Pure substances, and spotting an impurity from a melting point

Picture a bottle of spring water with the words one hundred per cent pure across the front. Is that water pure? Turn it round, and the same label lists the calcium, the magnesium and the nitrate dissolved in it. In everyday English, that water is as pure as water gets. In chemistry it isn't pure at all, and the cheap table salt in your kitchen comes far closer. The two verdicts come from two meanings of one word.

In chemistry, a pure substance is a single element or a single compound, with nothing else mixed in. Pure copper is a single element. Pure water is a single compound, H two O and nothing else. In everyday use, pure means something else: natural, untouched, nothing added. That's the meaning on the bottle. By the chemistry meaning, spring water is a mixture, because other substances are dissolved in the water. Now a check. Sugar is stirred into water until the water looks perfectly clear. Is it pure? No. It's still sugar plus water, two things together, and looking clear changes nothing. Now three labels to test against that meaning. Spring water, pure orange juice or pure gold: which passes the chemistry meaning of pure? Only the gold. Spring water and orange juice are both mixtures of many substances, while pure gold is, very nearly, one element. One examiner's report on a Higher paper says it is not enough to state that a pure substance is made up of one type of substance, and answers need to indicate a single element or compound. So say both options by name: a single element, or a single compound.

A pure substance has a property that follows from that definition: it melts at one fixed temperature, and it boils at one fixed temperature. Pure ice melts at nought degrees C, and pure water boils at one hundred degrees C, at normal air pressure, every time. In a pure substance every particle is the same, so they all break free at the same temperature. A mixture has more than one kind of particle, so it melts and boils over a range of temperatures instead, starting at one temperature and finishing at a higher one. Now a new case. Salt water starts boiling just above one hundred degrees C, and the temperature keeps rising as it boils. Is salt water pure, or a mixture? A mixture, because there's no single fixed point, only a band. On a thermometer scale, a pure substance is one tick, and a mixture is a band. That gives two separate facts about a pure substance. What it is: a single element or compound. And what it does: melts and boils at fixed temperatures. Now a student is asked what is meant by a pure substance, and writes: a pure substance is one that melts at a single temperature. Why does that answer miss, and how would you rewrite it? It gives the property, what it does, where the question wanted what it is. Rewrite it as: a single element or a single compound. One examiner's report on a Higher paper describes answers that failed by giving pure substances a fixed melting point, which it calls a property of a pure substance rather than what is meant by one. The habit is to tag the question before answering: is it asking what a pure substance is, or what it does.

Melting point data can now sort substances into pure and impure, where impure means a mixture. Impure isn't always a fault, though, and the video on formulations is about mixtures made on purpose. Four substances are heated. A melts at eighty degrees C, and B melts between seventy-one and seventy-eight degrees C. C melts at one hundred and thirteen degrees C, and D melts between one hundred and eight and one hundred and fifteen degrees C. Sort them into pure substances and mixtures, and write the reason using the words fixed and range. Try it on paper before I show you the answer. A and C are pure, because each melts at one fixed temperature. B and D are mixtures, because each melts over a range of temperatures. The size of the numbers doesn't decide it. B's band sits lower than A, but D's band reaches above C, and only the shape decides: one temperature, or a band. A Higher examiner's report from another year says students found it fairly straightforward to state which substances were pure or impure. The same report then notes where justifications went wrong: melting points described as accurate or precise, and those terms were ignored. The words that count are fixed and range. And a mixture doesn't have several melting points: it melts across one band.

To show in practice that a substance is pure, you measure its melting point or its boiling point. Now, once you have that measurement, what do you compare it with? The accepted value for that substance, the one in a data book. That makes three steps: measure it, quote the number with its unit, and compare it with the accepted value. Here's a bottle labelled propanone, and pure propanone boils at fifty-six degrees C. Describe how you would show whether it really is pure propanone, and say what result would prove it isn't. Have a go at this one before I show you. Heat a sample of the propanone and measure the temperature at which it boils, then quote that temperature with its unit. Compare it with the accepted value. If it boils sharply at fifty-six degrees C and stays there, it's pure propanone. If it boils over a range, it's a mixture. If it boils at one fixed temperature that isn't fifty-six degrees C, it's something else entirely. One examiner's report on a Higher paper gives the numbers for a question like this: fewer than a quarter of students gained at least one mark. Some realised a pure substance boils at a fixed temperature, but did not go on to measure and quote the boiling point. So every how-would-you-show answer gets all three steps written out: measure, quote, compare. The other purity test, a single spot on a chromatogram, is in the video Chromatography and Rf values.

Now three questions to close, starting with a sample that melts between one hundred and twenty-one and one hundred and twenty-six degrees C.

Is that sample pure, or a mixture? A mixture, since it melts across a band, not at one fixed point.

Now the definition. What is meant by a pure substance? A pure substance is a single element or a single compound, with nothing else in it.

Now a labelled bottle. A liquid labelled ethanol boils sharply at sixty-five degrees C, and ethanol's accepted boiling point is seventy-eight degrees C. Is the liquid in the bottle pure ethanol? No. It boils at one fixed temperature, so it may well be a pure substance, but it isn't ethanol.

Back to the bottle. Its label says pure in the everyday sense, natural, and the list on the back is the other substances in the mixture. Table salt is very close to a single compound, sodium chloride, which melts at one fixed temperature, eight hundred and one degrees C. So in chemistry the word pure belongs to the salt, and the water on the bottle is the mixture.

If you can sort a melting point table and define pure without mentioning melting, there's no need to see this twice: give it a thumbs-up. If not, say the definition out loud once and it sticks, then come back.

Next in the chapter is Formulations. Hopefully that's next in your feed, but if not, search for ScholaFly C H oh three, oh five.

Every other video is on scholafly.com too, easy to search by video code or by keyword. Whatever you're revising, go there and have a look, or, watch the next video.