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For simple metal + non-metal compounds an "-ide" name means just two elements: copper oxide, zinc sulfide, sodium chloride. Switch to "-ate" and the non-metal part carries oxygen as well: nitrate, sulfate, carbonate, chlorate. So zinc sulfate is zinc, sulfur AND oxygen; zinc sulfide is only zinc and sulfur. The exceptions to learn are the groups hydroxide (OH) and cyanide (CN), which end in "-ide" but are two atoms joined, so sodium hydroxide holds sodium, oxygen and hydrogen. Learn hydroxide, nitrate, carbonate and sulfate as whole groups and most names you meet will read themselves.

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 alloy

提示Bronze, brass and steel are all examples; pure copper and pure iron are not.

为什么Pure metals are soft because their atoms sit in neat layers that slide over one another. Mix in atoms of a different size — carbon in iron gives steel, tin in copper gives bronze, zinc in copper gives brass — and the layers snag, so the metal is harder and stronger. It is a MIXTURE, not a compound: the atoms are not chemically joined, there is no formula, and the recipe can be varied to tune the properties.

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Was there enough iron, and enough of a wait, to use up every last bit?

为什么Rusting stops when either the iron or the oxygen runs out. Too little iron wool, or too short a wait, and some oxygen is still up in the tube, so the water rises only by the share that was used and the reading comes out low. Leave it a week and it climbs towards a fifth. The candle-in-a-jar version cannot be trusted at all: the flame dies long before the oxygen is gone, and burning wax hands back about two molecules of carbon dioxide gas for every three of oxygen it uses, so most of the missing gas is quietly replaced. The rise you see there is mostly hot air cooling and contracting again.

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.

Deposition

提示The reverse of sublimation — frost forming on a lawn from water vapour on a cold clear night.

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). Deposition and sublimation are the two that skip the liquid stage altogether. Energy is given OUT as particles slow and lock into place, which is why frost appears on the coldest, clearest nights — and why a freezer furs up with ice from nothing but the air inside.

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: Na2CO3, with no hydrogen at all and two sodiums to every carbonate group. It is far more alkaline, which is why it is sold for cutting grease and softening water rather than for baking.