Top of the Bench 2022

Sublimation

HintDry ice "smoking" on a stage, or frost vanishing from a lawn on a cold sunny morning.

The whyMost solids melt, then boil. A few — solid carbon dioxide, iodine, and ice in very dry cold air — skip the liquid and go straight to gas. Dry ice is the everyday example: the white "smoke" is water vapour condensing in the cold gas. The reverse (gas straight to solid, as frost forms) is deposition.

Speeds it up

HintThink of the iron in the ammonia process — what does it do to the rate, and what happens to it afterwards?

The whyA catalyst makes a reaction faster and is left chemically unchanged at the end, so a small amount goes on working. A car’s catalytic converter is the everyday case: a thin coating of platinum turns exhaust gases into less harmful ones for the whole life of the car, and none of the platinum is used up. Enzymes in your cells do the same job at body temperature. A catalyst is NOT a reactant and does not appear in the equation.

black

HintThe same colour as the coating on a copper wire held in a Bunsen flame.

The whycopper carbonate → copper oxide + carbon dioxide. Three observations prove it: the colour change, the loss of mass (a gas left), and limewater turning milky if the gas is bubbled through it. Green-to-black is the classic "describe what you would see" answer; the reaction is a thermal decomposition and endothermic.

Fractional distillation

HintSimple boiling-and-condensing cannot tell them apart; this version adds a tall column.

The whySimple distillation copes with a liquid and a dissolved solid, or two liquids with very different boiling points. When the boiling points are close, the vapour is a mixture, so a tall fractionating column is added: the vapour condenses and re-boils many times on the way up, and each cycle enriches it in the lower-boiling liquid. Crude oil, liquid air and whisky are all separated this way.

Oxidised

HintThink of magnesium burning: it gains something from the air.

The whyMetals react by losing electrons, and losing electrons is oxidation — whether the partner is oxygen (burning, rusting), an acid, or a halogen like chlorine. The thing that takes the electrons is reduced. So in a displacement or an extraction, the metal that ends up in the compound has been oxidised and the metal that ends up as the free element has been reduced.

nitrogen; phosphorus; potassium

HintOne is made into ammonia, one glows in the dark, one is the metal in bananas.

The whyPlants take these three from the soil and put them into their leaves and seeds; harvest the crop and they leave the field with it. Fertilisers put them back, usually as soluble salts such as ammonium nitrate (nitrogen), ammonium phosphate (nitrogen and phosphorus) and potassium chloride. Soluble matters: the roots can only drink what is dissolved.

Carbon monoxide

HintIts formula is one letter short of the harmless combustion gas.

The whyIncomplete combustion: not enough oxygen, so each carbon gets one oxygen instead of two. Carbon monoxide binds to haemoglobin far more tightly than oxygen does, so the blood quietly stops carrying oxygen — and because the gas is colourless and odourless, nobody notices. Faulty boilers and blocked flues are the danger; the fix is ventilation and an alarm.

Chromatography

HintUsed to check whether a sweet’s colouring is one dye or several.

The whyThe solvent climbs the paper and carries the dyes with it; the more soluble a dye, the further it goes. One spot becomes a ladder of colours — and a pure dye gives only one.

Ductile

HintNot malleable (that is being hammered into sheets) — the other one, about being stretched.

The whyDuctile = can be pulled into wires; malleable = can be hammered into shape. Both come from the layers of metal ions sliding over each other on the sea of electrons. Copper is also a poor choice for anything that must stay light or cheap, which is why overhead power lines are aluminium.

reversible

HintLike a door that swings both ways.

The whyMaking ammonia from nitrogen and hydrogen is the famous example: as fast as ammonia forms, some of it splits back, so the mixture never fully converts. Chemists nudge the balance with temperature and pressure and remove the ammonia as it forms. Hydrated copper sulfate going white when heated and blue again with water is the KS3 version.

A Liebig condenser

HintCold water flows through its outer jacket.

The whyVapour runs down the inner tube; cold water runs through the outer jacket — in at the bottom, out at the top, so the jacket stays full. The thermometer bulb sits level with the side arm to read the boiling point of whatever is leaving. Quizzes ask its name, and ask what goes wrong if the water is connected the wrong way round.

A gas

HintPast the top of the heating curve, every particle has broken free.

The whyBelow the melting point: solid. Between melting and boiling points: liquid. Above the boiling point: gas. Room temperature is about 20 °C, so anything that boils below that is a gas in the lab, however cold its numbers look — ammonia and chlorine both boil well below 0 °C. Negative temperatures do not mean liquid; they only mean below zero.