Top of the Bench 2019

CFCs from aerosols and fridges

提示Chemicals once used as spray propellants and coolants, banned since the late 1980s.

为什么Two different problems that quizzes love to tangle. Global warming is extra carbon dioxide and methane trapping heat: rising seas, melting ice, wilder weather. The ozone hole was chlorine-containing gases (CFCs) breaking up the high-altitude ozone that screens out ultraviolet light. Banning CFCs is slowly closing the hole; it did nothing for the temperature. "Which is NOT a consequence of global warming?" — the ozone hole.

A polymer

提示Carrier bags and PVC pipes are made of them — one enormous chain after another.

为什么The small molecules are monomers; ethene molecules link up to make polythene, the plastic of carrier bags. Because a polymer is one enormous chain, you draw just the repeating unit with a bracket and an n. Plastics are unreactive — handy for storing food, awkward for the ocean, since most do not rot.

An alloy

提示Steel, brass and bronze are all examples.

为什么Pure iron is soft; add a little carbon and you get steel, which is far harder because the different-sized atoms stop the layers sliding. Stainless steel mixes in chromium and nickel to resist rust. An alloy is a MIXTURE, not a compound — no fixed formula, and the metals keep their own atoms.

neutrons

提示The nucleus holds two kinds of particle; only one of them shows up in the atomic number.

为什么The smaller number on a periodic-table tile (the atomic number) counts protons; the larger one (relative atomic mass, rounded) counts protons AND neutrons. Electrons weigh almost nothing and orbit outside, so they do not add to the mass number — but in a neutral atom there are exactly as many electrons as protons. Quiz question: "protons, neutrons, electrons?" is really two subtractions.

Magnesium sulfate

提示Metal first, then a surname borrowed from the acid.

为什么metal + acid → salt + hydrogen. Sulfuric acid makes sulfates, hydrochloric acid chlorides, nitric acid nitrates — so magnesium in hydrochloric acid gives magnesium chloride instead.

Its electrons are delocalised

提示Look at the "sea" in the bonding diagram — what is free to drift when a voltage is applied?

为什么In a metal the outer electrons are not tied to one atom; they are shared across the whole lump, and a voltage sets them drifting through it, carrying the charge. The same sea explains the other metal properties: it holds the positive metal ions strongly (high melting points) and lets layers of ions slide over one another without the structure breaking (malleable, ductile).

oxygen; water

提示One comes from the air, the other from rain; each control tube in the classic experiment removes just one of them.

为什么Rust is hydrated iron oxide: iron + oxygen + water → hydrated iron oxide. Take away either partner and nothing happens, which is exactly what paint, grease and galvanising do — they keep both away from the metal. Salt does not cause rust, it speeds it up.

Carbon dioxide

提示Not the squeaky-pop gas — the one that is heavier than air.

为什么acid + carbonate → salt + water + carbon dioxide. This is the reaction behind bath bombs, indigestion tablets and the way acid rain eats limestone. Metals give hydrogen with acid; carbonates give carbon dioxide — mixing the two up is the classic quiz slip. Bubble the gas through limewater and it goes cloudy.

Carbon

提示The reactivity series has one non-metal slotted in just above iron — that one.

为什么Carbon sits above iron (and zinc, lead and copper) in the reactivity series, so when heated together it grabs the oxygen: iron oxide + carbon → iron + carbon dioxide. Iron is reduced (it loses oxygen); carbon is oxidised. Metals above carbon — aluminium, magnesium, sodium — cannot be won this way and need electrolysis instead, which is why they were discovered so much later.

Citric acid

提示The sharpness your tongue meets in orange juice, not the one in vinegar.

为什么A weak acid, safe to eat, around pH 2–3 in fruit juice. It is the acid in sherbet: mixed dry with bicarbonate of soda nothing happens, but on a wet tongue they react and fizz. The everyday-acid list quizzes reach for: citric (citrus fruit), ethanoic (vinegar), lactic (sour milk), carbonic (fizzy drinks).

heavier

提示Ask which direction the atoms travelled — from the air onto the metal, or off it.

为什么Atoms are neither made nor destroyed, so a solid can only "gain" mass by capturing something that was not on the balance. Rust is iron plus oxygen plus water, so the flaky product weighs more than the shiny metal did. The same logic in reverse explains why a heated carbonate gets lighter — its carbon dioxide left.

Allotropes

提示Oxygen gas and ozone are another pair of the same kind.

为什么Carbon has several: diamond (every atom bonded to four others — hard, no free electrons, does not conduct), graphite (layers, each atom bonded to three — soft, slippery, conducts), plus graphene, nanotubes and the football-shaped fullerene. Same element, same atoms; only the arrangement differs, and that changes everything.