Chemistry challenge toolkit

Potassium

提示Think about where each of the three sits in its column — and which way the trend runs.

为什么All three alkali metals fizz on water, giving off hydrogen and leaving a metal hydroxide — an alkali, which is where the family name comes from. Lithium fizzes, sodium melts into a darting ball, potassium bursts into a lilac flame. Down group 1 the metals get more reactive, softer and lower-melting. The halogens in group 7 run the other way: fluorine is the most reactive, iodine the least.

12

提示The atomic number counts the protons on their own; the mass number counts the protons and the neutrons together.

为什么The atomic number is the proton count, and it is what fixes an element’s place in the periodic table — sodium is eleventh because it has 11 protons. The mass number counts protons plus neutrons, so the neutrons are 23 - 11 = 12. A neutral atom holds as many electrons as protons, so sodium also has 11 electrons, arranged 2, 8, 1 — and that single outer electron is exactly why sodium sits in group 1 and reacts so hard with water. Change the neutron count on its own and you have an isotope: same element, same chemistry, different mass number.

The halogens

提示Chlorine in a swimming pool and iodine on a cut are both members.

为什么Going down group 7 the members get LESS reactive — the opposite of group 1 — and their state changes: chlorine is a green gas, bromine a red-brown liquid, iodine a grey solid that gives off a purple vapour. Bromine is the only non-metallic element that is liquid at room temperature (mercury is the liquid metal), a pair every quiz-setter knows. Each halogen reacts with sodium to make a salt such as sodium chloride.

The noble gases

提示Helium balloons and argon-filled light bulbs are chosen because nothing happens.

为什么Unreactive is the fact to hold: helium fills balloons safely, argon fills light bulbs so the filament does not burn away, neon glows in signs. Because they are very unreactive, they usually exist as single atoms rather than molecules — unlike oxygen (O2) or nitrogen (N2). Group 0 is the one column with no metals and no reactive elements at all.

A compound

提示Each clue rules a different answer out: sharp melting kills one of the three, and splitting into simpler substances kills another.

为什么Take the clues one at a time, the way Curie does. A sharp, fixed melting point means the substance is pure, so it is not a mixture — a mixture melts over a range, and could anyway be pulled apart by physical means such as filtering, evaporating or chromatography. Being split by heat into simpler substances means it is not an element either, because an element cannot be broken down by any chemical change. Heating copper carbonate is the school example: one pure green solid becomes black copper oxide plus carbon dioxide. Neither clue settles it alone — the pair together pin the answer down.

Everything inside the bracket

提示Compare it with the small 2 in H2O, which belongs to one symbol only.

为什么A small number applies only to the symbol immediately before it; a number after a bracket multiplies the whole bracket. So Ca(OH)2 is one calcium, two oxygen and two hydrogen — five atoms — while CaOH2 would mean something different. Counting atoms this way is what "is this equation balanced?" questions really test: the same atoms must be counted on both sides of the arrow.

Oxygen from the air

提示Nothing was created — the metal captured something out of the surroundings.

为什么Atoms are never made or destroyed in a reaction, only rearranged, so the mass of everything that went in equals the mass of everything that came out. magnesium + oxygen → magnesium oxide: the oxygen was not on the balance at the start, so the solid seems to gain. Heat a metal carbonate and the opposite happens — carbon dioxide escapes, so the solid seems to lose. Ask "which gas came in or went out?" and the mass puzzle solves itself.

displacement

提示Ask what happened to the copper — the reaction is named after it.

为什么Iron sits above copper in the reactivity series, so it takes the sulfate and turns copper out as a brown solid: iron + copper sulfate → iron sulfate + copper. A metal can only displace one BELOW it.

salt

提示The crystals left behind when the water is evaporated off — sodium chloride is the everyday example.

为什么Hydrochloric acid + sodium hydroxide → sodium chloride + water. The salt takes its first name from the metal in the alkali and its surname from the acid: hydrochloric acid makes chlorides, sulfuric acid makes sulfates, nitric acid makes nitrates. So "which salt forms from potassium hydroxide and nitric acid?" wants potassium nitrate. In chemistry a salt is any compound made this way — table salt is just the famous one.

acidic; neutral; alkaline

提示Lemon juice, pure water and oven cleaner sit in three different places on the scale.

为什么pH runs from 0 to 14, with the most acidic at 0 and the most alkaline at 14 — so pH 2 is MORE acidic than pH 6, not less. Universal indicator paints the bands: red and orange for acids, green for neutral, blue and purple for alkalis; litmus only says acid (red) or alkali (blue). Lemon juice is about pH 2, milk about 6, bicarbonate of soda about 9 — the everyday values quiz-setters reach for.

Hydrogen

提示The lightest gas of all — the one that fizzes off when a metal meets an acid.

为什么Three tests, three results: hydrogen burns with a squeaky pop; oxygen relights a glowing splint; carbon dioxide turns limewater milky. Chlorine bleaches damp blue litmus paper. Quiz heats ask them in every direction — "which test?", "which gas?", "which result?" — so learn each as a pair. The pop is a tiny explosion: hydrogen + oxygen → water.

Melting

提示All the energy is going into loosening the particles apart, not into making them move faster.

为什么The flat stretch on a heating curve is a change of state: the energy arriving is spent pulling particles free of their neighbours, so it cannot raise the temperature until the change is complete. A second flat comes at 100 °C for boiling. A pure substance gives sharp plateaus at fixed temperatures; a mixture melts over a range — which is how a quiz asks "is it pure?" without ever using the word.