Top of the Bench 2024

The solute

提示In brine, it is the salt, not the water.

为什么Solute + solvent → solution. Salt (solute) in water (solvent) gives brine (solution). Water is the most common solvent but not the only one — nail varnish dissolves in propanone, not water. Sea water is a solution with sodium chloride as its main solute, which is why it tastes of salt and why distilling it gives pure water.

Ionic bonding

提示Sodium gives, bromine takes, and opposites attract.

为什么Three kinds of bonding to sort compounds into. Metal + non-metal (sodium bromide, calcium chloride): ionic — the metal hands electrons to the non-metal and the charged particles attract. Non-metal + non-metal (water, methane, ammonia): covalent — atoms share electrons. Metal alone (potassium, magnesium): metallic — a sea of shared electrons. The test on paper is simply "is there a metal in it, and is there a non-metal?"

green

提示The colour in the middle of the pH chart, between the reds and the blues.

为什么Universal indicator runs red (strong acid) → orange → yellow → green (neutral, pH 7) → blue → purple (strong alkali). Rainwater is slightly acidic from dissolved carbon dioxide, tap water is close to neutral, soap is mildly alkaline. Adding alkali to an acid a drop at a time and stopping at this colour is how you make a neutral salt solution; a pH meter reading 7 does the same job more precisely.

exclamation mark; flame; hand

提示Look at the three labels on a dilute alkali bottle, an ethanol bottle and a tin of oven cleaner.

为什么Dilute sodium hydroxide carries the exclamation mark; concentrated, it carries the corrosive hand. Ethanol carries the flame; a skull and crossbones means toxic; a dead tree and fish means harmful to the environment. Quizzes show the picture and ask the word, or name the chemical and ask which symbol belongs on the bottle.

Iron fluoride

提示The metal’s name first; the halogen gets a new ending.

为什么A compound of just two elements takes the ending "-ide": magnesium + oxygen → magnesium oxide, sodium + chlorine → sodium chloride. The metal always comes first, and spelling counts — fluoride, not flouride.

The range

提示Readings that agree closely give a small one; scattered readings give a big one.

为什么The mean tells you the best estimate; the range (biggest reading minus smallest) tells you how much to trust it. Three readings of 9.1, 8.9 and 9.2 have a small range, so their mean is solid; readings of 5.2, 4.1 and 4.7 spread much wider, so that mean is shakier. Repeats also let you spot an anomaly and leave it out before averaging.

Measuring cylinder

提示Read it at eye level, at the bottom of the curved surface.

为什么A beaker’s markings are only rough guides; this is graduated for volume and read at the bottom of the curved surface (the meniscus) at eye level. For greater accuracy a pipette or burette is used. Naming apparatus — conical flask, evaporating basin, crucible, pipette, Liebig condenser — is a regular Section A mark.

A billionth of a metre

提示Small enough that a virus is about a hundred of them across.

为什么One nanometre is 1 × 10⁻⁹ m: a thousand of them make a micrometre, a million make a millimetre. Atoms are about a tenth of a nanometre across, so a nanoparticle is only a few dozen atoms across — small enough that its size changes its colour, which is what quantum dots exploit. The 2023 Nobel Prize in Chemistry was awarded for them.

Evaporation

提示Let the water leave; the solid cannot follow.

为什么Warm the solution in an evaporating basin until the liquid has mostly gone, then let it cool and the crystals grow. Heating gently and stopping early gives bigger, cleaner crystals; boiling to dryness gives a powder. Filtering will not do it — a dissolved solid passes straight through the paper. If it is the WATER you want, distil instead.

A precipitate

提示The cloudiness when limewater meets carbon dioxide is one.

为什么Two soluble compounds swap partners and one of the new pairs will not dissolve, so it drops out as a solid. Chemists use this to identify what is dissolved in water: silver nitrate gives a white precipitate with chloride, barium chloride a white one with sulfate, sodium hydroxide a blue one with copper. The reaction type is precipitation.

shell

提示Layers of an onion.

为什么Atomic radius grows steadily down any group, so given the sizes of the neighbours above and below, you can predict the one in between — calcium sits between magnesium and strontium in size just as it does in the table. Bigger atoms hold their outer electrons more loosely, which is why metals get MORE reactive down a group.

Calcium and magnesium

提示The two group 2 metals that limestone and Epsom salts are made of.

为什么Rain trickling through chalk and limestone dissolves calcium compounds; through some other rocks, magnesium. Their ions (Ca²⁺ and Mg²⁺) react with soap to make scum, so a lather only forms once all of them have been used up — the more soap that takes, the harder the water. Boiling drops some of it out as the crust inside a kettle. Hard water is perfectly safe to drink.