Separating Mixtures…

A mixture is any situation when two or more substances are mixed together. In a mixture, the different particles are close to each other but they are not joined. This means it should be possible to separate most mixtures out into their component parts. To do this, you need to look at the physical properties of the different parts of the mixture to work out a method for separating them. Here are some examples…

Sand from water

Sand is insoluble in water so, even if it is very well mixed in, forming a suspension where the particles are hanging throughout the water, a filter can be used to separate the sand from the water. The water will pass through the filter (called the filtrate) and the sand will be trapped by the filter (called the residue). You can look here to see some SF boys folding filter papers to do just this.

Salt from water

Salt dissolves in water to form a solution. The salt is known as the solute and the water as the solvent. Filtration will not work in this case because the salt has broken down into individual molecules and will pass through the filter paper. However, if you boil the mixture, the water will evaporate leaving the salt behind.

Salt from sand

A mixture of salt and sand can be separated by combining the two techniques above. First add water to dissolve the salt, filter to remove the sand and finally evaporate the filtrate (salt solution) to recover the salt. The diagram below shows this process.

 

Water from salt solution

If it is the water you want to collect from a solution, then evaporation alone is not enough. The water would just escape into the atmosphere. In this case, you need to catch the water vapour and condense it back into a liquid. This technique is called distillation. The solvent collected in this process is called the distillate. A Liebig condenser can be used to make the condensation more efficient. The two diagrams below show simple distillation and distillation using a condenser.

Ethanol from water

Ethanol (alcohol) mixes with water and is also a liquid but it has a lower boiling point than water. If it is distilled at a lower temperature then it can be separated from the water. This process is not perfectly efficient because water molecules are quite ‘sticky’ and will hitch a lift with the ethanol as it evaporates and leaves the mixture. Fractional distillation can ensure that the water and ethanol are completely separated by providing a large surface area for condensation in a fractionating column. Other mixtures of liquids such as crude oil and liquid air can be separated in a similar way.

Mixtures of dyes

Another physical difference that can be used to separate mixtures is colour. Chromatography is a technique that uses the different solubility of different molecules to separate them out. The dyes are carried up a piece of paper by a solvent and the more soluble molecules travel further than the less soluble one.

Iron filings and sulphur

What physical difference between iron and sulphur could be used to separate them? Neither is soluble in water; they are both solids at room temperature; heating the mixture would cause them to react and produce iron sulphide. Iron is attracted to a magnet whereas sulphur is not so you could use this difference to separate the two.

Mixtures of soluble salts can also be separated by using differences in their solubility. Some salts dissolve more in hotter water (e.g. potassium nitrate) whilst others are not affected by the temperature of the water (e.g. sodium chloride). Look here for a more detailed explanation of this idea.

Questions…

  1. In sea-water, what is the solvent and what is the solute?
  2. How would you separate…
    • sulphur from water?
    • copper sulphate from  water?
    • water from a cup of coffee?
    • alcohol from wine?
    • copper sulphate and copper oxide?
  3. What are some of the physical differences between mixtures and compounds?

About SFScience

Head of Science at Summer Fields, Oxford

71 Responses to “Separating Mixtures…”

  1. How do you separate salt,copper filings and iron filings?

    • Well, what are the physical differences? Salt is soluble which makes it different from iron and copper. You should be able to separate out the salt from the other two with water, a little stirring and some filter paper. Once you have dried the residue, which should be a mixture of copper and iron, you need to think what are the physical differences between those two metal elements. What property does iron have that most other metals do not have? When you have answered that question, you should be able to separate the iron from the copper. (Ask again if I have not given enough clues!)

    • iron filing are separated by magnet after that mixed separated copper filing and salt with water and filter after that we obtain a copper filing on filter and salt solution in beaker in this way we can separate a given mixture

    • Aila (grade 7) Reply July 17, 2012 at 11:27

      by magnetization it is a proces of separating mixtures through a magnet

      • Yes – although there are a limited number of metals that respond to a magnet. Iron (or steel is one) – do you know any others?

        ps Are you in the Phillipines?

    • ScienceIsSo....Fun Reply February 22, 2014 at 15:51

      I would probably use a magnet to separate the iron, and then maybe mix the copper and salt with water, and then filtering the copper. At last evaporate the water it get salt.

  2. sir isn’t it dangerous to heat ethanol directly with a bunsen burner. arent you suposed to use a water bath or something?

    • Yes it is quite dangerous. We used an electric heater for that very reason – a waterbath would work too. The diagram above could be used if it was done behind a safety screen or in a fume cupboard.

  3. I wasn’t really asking 4 all these. I was searching for the use of these separation methods in daily lives. Can someone tell me???

  4. Glad it helps. Have a great weekend.

  5. What happens at the molecular level when one substance dissolves another? Why is it that copper sulphate dissolves in water?

    • As I understand it, there are two kinds of dissolving.

      1. Copper sulphate is an ionic compound, which means that when it dissolves it breaks up into positive copper ions and negative sulphate ions. This is why water with an ionic compound like a salt dissolved in it conducts electricity.

      2. Some molecules, like sugar or glucose, can break away from their crystal lattice and dissolve into water as entire molecules. The reason that they can do this is that they are “polar” molecules. That means that they have positive and negative regions to their structures. Water is a polar molecule so the positive and negative regions attract one another, enabling the dissolving process.

      Another useful guide for the solubility of salts is the relative reactivity of the metal involved (cation) or the particular anion (negative bit) attached. For example all nitrates dissolve in water. Most potassium salts are soluble (because potassium is very reactive). Copper is very unreactive so it is slightly peculiar that copper sulphate is so soluble – calcium sulphate is not, for example.

      I don’t know how much detail you need for your answer but the Wikipedia page on solubility is very good if you want to know more about how temperature, pressure and other factors affect solubility. http://en.wikipedia.org/wiki/Solubility

      • How do you separate copper sulfate from water?

        • Copper sulphate dissolves in water so it can’t be filtered out with a paper filter. However, if you warm the solution the solvent (water) will evaporate and the solute (copper sulphate) will crystallise out. (Don’t boil it dry or the copper sulphate will thermally decompose becoming anhydrous.) I hope this helps!

          • Thanks a lot!!! I have one more question though. How to separate caffeine from water?

            • Sorry for the slow reply – Chapel! This is an interesting one. How much information are you given about caffeine? It is very soluble in hot water but not very soluble in cold water. If you cool the solution down, the caffeine would come out of solution and then you could filter it out (the residue). There would probably still be a little caffeine dissolved in the filtrate so you could evaporate the water to leave white crystals of caffeine.

              • Thanks for the reply, but I don’t understand what you meant by “the caffeine would come out of the solution”

                • Sorry, that was a bit technical!

                  Caffeine is very soluble in hot water but not very soluble in cold water. Your question seemed to be about a caffeine solution in water so I assumed it was hot. As the solution cools, caffeine would start to settle out in the container as a solid because the cooler water would not be able to hold on to it. If you chilled it right down to just above freezing point, you would probably get most of the caffeine out and could simply filter the mixture.

                  Given the other questions you asked, I doubt that you were being asked anything as sophisticated as this but it might be something that would be fun to tease your teacher about! It would then open up the chance to discuss ‘solubility’ as a topic, which is surprisingly interesting in itself.

                  If you want a simple answer, caffeine dissolved in water can be separated by evaporating the water to leave the caffeine behind as a white powder.

                  I hope this is clearer!

          • However, I would like to collect the water after I separate the copper sulfate from water. What should I do?

  6. When heating iron and sulphur with a Bunsen burner, is the reaction exothermic or endothermic and can you give an explanation please?

    Thank you

    • Thanks for the question Claudine. My 11 yr olds were reacting iron wool and sulphur just yesterday. They heated them together in a test tube and initially the sulphur melts but at a high enough temperature a red glow rushes through the mixture as they combine together. It is definitely exothermic! If you are lucky you hear a loud whine as hot gas escapes from the tube. It’s really a pretty cool reaction.

      As to the explanation part of your question, do you mean “Why is it exothermic?” I’ll assume you do. The answer is that when atoms bond with each other, they release energy in the form of heat and sometimes light. Breaking bonds between atoms requires energy to be put in.

      If more energy is released by the forming of bonds than is required to break them in the first place, then the reaction is exothermic. If more energy is needed to break the bonds than is released when they reform, then the reaction is endothermic. The mixture between iron and sulphur has no bonds so the heat from the Bunsen just gives the atoms enough energy to encourage them to react, and as each bond forms a little heat and light is released.

      I don’t know if that explanation is any help, I have typed this in rather a hurry – you may find this previous post helpful as it explains bond energies in a little more detail.

      http://sfscience.wordpress.com/2011/10/25/breaking-the-bonds/

      Let me know if my reply has left you more confused than you started!

  7. Nope, your explanation was extremely helpful. Thank you ever so much.

  8. i am so weirded out right now!

  9. thanks for the information it was great for my homework
    thank you very much

  10. can someone tell me in a full process for how can i seperate iron filings from sand(given mixture is sand and iron filings..

    • It is a nuisance when you get your sand and iron filings all shamozzled – but there is a way out.

      First wrap a magnet in cling-film (this stops the magnet getting coated in iron). Then work the magnet through the mixture removing as much iron as possible. It will need to be pinched off the magnet when it becomes clogged up. Some sand may stick to the iron filings so you may need to repeat this several times. Carefully remove the cling-film from the magnet to separate the last shards of iron.

      Hopefully you now have iron and sand in separate containers and you can return to your normal life, glad of a job well done. Good luck!

  11. How do you separate iron from aluminium?

    • What is the main physical difference between iron and aluminium? The first is magnetic and the second is not. Not many metals are magnetic – iron is the most common one. I hope this helps.

  12. I have a question from my book I’m stuck on ” draw a particle diagram to show what happens when you filter salty water that has sand in it”

    Thanks

    • I don’t have a graphics program available so I will try to describe it – please excuse me if it doesn’t immediately make sense!

      Imagine a piece of filter paper as a V. Now draw it on the page with dotted lines (- – - – -). These gaps represent the microscopic holes in the filter paper that allow water to flow through. Next draw in a series of blobs, larger than the gaps in your dotted lines. These represent the grains of sand that are going to be trapped by the filter (the residue). Finally, draw lots of little pencil dots (smaller than the gaps in your V) to represent the particles of salt and water flowing through the filter (the filtrate). You should probably make the water molecules one colour and the salt molecules a different colour so that they can easily be told apart.

      Does that make any sense? It is quite hard to describe in words!

  13. yes homework done!!!:)

  14. i need help. how do make gasoline.

    • Crude Oil is a mixture of lots of different compounds like tar, oil and gasoline (petrol). They have different boiling points so can be separated out by ‘fractional distillation’. The crude oil is put into a tower that is hot at the bottom and cool at the top. The gasoline evaporates and travels up the column until it is cool enough to condense and can be piped away.

  15. Sir, can you do some more posts?

  16. Sir, for the “water from salt solution” why must the delivery tube reach far down into the test tube????.

    • This makes sure that as much of the vapour will condense as possible. If the tube only reaches the neck of the tube most of the vapour will escape into the air.

      Hope this helps and thanks for the question.

      • Thannkk you!! Sorry but i have another question again xD ” distillation is used rather than filtration, to purify sea water. Why?” (hwk question) ive been searching for a good answer to this all over the internet but i couldnt find one, i hope im not bothering you, but if you know please could you tell me :D if not then its okay :)

        • No problem! Filtration is usually only used to remove undissolved particles from water. The salt in seawater is dissolved so if you tried to filter it the salt molecules would pass straight through the tiny holes in the filter paper and you would still have salty water. When seawater is distilled, the water evaporates and leaves the solid salt particles behind completely. I hope this helps. Good luck!

  17. Thanks alot!! You’re a great help!!, :D

  18. What separation technique would you use to separate caffeine from water?

  19. How do you separate oil from water? Should I use a separating funnel?

    • Yes! Oil usually floats on water because it is less dense and the two don’t mix. Use the tap to drain off most of the water into a beaker. Then drain off the part where the water and oil meet into different container. Finally let the oil flow out into a third beaker.

  20. By which process we can seperate iron fillings and sand

    • Because iron filings are magnetic and sand is not, a bar magnet wrapped in cling film can pull the iron out of the mixture.

      You wrap the magnet in cling film to stop any of the iron filings sticking permanently to the magnet.

      Hope this helps!

  21. um what we talkin bout

  22. oh I also know that mechanical mixtures are stuff inside them you can see such as salad cereal and others but a sloution you really can’t SEE the juice and other substances

  23. what ways are there to separate iron fillings, copper and aluminium other than magnetism…? O.o

    • Sorry for the slow reply.

      I must confess this one has me slightly stumped! A magnet would pull the iron filings out of the mixture but then you would be left with a mixture of copper and aluminium. The only way I can think of to separate them would be to add something like hydrochloric acid which would react with the aluminium but not the copper. You would end up with copper in a solution of aluminium chloride and this could be filtered to remove the copper. You now need to get the aluminium back from the aluminium chloride solution so you could add magnesium. This would displace the aluminium to leave magnesium chloride solution and solid aluminium as a precipitate which could be removed by filtration.

      This is not a very satisfactory process as it would be hard to get the quantities right to prevent any contamination. I will have a think to see if there is another way to separate the aluminium and copper. Perhaps a process similar to panning for gold could be used, taking advantage of the copper’s higher density. I will let you know if anything occurs to me!

    • This video shows a machine that separates copper from aluminium.

      There is no description but it seems to wash the low density aluminium forwards with water whilst shaking the higher density copper backwards. Some aluminium also travels backwards but drops through holes into a tray beneath. I am not sure why it does this!

  24. Thank you sir! This page helped a lot with my revision

  25. how you can separate the mixture of water and ethanol

  26. ScienceIsSo....Fun Reply February 23, 2014 at 10:44

    what does emulsion mean?

    • Great question!

      An emulsion is a stable suspension of one immiscible liquid in another. Imagine tiny droplets of fat evenly distributed through water.

      A suspension is tiny undissolved particles distributed through a liquid.
      Suspensions tend to settle out over time – as do emulsions – which is why you have to stir emulsion paint thoroughly before using it.

      (Wasn’t it in last week’s prep?)

      • flynn here,
        these are my awnsers to your questions:
        1)salt is the solute, where the water is the solvent.
        2)
        1.filtration
        2.evaporation
        3.dsitillation
        4.put the wine in the freezer. as alcohol won’t freeze, after a while, one can pour out the remaining pure alcohol and boom goes the dynamite, you have vodka.
        5.put both in a solution and because copper oxide is insoluble in water it can be filtered out.
        3)a compound is chemical combined, whereas a mixture is not. compound has its components in fixed proportions, whereas a mixture does not necessarily. also, the separation of components in a compound is by chemical means, whereas separation of the components in a mixture is by physical means. that is all i can think of at the moment.

        thanks, mr.Randolph!

        • um, now that i think of it, i think that the answer to 4 is actually just fractional distillation.

        • Fab answers (although as you note below fractional distillation is my preferred answer). You would not get vodka, strictly speaking, because it is made from potatoes. You’d possibly call it hooch – and it would probably make you blind!

  27. thank you very much sir.
    i will try to get involved in as much science blogging as possible!

Leave a comment...

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out / Change )

Twitter picture

You are commenting using your Twitter account. Log Out / Change )

Facebook photo

You are commenting using your Facebook account. Log Out / Change )

Google+ photo

You are commenting using your Google+ account. Log Out / Change )

Connecting to %s

%d bloggers like this: