance}}{\text{distance moved by solvent}} $$
Note: $R_f$ values can only be compared if the same solvent and same paper are used. If two substances have the same $R_f$ in the same solvent, they may be the same compound.
Q1: How can chromatography identify a pure substance?
A pure substance will only show one spot on the chromatogram.
| Gas | Test Method | Result |
|---|---|---|
| Hydrogen ($H_2$) | Lit splint at mouth of tube | "Squeaky pop" sound |
| Oxygen ($O_2$) | Glowing splint inside tube | Splint relights |
| Carbon Dioxide ($CO_2$) | Bubble through Limewater | Limewater turns cloudy/milky |
| Chlorine ($Cl_2$) | Damp blue litmus paper | Turns red (acidic), then bleaches white |
The nichrome wire must be cleaned with concentrated HCl first. This removes traces of other metal ions that could contaminate the flame colour.
[Image of flame test colors for metal ions]| Ion | Flame Colour |
|---|---|
| Lithium ($Li^+$) | Crimson |
| Sodium ($Na^+$) | Yellow |
| Potassium ($K^+$) | Lilac |
| Calcium ($Ca^{2+}$) | Orange-red |
| Copper ($Cu^{2+}$) | Green |
Aluminium, Calcium, and Magnesium ions all form white precipitates with $NaOH$.
Confirmation Rule: Aluminium hydroxide dissolves in excess sodium hydroxide, confirming the presence of $Al^{3+}$ ions.
Add dilute acid → Effervescence → Gas turns Limewater cloudy.
Add dilute Nitric Acid then Silver Nitrate solution.
Why Nitric Acid? It is added first to remove carbonate ions, which would otherwise react with the silver nitrate to form a false-positive white precipitate.
Add dilute HCl then Barium Chloride. Result: White precipitate.
A sample is passed through a flame and the light emitted is passed through a spectroscope. Unlike flame tests, flame emission spectroscopy can identify ions in mixtures and measure their concentration.
1. Don't forget to add nitric acid before silver nitrate in halide tests; it removes interfering carbonates.
2. Don't say $R_f$ is distance of solvent divided by substance; it is always substance divided by solvent.
3. Don't forget to clean the wire in HCl before flame tests to prevent contamination.
4. Don't assume all white precipitates are the same; use "excess NaOH" to confirm Aluminium.