Mixtures, and choosing a separation technique

What comes out of the bottom of a coffee filter when you pour sea water through it? It's tempting to expect the salt to stay behind in the paper, the way coffee grounds do. It doesn't. What drips out is still sea water, every bit as salty as it went in. Yet boil the same sea water dry in a pan on a camping stove, and a white crust of salt is left behind. One method does nothing and the other works, and there's a way to tell which before you start.

A mixture is two or more substances that are together but not chemically combined, so each one keeps its own properties. Sea water is a mixture of salt and water. The salt is still salty, and the water still boils away when it's heated. Because nothing in a mixture is chemically joined, it can be separated by physical methods, with no chemical reaction needed. A compound is different: its elements are chemically combined. If that word is shaky, the video Atoms, elements and compounds covers it. Salt itself is a compound, sodium chloride. Dissolve it, filter it or boil it, and you get salt back every time, never sodium and chlorine. Now a first check. Could filtering or distilling get the oxygen out of copper oxide? No. Copper oxide is a compound, with the copper and the oxygen chemically combined, so no separating technique can touch it. One examiner's report on a Higher paper notes responses that suggested methods to separate mixtures rather than a compound, such as filtering, crystallisation and distillation. The fix is a gate before any technique: check the substance is a mixture at all, because a compound gets no separating method. There are five techniques to choose from. Four of them separate a mixture into its parts: filtration, crystallisation, simple distillation and fractional distillation. The fifth, chromatography, separates too, but its main use is to identify: to show what's in a mixture, or whether a substance is pure.

Filtration separates an insoluble solid, one that doesn't dissolve, from a liquid. Step one: fold a filter paper into a cone and sit it in a filter funnel, over a beaker or a conical flask. Step two: pour the mixture into the filter paper, and the liquid runs through. Now think about the holes in the paper. Why does the solid stay behind? The pieces of solid are too big to pass through the tiny holes in the paper, while the liquid's particles are small enough. What stays in the paper is called the residue, and the liquid that runs through is called the filtrate. Now apply that to sand stirred into water. Which is the residue, and which is the filtrate? The sand is the residue, left in the paper, and the water is the filtrate. A dissolved solid is different. It's spread through the water as particles small enough to pass through the paper with it, which is why filtration only takes out a solid that hasn't dissolved. One examiner's report on a Foundation paper describes students planning a separation who dissolved the mixture in the filter funnel instead of a beaker, and some lost credit for never mentioning the filter paper. The habit is to name the apparatus on every step: dissolve in a beaker, then pour through filter paper in a funnel. Crystallisation gets a soluble solid, one that does dissolve, back out of its solution as crystals. Step one: pour the solution into an evaporating basin and heat it gently, so some of the water evaporates. Step two: stop heating before it's dry, and leave the rest of the water to evaporate slowly. Crystals grow as it does. Now a reason to find. Why stop heating before the basin is dry? Heated all the way dry, you get a crust of tiny crystals instead of good ones, and some substances break down if they're overheated. Step three: pour off any liquid that's left, and dry the crystals on filter paper or somewhere warm. One examiner's report on a Foundation paper says the crystallisation phase of a long method question was often described very well, and was where most students scored. So if crystallisation already feels familiar, you're with a lot of students on it, and the work left is choosing it at the right moment. Here's a mixture with two things to get out of it. A beaker holds powdered chalk stirred into blue copper sulfate solution. You want the chalk in one pile and dry copper sulfate crystals in another. Name the two techniques, their order, and what you're holding after each. Have a go at this one before I show you. First, filtration. The powder doesn't dissolve, so it stays in the paper as the residue, and the blue solution runs through as the filtrate. Second, crystallisation of that filtrate in an evaporating basin, which leaves copper sulfate crystals. Filtering has to come first, or the chalk would end up mixed into the crystals.

Simple distillation gets the solvent back out of a solution. The solvent is the liquid doing the dissolving, like the water in sea water. Step one: heat the solution in a flask until the water boils. A thermometer at the top reads the temperature of the vapour leaving. Step two: the vapour passes into a condenser, a tube with cold water flowing around the outside, where it cools and condenses back into liquid. Step three: that liquid drips into a beaker, and the collected liquid is called the distillate. Now a reason check. The water has boiled off and come over, so where is the salt, and why? Still in the flask. Salt doesn't boil at one hundred degrees C, so it stays behind while the water leaves. One examiner's report on a Higher paper says answers needed more detail, and often didn't describe the solid sodium chloride staying in the flask once the pure water had evaporated. So end every distillation answer with where the solid finishes: left behind in the flask. Now a choice. With sea water, when do you crystallise it, and when do you distil it? Crystallise it when you want the salt. Distil it when you want the water. Fractional distillation separates two or more liquids that are mixed together, using the difference in their boiling points. It needs all three conditions: the parts are liquids, they're mixed together, and they boil at different temperatures. The mixture is heated in a flask with a fractionating column above it, a tall glass tube packed with glass beads, and a condenser off the top. The column is hottest at the bottom and coolest at the top, and a vapour that cools below its boiling point condenses and runs back down. Now predict the order. Which liquid's vapour makes it to the top of the column first? The one that boils at the lowest temperature. It passes into the condenser and is collected first. The thermometer stays at that first liquid's boiling point until all of it has come over, then the temperature rises and the next liquid follows. On sand in water, the only thing that comes over is the water. Sand needs a filter, not a column. Here's a pair of liquids. Propanone boils at fifty-six degrees C, water at one hundred degrees C, and the two are mixed. Which technique, which liquid comes off first, and why would filtering achieve nothing? Fractional distillation, because the parts are liquids, mixed together, with different boiling points. Propanone comes off first, because fifty-six degrees C is the lower boiling point. And filtering achieves nothing, because both are liquids, and both run straight through the paper.

The fifth technique, chromatography, separates substances that are dissolved together in a solvent, like the dyes in an ink. It also identifies what's in a mixture, and how it does that is the job of the video Chromatography and Rf values. Not every course stops at paper chromatography. Some also name thin layer and gas chromatography, and the screen shows which. Now a question about jobs. To show which colourings are in an orange fizzy drink, would you distil it? No. That question wants the colourings identified, not separated out, so it calls for chromatography. One examiner's report on a Foundation paper, on showing what was in a mixture, says the successful answers described chromatography or measuring a boiling point, and distillation, filtration and crystallisation were common incorrect responses. The fix is to decide the job before the technique: separating, or identifying. With chromatography placed, the choosing rule runs like this. Check it's a mixture at all, decide the job, then look at the parts, and an undissolved solid means filtration. Now, with the solid dissolved, what decides between crystallisation and distillation? Which part you want to keep: the solid by crystallisation, or the solvent by distillation. Liquids mixed together with different boiling points mean fractional distillation. And whatever you choose, write its name first, then describe it. Now a last worked case, with the early steps done. Two clear solutions are mixed, and a white solid appears and settles. A solid made like this is called a precipitate, and it doesn't dissolve. So it's a mixture, the job is separating, and an undissolved solid means filtration. The solid is wanted clean and dry. What are the residue and the filtrate, and what do you do to the solid next? The white solid is the residue, and the solution that runs through is the filtrate. Then rinse the solid with a little pure water to wash off any solution, and leave it somewhere warm to dry.

Now four to finish. Sand in salt water, and you want dry salt: which techniques, in order?

Filtration first, leaving the sand as the residue, then crystallisation of the salty filtrate.

Now two liquids. They're mixed, one boiling at sixty-five degrees C and one at one hundred and eighteen degrees C. Which technique, and which liquid comes off first? Fractional distillation, and the lower boiling one comes off first.

Now a compound. Iron sulfide is made of iron and sulfur. Which technique gets the iron out? None of them. In a compound the iron and sulfur are chemically combined, so no separating technique applies.

Now a definition to finish. What is a mixture? Two or more substances together but not chemically combined, each keeping its own properties.

Back to the coffee filter. The salt in sea water is dissolved, spread out as particles small enough to pass straight through the paper. Boiling it dry worked because the water boils away and the salt can't, and done gently, that's crystallisation. And if it was the water you wanted, you'd distil it instead. The mixture tells you which one, before you start.

If you can run the choosing rule on a mixture you've never seen, that's in the bag, so give it a thumbs-up. If not, don't worry: the rest of the chapter keeps using this, so it comes back round.

Next in the chapter is Pure substances, and spotting an impurity from a melting point. Hopefully that's next in your feed, but if not, search for ScholaFly C H oh three, oh four.

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