Salters' practical toolkit

The filtrate

提示The paper traps the gritty bits; this is the name for whatever got past it.

为什么Two words, one for each half of the split: what the paper holds back is the residue, what passes through is the filtrate. Filtering works because sand grains are far bigger than the gaps in the paper, while water molecules slip straight through — so it separates an INSOLUBLE solid from a liquid and nothing else. Rinse the residue with a little clean water before drying it, or the trapped liquid dries onto it and spoils the mass.

Chromatography

提示A single spot of black ink on damp paper opens out into a row of separate colours.

为什么A pencil line is drawn near the bottom of the paper (pencil, because ink would run and join in), a spot of the mixture goes on the line, and the paper stands in a shallow solvent that must start BELOW the line. Each dye is carried a different distance, so a mixture of four dyes gives four spots. It is how an analytical challenge shows that a "pure" colour was really a blend — and how two samples can be matched by comparing how far each spot travelled.

Distillation

提示How a ship at sea turns seawater into something the crew can drink.

为什么It collects the SOLVENT, not the dissolved solid: boil salty water, cool the steam in a condenser, and pure water drips out while the salt stays behind in the flask.

cylinder; balance; thermometer

提示Three jobs, three instruments — and none of them is a beaker, whose printed marks are only a rough guide.

为什么Each instrument answers one question, and the marks on a beaker answer none of them properly — they are moulded in for guidance, not for measuring. Read a liquid level with your eye level with the meniscus, taking the reading from the BOTTOM of the curve. Zero a balance with the empty container on it before adding the solid, so you weigh the solid and not the pot. Let a thermometer settle before reading, and leave it in the liquid while you read it.

Its boiling point

提示A pure substance changes state at one sharp fixed temperature; a mixture drifts over a spread of them.

为什么Pure water turns to steam at 100 °C at normal pressure and stays there until it has all gone. Dissolve something in it and the liquid boils higher, and over a range rather than at a fixed value — so salty water boils above 100 °C. The same trick works downwards: pure water freezes at 0 °C, salty water below it, which is why grit is spread on roads. Looking at the liquid tells you nothing at all, because a salt solution is every bit as clear as water.

solvent

提示It does the dissolving, and the solute is the thing that seems to vanish into it.

为什么Solute + solvent = solution, and the three words are worth keeping straight because a challenge question will use them instead of naming the substances. Water is the everyday one, but it is not the only one: nail-varnish remover and the liquid at the bottom of a chromatography tank are doing the same job. A substance that dissolves is soluble; one that does not, like sand, is insoluble — which is exactly why sand can be filtered out and salt cannot.

Saturated

提示The water is holding all it can at this temperature — there is one word for a solution sitting at its limit.

为什么Every solvent has a limit, and that limit is called the solubility — usually quoted as grams that will dissolve in 100 g of water at a stated temperature. Warm the water and most solids dissolve more, so a solution that was full at room temperature will take more sugar at 60 °C. Cool it back down and the extra can no longer stay in, so it comes out as crystals — which is how a chemist grows crystals on purpose.

physical

提示No new substance was made and the change can be undone — the opposite of a chemical one.

为什么The test is whether a NEW substance has appeared. Dissolving, melting, boiling and mixing make none, and each can be reversed, so all four are of this kind. Burning, rusting and fizzing an acid onto a carbonate make new substances that cannot simply be un-made, so those are chemical. It is why "the salt disappeared" is a poor observation and "the salt dissolved" is a good one — nothing was destroyed, only spread out among the water.

The safety flame

提示Why leave a burner yellow and easy to spot when nobody is heating anything?

为什么Air hole closed gives a yellow, luminous, wavering flame that is cool and, above all, visible — so nobody puts a hand through it. Air hole open gives a blue roaring flame with a pale inner cone; that is the one that heats, and the hottest part is just above the tip of the inner cone. Turn the collar to close the hole the moment you stop heating, and light the burner with the hole closed so it does not strike back down the tube.

Corrosive

提示Think what a strong acid does to skin, to cloth and to the wood it is spilled on.

为什么It means the liquid attacks living tissue and materials on contact, so it goes nowhere near skin or eyes — goggles on before the stopper comes off, and wash any splash off at once with plenty of water. Do not confuse it with the exclamation-mark symbol, which means irritant: unpleasant, but far less serious.

An acid

提示Metals bubble in only one of the two families of solution, and it is not the family that feels soapy.

为什么Metal + acid gives a salt and hydrogen, and the hydrogen is what you see streaming off the ribbon. The more reactive the metal, the harder it fizzes, so magnesium is lively where copper does nothing at all — copper sits below hydrogen in the reactivity series and will not push it out. This is the sort of "which is which?" job an identification challenge is built from: pick a test whose two possible outcomes look completely different.

gas

提示Something is leaving the mixture and rising through the liquid — and the beaker never got hot enough to boil.

为什么The signs that a chemical reaction has happened are worth knowing as observations, not as a list to recite: bubbles where nothing is boiling, a colour change, a solid appearing from two clear liquids, and the beaker warming or cooling in your hand. Write down what you actually saw and when — "steady bubbles for about ten seconds, beaker stayed cool" is a usable record, while "it went a bit funny" is not.