Top of the Bench 2025

78; 21; 0.04

提示Roughly four-fifths, one-fifth, and a trace.

为什么Four numbers worth knowing: 78, 21, 0.9 (argon) and 0.04. The last is tiny yet it is the one that matters for climate. Air is a MIXTURE — the gases are not bonded to each other — which is why they can be separated: cool air until it is a liquid, then let it warm and collect each gas as it boils off at its own temperature.

Nitrogen

提示Two identical atoms bonded together cannot absorb the infrared that three-atom molecules can.

为什么Greenhouse gases absorb infrared radiated by the ground and send some of it back. Carbon dioxide, methane and water vapour all do; the two-atom nitrogen and oxygen, and the single-atom noble gases, do not. Water vapour is the one people forget — it is actually the biggest contributor; carbon dioxide matters because human activity is raising it.

Oxygen

提示Chlorides and sulfides have two elements; chlorates and sulfates have three — what is the third?

为什么An "-ide" name means just two elements: copper oxide, zinc sulfide, sodium chloride. An "-ate" name means the non-metal part also carries oxygen: nitrate, sulfate, carbonate, chlorate. So zinc sulfate is zinc, sulfur AND oxygen; zinc sulfide is only zinc and sulfur. It is the quickest way to read a name you have never seen before.

Copper

提示The metal of the 2p coin and of the Statue of Liberty’s green skin.

为什么Flame colours: lithium red, sodium yellow, potassium lilac, calcium orange-red, copper blue-green. Blue fireworks are usually copper; green ones are barium.

An element

提示If it has its own square on the periodic table, it is one.

为什么Elements are the entries on the periodic table — about 118 of them, and everything else is built from them. Oxygen gas is an element even though it comes as O2 molecules, because both atoms are the same kind. Water, with hydrogen and oxygen bonded together, is a compound; air, with several gases merely mixed, is a mixture.

density

提示The same stuff spread through a bigger space — which property drops?

为什么Warm gas particles move faster and spread further apart, so the same mass now fills a larger volume. Nothing has been lost or gained. A hot-air balloon works the same way: the burner warms the air inside, it expands, and the balloon holds air that is lighter for its size than the cold air around it — so it floats, as a cork floats on water.

Not all the oxygen reacted

提示Did the flame use up every last bit before it went out?

为什么A flame goes out well before the oxygen is gone, so the water only rises by the share the candle managed to burn — the rest is still inside the jar and the answer reads low. The same logic applies to iron wool rusting in a tube of air: too little iron, or too short a time, and some oxygen is left. Reading the volumes while the gas is still warm and expanded also reads low.

Conical flask

提示Its shape is the clue — think of what a cone looks like from the side.

为什么Its narrow neck makes it the flask for titrations and for reactions that fizz. A beaker has straight sides and a lip; a round-bottomed flask needs a stand; a volumetric flask has a single mark on a long neck.

Sulfur dioxide

提示It comes from an impurity in the coal, not from the carbon.

为什么Coal is never pure carbon; the sulfur in it burns too, and the gas that forms reacts with water and oxygen in the air to give sulfuric acid, which falls as acid rain. It damages limestone buildings, acidifies lakes and harms trees. Modern power stations scrub it out with lime; low-sulfur fuels are the other fix. Carbon dioxide makes rain only mildly acidic — sulfur oxides do the real damage.

Condensation

提示Droplets forming on the inside of a cold window on a winter morning.

为什么Six changes of state: melting (solid → liquid), freezing (liquid → solid), boiling or evaporating (liquid → gas), condensing (gas → liquid), subliming (solid → gas) and depositing (gas → solid). Condensing is what the Liebig condenser does to steam, and what clouds are made of. Energy is given OUT as particles slow and cluster.

diffusion

提示The same word describes a drop of ink slowly spreading through a glass of still water.

为什么Particles in a gas move constantly and randomly, bumping into each other, so they gradually spread out. Lighter molecules diffuse faster: ammonia (light) outruns hydrogen chloride (heavy) along a glass tube, so the white ring where they meet forms nearer the hydrogen chloride end. Diffusion is slower than the molecules’ own speed because they collide and zigzag rather than travel straight.

Sodium hydrogencarbonate

提示Not the same as washing soda — this one has an extra H in the formula.

为什么Formula NaHCO3, also called bicarbonate of soda. Warm it or add acid and it releases carbon dioxide, which is what lifts the cake — and what makes sherbet fizz on a wet tongue. Washing soda, sodium carbonate, is its harsher cousin: the same elements, one hydrogen fewer, and far more alkaline.