The full, readable lecture — chromatography and the Rf value, solvent extraction, and the spectroscopy of light: the EM spectrum, flame tests, absorption and UV-visible. As you scroll, the panel on the right turns each idea into something you already know — a marker pen blooming on wet kitchen roll, fireworks in the sky, a prism splitting sunlight, oil floating on water.
Analytical chemistry answers two questions about any sample: what is in it (qualitative analysis) and how much (quantitative analysis). The first great tool for the "what" is chromatography.
Try it at home: put a dot of black marker on a strip of kitchen roll and dip the bottom in water. The water climbs by capillary action, and the single black dot blooms into separate colours — black ink is really a mixture of dyes. Components held tightly to the paper move slowly; those that prefer the water race ahead, so the mixture spreads out. Separation is by partition (dissolving between liquids, as in paper) or adsorption (sticking to a surface, as on a silica plate).
To turn the bands into evidence we measure the retardation factor, Rf. Draw a pencil base line (ink would run), spot the sample, let the solvent climb, then mark the solvent front.
Rf has no units, lies between 0 and 1, and for a given compound, solvent and paper it is constant — so it identifies the substance. A small Rf means it clings to the paper; near 1 means it travels almost with the solvent. This is exactly how a forensic scientist matches the ink on a forged cheque to a particular pen: run the questioned ink beside known pens and look for the same Rf bands. Thin-layer chromatography (TLC) does the same on a silica plate — faster and sharper — and column chromatography lets weakly bound parts elute first so each can be collected.
Solvent extraction separates a solute from one liquid by shaking it with a second liquid that does not mix — an immiscible solvent in which the solute dissolves better. You see the layering every time oil floats on water or cream rises on tea.
Use a separating funnel: shake, let the layers settle, and run off the lower (denser) layer through the tap. A single extraction never takes everything, so the same total solvent in several small portions removes more than one big one. The same trick decaffeinates coffee and pulls fragrance oils from flowers.
Now to the "how much" and the second great toolkit — light. Heat a metal salt in a flame and its electrons jump up; as they fall back they emit a characteristic colour. That is the flame test, a quick qualitative test for metal ions — and exactly why fireworks burst in different colours.
| Metal ion | Flame colour |
|---|---|
| Lithium (Li⁺) | crimson red |
| Sodium (Na⁺) | golden yellow |
| Potassium (K⁺) | lilac |
| Copper (Cu²⁺) | blue-green |
Flame photometry makes it quantitative: the intensity of the colour gives the concentration of the ion (used for Na⁺ and K⁺ in clinical and water testing). Because an electron only moves between fixed levels, each element emits its own set of wavelengths — a fingerprint.
Hold a glass prism in sunlight and the white beam fans into a rainbow — proof that "white" light is a blend of colours from violet to red. That rainbow is only a sliver of the full electromagnetic spectrum, which runs from long, low-energy radio waves to short, high-energy gamma rays.
The narrow visible window (≈ 400–700 nm) is the only part our eyes detect; everything else — infrared warmth, ultraviolet, X-rays — is invisible but obeys the same law.
The EM spectrum is not abstract — your house is full of it. Each device works at a different band, and the band fixes its energy:
| Region (low → high energy) | Everyday device |
|---|---|
| Radio / Microwave | radio, Wi-Fi, microwave oven (MRI in hospitals) |
| Infrared (IR) | TV remote, heat lamp, night-vision |
| Visible | your eyes, a phone screen, a torch |
| Ultraviolet (UV) | sunburn, sterilising lamps, UV "money checker" |
| X-ray / γ-ray | dental X-ray badge, cancer radiotherapy |
Sliding right along the scale means shorter wavelength and higher energy — which is why X-rays pass through skin while a TV remote's infrared cannot. Same physics, different rung on the ladder.
Pass light through a coloured sample and some wavelengths are absorbed as electrons jump to higher levels, leaving dark gaps in the light that comes out — the absorption spectrum. (When the electrons fall back they emit those same wavelengths as bright lines.)
| Technique | What it probes | What it tells you |
|---|---|---|
| UV-visible | electronic transitions | coloured species & concentration (quantitative) |
| Infrared (IR) | bond vibrations | which functional groups are present (O–H, C=O) |
A spectrometer runs source → monochromator → sample cell → detector → recorder, plotting absorbance against wavelength. By the Beer–Lambert idea, absorbance is proportional to concentration — which is how a hospital blood analyser reads your glucose or haemoglobin in seconds.