Add a drop of pale yellow iron(III) chloride solution to a little salicylic acid in water and the tube turns violet almost at once. Do the same with clean aspirin and the colour barely changes. That contrast is the whole basis of the salicylic acid iron III chloride test, one of the most familiar quick checks in an undergraduate lab. It’s simple to run, but the chemistry behind the colour is worth a closer look, and so are the limits […]
Take one carbon atom, bolt three benzene rings onto it, and add a single OH group. The result, triphenylmethanol, is an unremarkable-looking white powder. Drop it into concentrated sulfuric acid, though, and it dissolves to give a bright yellow solution. That colour change helped chemists realise, more than a century ago, that a positively charged carbon could be stable enough to sit in a flask. The same crowded molecule also sits at the root of the first organic free radical […]
A brass doorknob, a trumpet and a plumbing valve can look like the same golden metal, but they aren’t the same recipe. Brass is copper with zinc mixed in, and the amount of copper in brass decides a surprising amount: whether it glows reddish or pale yellow, whether it bends cold or cracks, and how cleanly it cuts on a lathe. Chemists have reliable ways to measure that copper share, and the methods are a tidy tour of analytical chemistry. […]
Stir a white powder into a colourless liquid, add a few drops of strong acid, warm it for ten minutes, and you have aspirin, plus a lab that smells faintly of vinegar. The salicylic acid and acetic anhydride reaction is one of the most familiar syntheses in chemistry teaching. It’s also a compact lesson in how esters form, why chemists choose some reagents over others, and where the leftover smells and gunk come from. A safety note before the chemistry: […]
Most purified proteins dissolved in water look like water: clear and colourless. Shine ultraviolet light through it, though, and the solution soaks up a band of that light near 280 nanometres. The protein absorbance wavelength of 280 nm comes from a handful of amino acids with ring-shaped side chains. It’s also why one reading on a spectrophotometer, called A280, can tell you how much protein is in the tube without adding any reagent. Why the protein absorbance wavelength is 280 […]
A big drop falling through a cloud ought to sweep up every small droplet in its path. Mostly it doesn’t. The smallest droplets get pushed aside by the air flowing around the falling drop and slip past without touching it. Whether a particle rides the air like that or ploughs straight through it is the question the Stokes number answers. It’s a single dimensionless figure that predicts whether a particle carried by a moving fluid will follow the flow or […]