Top of the Bench 2016

Exothermic

HintThe prefix means "out" — think of energy leaving the chemicals and arriving in the beaker, the bench and your hand.

The whyCurie always asks "where did the energy go?" before naming anything. If the surroundings got warmer, the reacting particles handed energy over: exothermic. Burning, neutralisation, respiration and most metal-plus-acid reactions all do this. The clue on paper is a temperature RISE; the clue on an energy diagram is products drawn lower than reactants.

Thermal decomposition

HintLimestone roasted in a kiln to make quicklime is the industrial example.

The whyNothing is added — the heat itself pulls the compound apart. Metal carbonates split into a metal oxide and carbon dioxide; that is why the solid loses mass and why limewater turns milky above it. Metal-carbonate decompositions like this one are endothermic — they stop the moment you stop heating — which is a favourite quiz pairing.

endothermic

HintThe prefix means "in" — the process is a taker, not a giver.

The whyChanges of state that loosen particles (melting, evaporating, boiling) need energy in; so does photosynthesis, which stores sunlight as sugar; so does the thermal decomposition of a carbonate. The tell-tale on the bench is that the surroundings get COLDER — a cold pack, or a beaker that feels chilly after two solids are stirred together.

Filtration

HintThe paper has tiny holes — the solid stays behind, the liquid runs through.

The whyIt only works for an INSOLUBLE solid. The liquid that runs through is the filtrate; what stays on the paper is the residue. Dissolved salt would sail straight through.

Distillation

HintBoil it off, then cool the vapour back to a liquid in a condenser and collect it.

The whyA dissolved solid cannot be filtered out — the particles are far too small — so you separate on boiling point instead. Water boils at 100 °C and leaves; the salt, which would need over 1400 °C, stays in the flask. If you only want the SOLID, evaporate instead; if you want the LIQUID, distil. Quiz setters ask both directions in the same section.

fuel; oxygen; heat

HintRemove any one corner of the triangle and the flame dies: the cloth, a bucket of water and turning off the gas each remove a different one.

The whyThe fire triangle is really three conditions for combustion. A fire blanket cuts off the air; water takes the heat away; turning off a gas tap removes the fuel. It explains the odd-looking facts too — a chip-pan fire must NOT be doused with water because the water flashes to steam and flings burning oil about.

Carbon dioxide

HintEvery atom of the fuel’s black element leaves with two oxygens attached.

The whyComplete combustion of anything made of carbon and hydrogen gives just two products: carbon dioxide and water. Every carbon atom picks up two oxygens, every pair of hydrogens picks up one. With too little air the carbon cannot get its full share and you get carbon monoxide and soot instead — incomplete combustion, and the reason gas boilers need ventilation.

Ethanoic acid

HintIts older name was acetic; it forms when wine is left open and goes sour.

The whyA weak acid at around pH 3 — safe enough for chips, yet it still fizzes gently with bicarbonate of soda. Quiz rounds love "the chemical name for…": vinegar → ethanoic acid, lemons → citric acid, dry ice → solid carbon dioxide, table salt → sodium chloride, bicarbonate of soda → sodium hydrogencarbonate.

Mercury

HintThe silver liquid once inside thermometers — dense enough that an iron nail floats on it.

The whyBromine (a red-brown non-metal) and mercury (a silver metal) are the pair every chemistry quiz asks for. Mercury is a liquid at room temperature yet still a metal: it conducts, it is shiny, and it is so dense that steel sinks in it only with difficulty. Its vapour is poisonous, which is why thermometers switched to alcohol.

Hydrogen

HintThe lightest element of all.

The whymetal + acid → salt + hydrogen, every time the metal is above hydrogen in the reactivity series. The salt takes its surname from the acid: sulfuric gives a sulfate, hydrochloric a chloride, nitric a nitrate. The reaction is exothermic, which is why the acid warms up.

excess

HintThe opposite of running out — there is spare left in the flask at the end.

The whyWhichever reactant runs out first decides how much product you get; the other is in excess. Drop marble chips into a little acid and the fizzing stops while chips remain — the acid was the limiting reactant and the carbonate was in excess. Add far more acid than the chips need and the chips vanish, the acid is in excess, and the volume of gas is fixed by the carbonate alone.

Heat is lost to the surroundings

HintNot all of the flame’s energy ends up in the water — think about where else it can go.

The whySome energy warms the air, the can and the stand; some fuel burns incompletely; some vapour escapes. So calorimetry under-reads. A copper can, a draught shield and a short gap between flame and can all reduce the loss but never remove it. "Explain why the experimental value is lower" wants heat loss, not "the student made a mistake".