Top of the Bench 2023

Neutralisation

提示A base cancels out an acid — even when the base is an insoluble black powder rather than an alkali.

为什么acid + base → salt + water. A base is any substance that neutralises an acid; an alkali is just a base that dissolves in water. Metal oxides and hydroxides are bases, so copper oxide in an acid is a neutralisation exactly as sodium hydroxide in an acid is — the coloured solution is the copper salt. Nothing insoluble forms and no element swaps places, so it is not precipitation or displacement.

Chlorine

提示The element that keeps tap water safe too — group 7, second row down.

为什么The same element sterilises tap water, killing microbes by reacting with them. The "swimming-pool smell" is not the element itself but chloramines — compounds it forms with sweat.

Dissolved in water

提示The fourth state symbol, after s, l and g.

为什么Four state symbols: (s) solid, (l) liquid, (g) gas, (aq) aqueous — dissolved in water. So HCl(aq) is hydrochloric acid, the solution, while HCl(g) is hydrogen chloride gas. Water itself is (l), never (aq). Equations with state symbols earn an extra mark on the Older paper most years.

2

提示Helium’s only shell is completely full.

为什么The first shell holds 2, the second 8, the third 8 — so lithium (3 electrons) is 2,1, sodium (11) is 2,8,1 and argon (18) is 2,8,8. The number in the OUTER shell is what decides how an element behaves: one outer electron gives a group 1 metal, a full outer shell gives a noble gas. Drawing these dot-diagrams is a regular Younger-paper mark.

corrosive; 14; purple

提示Oven cleaner: the pictogram on the tin, the end of the scale, the colour on the paper.

为什么A strong alkali sits at the top of the pH scale, and universal indicator shows it as deep purple. Concentrated, it attacks skin, which is why oven cleaner carries the hand-and-surface pictogram and why you wear goggles. Alkaline solutions like this are used to scrub acidic gases such as carbon dioxide out of a gas stream.

lilac

提示Not sodium’s yellow, not lithium’s red.

为什么Lithium fizzes; sodium melts into a ball that skates about; potassium is so reactive that the hydrogen ignites at once and burns with the metal’s own colour, sometimes with a small bang. Those three observations are the standard evidence that reactivity increases down group 1. Flame colours to know: lithium red, sodium yellow, potassium lilac, copper blue-green.

Electrolysis

提示How aluminium is won from its ore, and how a spoon gets its silver plating.

为什么Electricity can pull a compound apart that heat alone cannot. Water gives twice as much hydrogen as oxygen, because each molecule has two hydrogens to one oxygen. Store the hydrogen and burn it later and you have a way of saving energy from a windy day for a still one — the idea behind hydrogen cars.

Saturated

提示Full up — like a sponge that cannot soak up another drop.

为什么The sugar (solute) breaks into particles that slip between the water (solvent) particles, but a solvent can only hold so much at a given temperature. Warm the tea and more will dissolve — solubility rises with temperature for most solids — so a solution that was saturated when hot drops crystals as it cools. Mass is still conserved: the solution weighs sugar plus tea.

Methane

提示The simplest hydrocarbon — cows and landfill sites leak it.

为什么One carbon, four hydrogens: CH4. Burn it in plenty of air: methane + oxygen → carbon dioxide + water. It is also a powerful greenhouse gas, which is why capturing it from rotting food waste (biogas) and burning it is better than letting it escape. Biogas counts as renewable because the waste that makes it is produced continuously; natural gas from underground took millions of years.

Absorbs it and re-emits some downward

提示The Sun’s light gets in fine; the problem is what happens to the warmth on its way back out.

为什么Sunlight arrives as visible light, which the air lets straight through; the warmed ground gives its energy back as invisible infrared. A greenhouse gas is one whose molecules can soak up that infrared and then give it out again in every direction — including back towards the ground. The more of those molecules there are, the more of the outgoing warmth gets returned, and the planet settles at a higher temperature. Nitrogen and oxygen cannot do this; carbon dioxide, methane and water vapour can.

Carbon and hydrogen only

提示Methane and propane qualify; ethanol, with its oxygen, does not.

为什么The word "only" is the mark-earner: ethanol contains carbon, hydrogen AND oxygen, so it is not a hydrocarbon. Methane (natural gas), propane (camping gas) and octane (petrol) are. Because they contain nothing else, complete combustion gives just two products — carbon dioxide and water.

one

提示Count the electrons in its outer shell.

为什么Losing that outer electron leaves the atom with three protons and only two electrons, so it carries a single positive charge. Every group 1 metal does the same; group 2 metals lose two and become 2+. This is what happens inside a lithium battery as it powers a phone — the metal gives up its electron and the electron does the work.