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I'm a student and I've been getting interested in chemistry. I really like experiments and was brainstorming of possible things to try out, one of them was separating a homogeneous mixture. I thought of sugar water and tried looking it up online but couldn't find anything like I wanted. Everything says to use crystallization but I don't understand why you can't just boil away water and be left with sugar like you can with salt water. Anyways, what methods/ways can I separate sugar from water?

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    $\begingroup$ You are more or less right about boiling water away. Crystallization per se is not a method (rather, it is what happens when you use some actual methods). You can't point a wand at your solution and say "Crystallize!" $\endgroup$ – Ivan Neretin Sep 27 '16 at 21:39
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    $\begingroup$ @IvanNeretin Nah, you have to say something like "expecto crystallize". I am not sure, never quite got my letter from hogwarts. $\endgroup$ – getafix Sep 27 '16 at 23:55
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If you start with a known 'pure' mixture of sugar and water, then what you propose is fine, to an extent, if you are just after separating the two ingredients.

Crystallisation is a purification method for extracting a solid from solution; it will allow you to separate pure solute from the solvent. The remaining solution would still contain a small amount of dissolved material, and hopefully all of your impurities.

If you wanted to isolate pure solvent from the solution, you would need to use distillation. After collecting the distillate, you would be left with a gummy residue of semi-decomposed sugar (and your impurities).

Sugar is hygroscopic, and starts decomposing at just over 180°C (~365°F). The problem that you would have with your proposed method is that in order to remove the final remnants of the water, you will need to heat well above the boiling point of water. Google 'making caramel' if you can't think of what will happen. Even a very small amount of decomposition of your sugar will mean you are left with an impure solid. Back to square one. The reason that you can simply boil the water off salt is that it is far more stable and can be heated safely to much higher temperatures.

So, there are a few ways to separate these two ingredients, depending on your end goals:

  • To collect just solute (sugar): heat over very low heat to evaporate as much water as possible to give yourself a saturated solution. Then crystallise sugar and filter (in the lab you'd wash with a suitable solvent and dry under vacuum)
  • To collect just the solvent (water): distill over moderate heat to collect water. To collect as much as possible, the heat used to boil off the solvent needs to be slowly increased, especially as you approach the final portion.
  • To collect both solute (sugar) and solvent (water): You would remove solid first, then distil solvent. Unless you are starting with a saturated solution, you would need to do a partial distillation to remove some solvent, then crystallise your sugar, then complete the distillation. You'd still be left with a horrible brown gummy residue.

Sounds like a great home project, though. Just be sure to have done some suitable research to allow you to make use of your materials if it doesn't quite go to plan :-)

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  • $\begingroup$ Amazing answer! Just what I was looking for $\endgroup$ – Ramon Sep 28 '16 at 19:15

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