4.2 — Chemistry Around You

Chemistry is mostly one question repeated: which electrons want to move, and what happens when they do. Everything on this page follows from that.

Metals

Why does iron rust and gold does not?

Because iron gives up electrons easily and gold does not.

Rusting is iron reacting with oxygen and water. Iron atoms lose electrons to oxygen, forming hydrated iron oxide — the flaky orange solid. The reason it destroys the metal completely, rather than stopping, is that rust is bigger than the iron it came from and does not stick well. It flakes off, exposing fresh metal, and the process eats inward until there is nothing left.

Compare aluminium, which is chemically more reactive than iron. It oxidises the instant it meets air — but aluminium oxide is dense, hard, and bonded tightly to the surface, forming an invisible layer a few nanometres thick that seals the metal. The reaction stops because its own product blocks it. Chromium does the same thing, which is what makes stainless steel stainless: it is iron with enough chromium to grow a self-repairing oxide skin.

Gold sits at the other end. Its electrons are held tightly enough that oxygen cannot take them under ordinary conditions, so gold objects buried for three thousand years come out of the ground bright. That is precisely why it became money, covered in Volume VIII.

Why does the Iron Pillar of Delhi not rust?

The pillar at the Qutb complex is about seven metres of wrought iron, raised around 400 CE, and it has stood in the open for sixteen centuries with only light surface corrosion.

The explanation is chemical, not mysterious. The iron contains an unusually high level of phosphorus, roughly one per cent, because it was made by a process that did not remove it. In Delhi's climate — dry for most of the year, humid in the monsoon — the phosphorus reacts to form a thin, dense compound layer of iron hydrogen phosphate hydrate on the surface, which seals the metal in the same way aluminium's oxide does.

The layer needs the wet-dry cycling to form properly, so the same iron in a permanently humid or permanently wet environment would not be protected. It is a fortunate combination of an unrefined material and a particular climate, and it was analysed in detail by R. Balasubramaniam at IIT Kanpur.

Why do copper roofs go green and copper wire does not?

Both oxidise. The difference is time and exposure.

Copper exposed to air, moisture and traces of sulphur and carbon dioxide over decades forms a mixture of copper carbonates and sulphates — the green patina, which like aluminium's oxide is stable, adherent and protective. The Statue of Liberty is copper and was originally the colour of a new coin; it took about thirty years to turn green.

Copper wire is either insulated, indoors, or in contact with air only briefly, and it darkens to brown rather than green because the process stops at the first oxide stage.

Food

Why does a cut apple go brown?

Because you broke the cells open and let an enzyme meet oxygen.

Apple cells contain phenolic compounds and, separately, an enzyme called polyphenol oxidase. Cutting ruptures the walls, the two mix, and the enzyme uses oxygen from the air to convert the phenols into compounds that react further and polymerise into brown pigments — chemically close to melanin, which is what browns your skin in sunlight.

Everything that prevents it attacks one of the three requirements. Lemon juice works two ways: the acid slows the enzyme, and vitamin C reacts with the intermediate products before they can brown. Water or cling film excludes oxygen. Heat destroys the enzyme, which is why cooked apple does not brown further. Cold slows it. Salt water works too.

The same reaction browns potatoes, bananas, avocados and mushrooms, and it is deliberately encouraged in tea and cocoa, where the brown compounds are most of the flavour.

Why does browning meat or bread taste so good?

Because at high temperature, sugars and amino acids react together in a cascade that produces hundreds of new compounds — the Maillard reaction, named after Louis-Camille Maillard, who described it in 1912.

It is not caramelisation, which is sugar alone breaking down. Maillard needs both a sugar and a protein fragment, and the products include the flavours we call roasted, meaty, malty, nutty and bready. A single reaction pathway is responsible for the taste of seared steak, toast crust, roasted coffee, fried onions, soy sauce, chocolate and the top of a naan.

It needs a surface above about 140 °C and, critically, a dry surface — which is why patting meat dry before searing works, why a crowded pan steams instead of browning, and why nothing in a microwave browns, as covered in 1.2.

What actually makes chilli hot, and why does water not help?

Capsaicin, an oil-soluble molecule that binds to a receptor in your nerve endings called TRPV1.

That receptor's normal job is to detect heat — it fires when tissue exceeds about 43 °C, which is the temperature at which damage begins. Capsaicin binds to it and makes it fire at ordinary body temperature. So the burning is not a metaphor and not an irritation: your brain is receiving the exact signal it would receive from a real burn, from a nerve that has been chemically tricked.

Water does not help because capsaicin is oil-soluble and barely dissolves in it; you spread it around. Milk works because casein, a protein in it, binds the molecule and lifts it off the receptor, and the fat dissolves it. Yoghurt, and any oily or fatty food, do the same. Sugar helps somewhat by competing for attention.

The plant's reason for making it is precise. Capsaicin affects mammals and not birds, because the bird version of the receptor does not respond. Mammals grind seeds with their teeth and destroy them; birds swallow them whole and deposit them far away. The chilli is discouraging the wrong customer.

Menthol is the same trick in reverse — it binds the cold receptor, TRPM8, so mint feels cool without lowering any temperature.

Why do onions and garlic behave the way they do?

Onions are in 1.2. Garlic is the same design with a different product: an intact clove contains alliin and, kept separate from it, the enzyme alliinase. Crushing mixes them and produces allicin, which is the smell, the taste and the antibacterial activity.

The consequence in cooking is practical. Crush garlic and wait a minute before it hits the pan, and you get more of the compound; throw it straight into hot oil and the enzyme is destroyed before it has worked. Whole roasted garlic is mild and sweet for the same reason — the cells were never broken while the enzyme was alive.

Why does soda fizz, and why does it go flat?

Carbon dioxide is forced into the liquid under pressure. Gas dissolves in liquid in proportion to the pressure above it — Henry's law — so a sealed bottle holds several times as much dissolved gas as the liquid could hold at ordinary pressure.

Open it and the pressure drops. The liquid is now holding more gas than it can, and the excess comes out. But it needs somewhere to start. Forming a bubble from nothing in a smooth liquid requires overcoming surface tension, which is surprisingly hard, so bubbles form on nucleation sites — scratches in the glass, dust, a straw, the rough surface of a sugar crystal.

That is the whole mechanism behind the mint-and-cola fountain: the mint's surface is covered in microscopic pits, each of which is a ready-made bubble factory, so an enormous amount of gas comes out at once.

Flat soda is simply soda that has finished doing this. Cold liquid holds more gas, which is why a warm bottle fizzes violently and why soda tastes flat sooner in the heat.

Cleaning and reacting

What does bleach actually do?

It destroys colour by breaking chemical bonds, and it destroys germs by the same brutality.

Most colour comes from molecules with a long alternating chain of double and single bonds — a chromophore — which absorbs particular wavelengths of light. Chlorine bleach attacks those double bonds by oxidation, chopping the chain. The molecule is still there. It simply no longer absorbs visible light, so the stain is invisible rather than removed.

Oxygen bleaches, like hydrogen peroxide and the sodium percarbonate in colour-safe laundry powders, do the same thing more gently. Optical brighteners in detergent do something different and slightly dishonest: they absorb ultraviolet and re-emit it as blue, so the fabric emits more visible light than falls on it and reads as "whiter than white".

Never mix bleach with anything. Bleach plus an acid, including toilet cleaners and vinegar, releases chlorine gas. Bleach plus ammonia, including some glass cleaners and urine, releases chloramine vapour. Both have killed people in bathrooms.

Why does baking soda make cakes rise?

Because it releases carbon dioxide when it meets an acid, and the gas is trapped by the batter as it sets.

Sodium bicarbonate plus any acid gives a salt, water and CO₂. In a recipe the acid is buttermilk, yoghurt, lemon juice, honey, cocoa or vinegar. If there is no acid, the soda does not fully react, and what is left tastes soapy and metallic — which is the usual cause of a bitter home-baked cake.

Baking powder solves that by being a complete kit: bicarbonate, a powdered acid, and starch to keep them apart while dry. "Double acting" means two acids, one reacting on contact with liquid and one only on heating, so some rise happens in the bowl and the rest in the oven.

Yeast does the same job biologically and slowly, producing CO₂ by fermentation — and, unlike soda, also producing flavour compounds and alcohol, which is why bread and cake taste fundamentally different.

Why does salt melt ice?

Because dissolved particles make it harder for water to freeze.

Ice and liquid water at 0 °C are in balance — molecules leave the ice and join it at the same rate. Dissolve salt in the water and the water molecules are now diluted by ions, so fewer of them are available at the surface to join the ice, while the ice still loses molecules at the same rate. Melting wins, and it keeps winning until the temperature drops far enough to restore the balance.

This is freezing point depression, and it depends only on how many dissolved particles there are, not what they are. Table salt gives two particles per unit, sodium and chloride, which is why it is efficient. Calcium chloride gives three and works to lower temperatures, which is why it is used on very cold roads.

The limit is real: a salt-water mixture cannot stay liquid below about −21 °C however much salt you add, so gritting stops working in extreme cold.

Materials

Why does glue stick, and why does superglue set so fast?

Two different mechanisms, which is why they behave so differently.

Ordinary glue works by drying or curing into a solid that has flowed into the surface's roughness first. Wood glue soaks into the fibres and hardens. Contact adhesive forms a tacky film that grips by molecular attraction over a large area. What matters in both cases is intimate contact — which is why roughening a surface and clamping it helps.

Superglue is a chemical reaction, and its trigger is water. Cyanoacrylate monomers are small molecules that link into long chains the moment they meet a trace of moisture. Every surface in normal air, including your skin, has a film of water on it a few molecules thick, and that is all it needs. It sets in seconds, and it sticks to fingers instantly and enthusiastically because skin is damp.

It also explains why superglue is useless on large gaps — with no surface to trigger it in the middle, the bulk stays liquid — and why it was used in Vietnam to seal wounds in the field.

Why does non-stick coating not stick to the pan?

An excellent question, because PTFE — the polymer sold as Teflon — is famous for sticking to nothing.

The answer is that it is not glued on. The pan's aluminium surface is roughened first, by sandblasting or by etching with acid, and the coating is sprayed on and baked so that it flows into the pits and locks there mechanically. Modern pans also use a primer layer and sometimes a hard particle matrix.

That is exactly why scratching a non-stick pan ends it: the coating is held by geometry, and a scratch gives it an edge to peel from.

PTFE was discovered by accident in 1938 by Roy Plunkett at DuPont, who found a cylinder of gas that appeared empty but still weighed full; the gas inside had polymerised on the walls into a waxy solid. Its non-stick property comes from fluorine atoms wrapped tightly around a carbon backbone, presenting a surface that has almost no chemical interest in anything.

What is the difference between a diamond and a pencil lead?

Nothing but arrangement. Both are pure carbon.

In diamond, each carbon atom is bonded to four others in a rigid three-dimensional lattice, so the whole crystal is one enormous molecule. Breaking it means breaking covalent bonds in every direction, which is why it is the hardest natural substance.

In graphite, each carbon bonds to three others in flat hexagonal sheets, with the fourth electron free to roam across the sheet. The sheets stack but are held to each other only weakly, so they slide. That is why graphite marks paper — layers shear off — and why it conducts electricity, thanks to the roaming electrons, while diamond is an insulator.

Diamond is not even the stable form at room temperature; graphite is. Diamonds are slowly turning into graphite everywhere on Earth, at a rate so slow that "forever" is a fair advertising claim.

A single sheet of graphite one atom thick is graphene, isolated in 2004 by Geim and Novoselov, reportedly using sticky tape to peel layers off a block of graphite. They received the Nobel Prize in 2010.

What comes next

The next page turns the chemistry inward, to the body — why you get goosebumps, why yawning is contagious, why hiccups exist at all, and why time seems to speed up as you get older.