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Separating mixtures

Choosing a separation technique from the property the parts of a mixture differ on, rather than from memory.

Paper packet. Every task here also exists on screen, where it is checked automatically; answers written on paper are not assessed by Nydus. When you are back at a device, enter your answers there.

1. What you will learn

By the end of this lesson you will be able to choose a separation technique for a described mixture and say which property difference it makes use of — size, boiling point, density, solubility or magnetism. You will be able to order the stages of a multi-step separation, and to say why filtration will never separate a solution and why evaporation and distillation are opposites rather than alternatives.

2. What you already have

You can tell a mixture from a compound, and you know that the substances in a mixture are not joined to each other. That is the fact this whole lesson rests on: because nothing is joined, nothing has to be broken, and a mixture can be taken apart by physical means alone. A compound cannot, which is why no technique in this lesson will ever separate water into hydrogen and oxygen.

3. Words for this lesson

4. Find the property they differ on

There is no list of techniques to memorize. There is one question — on what property do the parts of this mixture differ? — and each answer has a piece of apparatus attached to it.

And one warning that saves more marks than the whole table: a solution cannot be filtered. A dissolved solute is no longer in grains. It goes through the paper with the solvent, every time.

5. The method, step by step, and how to check it

Name the parts and the product. Write down what is in the mixture and which part is wanted. A separation with no named product has no right answer.

Ask whether anything is dissolved. This one question splits the techniques into two families. If nothing is dissolved, the parts are separate objects and a mesh, paper, magnet or stopcock can pick them apart. If something is dissolved, only a boiling point or a solubility change will.

Find the property difference. Size, boiling point, density, solubility, magnetism, or how strongly something clings to paper.

Choose the technique that uses it. And where the technique depends on which half is kept, choose by the product.

Order the stages. Dry, coarse steps first; dissolving next; filtering before evaporating.

Check the work. Can you name the property your technique uses? Does it keep the part that was wanted? Would a dissolved solid get through any filter in your plan? And at the end, has every part of the mixture been accounted for — does the mass of the parts add up to the mass you started with?

6. Why each step is allowed

A mixture can be separated physically because its parts are not joined. Each keeps its own properties — its own boiling point, its own density, its own solubility — and those differences are what every technique works on.

Asking whether anything is dissolved first is allowed because dissolving changes which properties are available. A dissolved solute is spread out as single particles among the solvent's particles, so it has no size a paper could catch. Only a property such as boiling point still tells it apart.

Ordering the stages by what each needs is needed because a step changes the mixture for every step after it. Evaporating before filtering would dry the sand into the salt; dissolving before using a magnet would wet the iron filings and make them harder to lift.

Checking the masses is allowed because a physical separation creates and destroys nothing. If 50 g of rock salt went in and 14 g of sand and 36 g of salt came out, nothing was lost; a shortfall means some product was left behind on the paper or in the dish.

7. Evaporation and distillation are opposites

Both separate exactly the same mixture: a solid dissolved in a liquid. They are not two ways of doing the same job, and choosing between them is not a matter of taste.

Evaporation heats the solution until the solvent has left as vapor, into the room. You keep the solid. The solvent is gone.

Simple distillation heats the same solution, but the vapor is led into a condenser — a tube with cold water running round the outside of it — where it cools back to a liquid and drips into a second flask. You keep the liquid. The solid stays behind in the first flask, and you could have it too.

So the question which one? is really the question which half is the product? A salt works evaporates seawater, because it wants salt. A desalination plant distills seawater, because it wants water. Same mixture, same property difference, opposite apparatus, and each one throws away what the other was after.

Crystallization is evaporation's careful cousin. Evaporating a solution to dryness gives a powder. Driving off only some of the solvent and then letting the solution cool slowly gives large, clean crystals, because a crystal that grows slowly excludes whatever else was in the solution. That is why a chemist purifying a new solid crystallizes it rather than evaporating it.

8. What chromatography is really measuring

Chromatography is the one technique here that is not obviously about a physical property, so it is worth saying plainly what it uses.

There are two things in the experiment, and they compete. The paper stays where it is. The solvent climbs the paper and moves. Every substance in the spot divides itself between the two: some of it sits on the paper, some of it rides in the solvent. A substance that clings hard to the paper spends most of its time not moving, and ends up near the start line. A substance that stays dissolved in the solvent spends most of its time moving, and ends up near the solvent front.

So what is being separated is not color — the dyes are colored only so you can see where they got to — but the ratio in which each substance splits itself between a thing that moves and a thing that does not. That idea is the whole of chromatography, including the machines that separate colorless things in an industrial laboratory.

The practical rules follow from it. The start line is ruled in pencil, because pencil is graphite and does not dissolve and travel; an ink line would separate into its own dyes and run up the paper across the sample. And the solvent level must start below the line, because a spot sitting in the solvent dissolves off the paper into the tank instead of climbing it.

9. In the world: the salt ponds of San Francisco Bay

The colored ponds along the southern edge of San Francisco Bay are salt works, and they run the evaporation this lesson describes at a scale of square miles. Seawater is let into the first pond, where it holds about 35 g of dissolved salt in every liter. The sun and the wind evaporate the water, and the brine is moved from pond to pond as it concentrates, turning pink and red from salt-loving microorganisms that thrive in it.

When the brine reaches about ten times the salt content of seawater, the salt begins to crystallize on the pond floor, and it is harvested in the late summer. A pond of a million liters of seawater holds about 35 tons of salt, which is why the ponds are so large: the separation is cheap, using only sunlight, but slow.

The choice of technique follows the lesson's rule exactly. The salt works wants the salt, so it evaporates the water and lets it go. A desalination plant, such as the one in Carlsbad, California, separates the same mixture and wants the water, so it uses a different physical method that keeps the water and sends the concentrated brine back to the sea. Same mixture, same property, opposite products.

10. In the world: a refinery's fractionating column

Oil refineries in Texas and Louisiana separate crude oil by fractional distillation. Crude oil is a mixture of many liquids with different boiling points, and a tall column lets each fraction condense at its own height, from gasoline near the top to heavy fuel oils near the bottom.

11. Where this goes wrong

Filtration is used on a solution. This is the big one. Salt water poured through filter paper gives salt water. A dissolved solute has no grains for the paper to catch — the separation you need is a boiling point.

Evaporation and distillation are treated as interchangeable. They keep opposite halves. Before choosing either, say which part of the mixture is the product.

A separating funnel is reached for whenever there are two liquids. It needs two layers, and two layers need the liquids to be immiscible. Ethanol and water make one liquid and no funnel will help.

Chromatography's start line is drawn in ink. The line separates too, and runs up the paper through the sample.

A technique is expected to separate a compound. Nothing here will: the parts of a compound are joined, and undoing that needs a chemical change. If you find yourself distilling water to get oxygen out of it, the mistake is one lesson further back.

The coarse step is left until last. Magnets and sieves work on dry mixtures and take out a lot of material for almost no effort. Doing them first means everything afterwards has less to handle.

12. Getting pure dry salt out of rock salt

1. Name the parts and the product.

   $\text{salt, sand and grit; salt is wanted}$

   Name the product before any apparatus.

2. Find the property difference.

   $\text{salt dissolves; sand does not}$

   Solubility is what the separation will use.

3. Dissolve the salt.

   $\text{crush and stir into warm water}$

   All the salt goes into solution.

4. Filter the mixture.

   $\text{sand is the residue; salt water the filtrate}$

   Filtration catches what never dissolved.

5. Evaporate the filtrate.

   $\text{dry salt left in the dish}$

   The salt is kept, so the water is the half to lose.

13. Separating sand, salt and iron filings

1. Name the parts.

   $\text{three parts, so two separations at least}$

   Each separation splits off one part.

2. Use the magnet first.

   $\text{iron filings lifted from the dry mixture}$

   The coarse, dry step goes first.

3. Add water to the rest.

   $\text{salt dissolves; sand does not}$

   Adding a solvent creates a difference to exploit.

4. Filter out the sand.

   $\text{sand on the paper, rinsed and dried}$

   The paper catches the undissolved part.

5. Evaporate the filtrate.

   $\text{salt left in the dish}$

   Boiling point separates salt from water.

6. Check the masses.

   $12 + 30 + 18 = 60 \text{ g}$

   The parts add up to the 60 g sample.

14. Fresh water from seawater on a ship

1. Name the parts and the product.

   $\text{salt dissolved in water; water is wanted}$

   The product decides between two techniques.

2. Ask whether anything is dissolved.

   $\text{yes: the salt}$

   So no filter will catch it.

3. Find the property difference.

   $\text{water boils at } 100^\circ\text{C; salt far higher}$

   Boiling point is the difference to use.

4. Choose between the boiling-point techniques.

   $\text{simple distillation}$

   Distillation keeps the liquid; evaporation would lose it.

5. Condense the vapor.

   $\text{steam cooled to fresh water}$

   The condenser turns the vapor back to liquid.

6. Find the water from 10 L.

   $10 \text{ L of seawater} \to \text{about } 9.6 \text{ L of water}$

   Seawater is about 3.5 percent salt by mass.

7. Name what is left.

   $\text{brine and salt in the boiler}$

   Nothing was lost; the salt stayed behind.

15. Your turn: ethanol is to be separated from water. Which technique, and why is a separating funnel wrong?

1. Check for layers.

   $\text{miscible: one liquid, no layers}$

   A funnel needs two layers to pour off.

2. Find the property difference.

   $\text{ethanol boils at } 78^\circ\text{C, water at } 100^\circ\text{C}$

   Boiling point is what differs.

3. Your turn: work this step out. Its working is at the end of the packet.

   Choose the technique.

16. Guided practice

Match each technique to the difference between the parts of a mixture that it makes use of. The mixture it is usually shown on is not the answer; the property is.

17. Guided practice

Complete the worked solution: $50$ g of rock salt is stirred into warm water and filtered. The sand left on the paper weighs $12$ g once dry. Find the mass of salt that dissolved, and the salt as a percentage of the rock salt.

1. Find the salt that dissolved.

   $(\text{rock salt}) - (\text{dry sand}) =$ _d g

   Everything that is not sand went through the paper in solution.

2. Find the salt as a percentage.

   $(\text{salt}) \div (\text{rock salt}) \times \text{a hundred} =$ _p

   A share of the whole, out of a hundred.

3. Name the next step.

   $\text{Evaporate the filtrate to get the salt back.}$

   The salt is the product, so the water is the half to lose.

18. Guided practice

Put these five steps into the order they have to be done in, to get sand, salt and iron filings separated from one another.

Number the steps in order (write the number in the box):

• Draw a magnet just above the dry mixture to lift the iron filings out
• Stir what is left into water so that the salt dissolves and the sand does not
• Filter the mixture so that the sand is caught on the paper
• Rinse the sand on the paper with a little water and leave it to dry
• Evaporate the liquid that came through the paper to get the salt back

19. Guided practice

A technician has salt dissolved in water, where it is the salt that is wanted. Which of these four techniques should be used?

• filtration
• evaporation
• simple distillation
• a separating funnel

20. Practice

Three mixtures are to be separated. For each, give the technique and say whether anything in the mixture is dissolved.

21. Practice

A salt works on San Francisco Bay lets seawater evaporate in shallow ponds. Each liter of the seawater holds about $35$ g of dissolved salt. A small test pond holds $4$ thousand liters. How many kilograms of salt are left when all the water has evaporated?

The answer: _a.

22. Somewhere new

This is not a laboratory bench: it is a works, running the same separations at a scale of tons. Put the five stages into the order they run in, to get drinking water from river water at a treatment works.

Number the steps in order (write the number in the box):

• Pass the raw river water through a coarse screen to take out sticks and weed
• Let the water stand in a settling tank so that the heavy grit sinks to the floor
• Add a substance that makes the fine clay particles clump into larger ones
• Pass the water down through a deep bed of sand and gravel
• Treat the filtered water so that it is safe to send out to houses

23. Lesson test

Lesson test: one question per skill, one attempt each, no hints. Your answers are checked when you submit.

24. Test question

Three mixtures are to be separated. For each, give the technique and say whether anything in the mixture is dissolved.

25. What you can do now

You can choose a technique from the mixture rather than from memory, and name the property difference behind it. Say out loud why a salt works evaporates seawater and a desalination plant distills it, when both are separating the same mixture. Next: how a chemical formula is read as a count of atoms, which is where the equations begin.

Working for the steps left to you

15. Your turn: ethanol is to be separated from water. Which technique, and why is a separating funnel wrong?, step 3