Top of the Bench 2016

Exothermic

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

为什么Curie 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

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

为什么Nothing 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

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

为什么Changes 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

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

为什么It 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

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

为什么A 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

提示Remove 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.

为什么The 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

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

为什么Complete 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

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

为什么A 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

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

为什么Bromine (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

提示The lightest element of all.

为什么metal + 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

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

为什么Whichever 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

提示Not all of the flame’s energy ends up in the water — think about where else it can go.

为什么Some 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".