How to Separate Salt and Sugar


You separate a mixture a salt and sugar by using differences between the two compounds, like boiling solubility and boiling point.
Separate a mixture a salt and sugar by using differences between the two compounds, like boiling solubility and boiling point. (photo: C Drying)

If you spill sugar and salt together in your kitchen, it’s not worth the effort to separate them. But, you can separate salt and sugar mixtures as a science project to learn about chemical and physical properties and separation chemistry. Here are three ways to separate salt and sugar, plus one that seems like it should work, but really doesn’t.

Separate Salt and Sugar Using Solubility

Both salt and sugar dissolve in water. However, sugar (sucrose) is much more soluble in alcohol than salt (sodium chloride) is. For all practical purposes, sugar is insoluble in alcohol. The solubility of salt is 14 g/kg in methanol (25 °C or 77 °F) and 0.65 g/kg in ethanol (25 °C or 77 °F). If you ever plan on eating the salt or sugar, use ethanol to separate the components of the mixture because methanol is toxic. If efficiency is your goal, use methanol because you’ll need less of it to dissolve the salt, leaving the sugar behind. Evaporate or boil off the alcohol to recover the salt.

Be aware this method doesn’t work nearly as well if you don’t use absolute alcohol. If you try to separate sugar and salt using 50% alcohol, it’s likely there will be enough water in the liquid to dissolve both components of the mixture!

Separate Salt and Sugar Using Density

The density of pure table salt (NaCl) is 2.17 g/cm3, while the density of pure table sugar (sucrose) is 1.587 g/cm3. So, to separate the pure solids, you could shake the mixture. The heavier salt will sink to the bottom of the container. While the material at the top of the container will be almost pure sugar and that at the bottom will be almost pure salt, it may be hard to tell where one compound ends and the other begins. You won’t be able to get 100% separation using only this method.

Separate Salt and Sugar Using Crystal Shape

If you have infinite time and patience, you can separate sugar and salt in a mixture with a magnifying glass and pair of tweezers. Salt crystals are cubic, while sugar crystals are monoclinic hexagons.

What About Using Melting Point?

Sugar is a covalent compound, while salt is an ionic compound. So, you might predict you can separate sugar and salt using melting point. The melting point of salt is very high (800.7 °C or 1473.3 °F). The problem is sugar decomposes at 186 °C (367 °F) rather than melts. If you try to separate the components of the mixture using heat, all you’ll get is burned sugar (carbon) and salt. Save this method for separating salt and sand (although there are better options).

References

  • Burgess, J (1978). Metal Ions in Solution. New York: Ellis Horwood. ISBN 978-0-85312-027-8.
  • Rumble, John (ed.) (2019). CRC Handbook of Chemistry and Physics (100th ed.). CRC Press. ISBN:978-1138367296.
  • Westphal, Gisbert et al. (2002) “Sodium Chloride” in Ullmann’s Encyclopedia of Industrial Chemistry. Wiley-VCH, Weinheim. doi:10.1002/14356007.a24_317.pub4
  • Wilson, Ian D.; Adlard, Edward R.; Cooke, Michael; et al., eds. (2000). Encyclopedia of Separation Science. San Diego: Academic Press. ISBN 978-0-12-226770-3.

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