How to Separate Sand and Salt


Separate sand and salt by using the difference in solubility, density, or melting point.
Separate sand and salt by using the difference in solubility, density, or melting point. (photo: Castorly)

You can separate sand and salt either to obtain the components or to explore separation chemistry. Separation is a method of purification that based on physical or chemical differences between two or more materials. Here are four ways to separate sand and salt and a look at the principles involved. This is a great science project for students at any grade level because it gets them thinking about properties of matter using familiar materials. They can apply the scientific method to test different separation methods.

Separate Sand and Salt Mechanically

Components of any mixture, such as sand and salt, are separated based on their differences. Both sand and salt are solids with similar-size particles. However, the particles look different under magnification. One method to separate sand and salt is to use tweezers to mechanically separate the grains, piece by piece. It’s not an efficient separation method, but it works.

Separate Sand and Salt by Density

You can separate sand and salt much more quickly using the different densities of the two substances. The density of salt is 2.16 g/cm³ while the density of sand is 2.65 g/cm³. Put another way, if you filled a bucket with salt and another with sand, the one with sand would weigh more. Sand is slightly heavier than salt.

So, if you shake a pan of sand and salt, the salt will rise to the top of the pan. You can scrape off the top layer of salt to separate it from the sand. The same principle is used to pan for gold. Gold is heavier than most other substances, so it stays at the bottom of the pan.

Separate Sand and Salt by Solubility

These women separate sea salt from sand and water by letting the sun evaporate the water.
These women separate sea salt from sand and water by letting the sun evaporate the water. (photo: Quang Nguyen Vinh)

Salt dissolves in water, but sand does not. In other words, salt is soluble in water, while sand is not. So, you can use the difference in solubility to separate a mixture of sand and salt. Add water to the mixture until the salt dissolves. Solubility increases with temperature, so you’ll need less water if it’s hot. Once the salt dissolves, you can collect the sand. One way to do this is to pour the sand and salt water through a coffee filter or fine strainer. You can recover the salt by boiling the salt water until all of the water evaporates. Alternatively, you could just pour the salt water into a pan and let the water slowly evaporate. People get sea salt by spreading sea water over a large area and letting the sun evaporate the water.

Separate Sand and Salt by Melting Point

Salt (sodium chloride) and sand (silicon dioxide) have different melting points. The melting point of salt is 1474°F (801°C), while that of sand is 3110°F (1710°C). So, if you apply heat, salt melts before sand. You can use the melting point difference to separate a mixture of sand and salt by heating the mixture above 801°C, but below 1710°C. Pour the molten salt off, leaving the sand. While it’s possible to separate the components of the mixture using melting point, it’s not very practical. High temperatures are involved and the sand will be contaminated with a bit of salt. When you pour away the liquid salt, a little will contaminate the sand, like when you pour water off of sand.

Questions and Answers for Students

Here are some questions to ask students to get them thinking about how separation works:

  • Which method does the best job at separating sand and salt? Answer: Solubility works best because no sand remains in the salt water, while very little salt remains on the sand. If you rinse the remaining sand with water, you can boil it off to recover the remaining salt.
  • Does any method recover 100% of the salt and sand from the mixture? Answer: Not really. Although it’s slow, picking up individual sand and salt particles probably gives the best separation. However, at the molecular level, there’s probably a little salt on the surface of the sand. Purification always involves a little loss.
  • How can you make separation using solubility more efficient? Answer: Use boiling water to dissolve the salt to minimize the amount of water that is needed. To recover the salt, pour the salt water in a thin layer to increase surface area and speed the rate of evaporation.
  • Using the solubility method, why doesn’t salt evaporate with the water? Answer: Water has a much higher vapor pressure than salt. Water also has a much lower boiling point than salt. Using different boiling points is the basis of the separation method called distillation.
  • Would any of these methods work to separate salt and sugar? Answer: You could separate them using tweezers under magnification (although you’d hate your life). Salt crystals are cubic, while sugar crystals are hexagonal prisms. You could use solubility. Sugar dissolves in alcohol, while salt does not. While the two compounds have different boiling points, you’d have to be careful to avoid burning the sugar.

References

  • National Academies of Sciences, Engineering, and Medicine (2019). A Research Agenda for Transforming Separation Science (Report). Washington, DC: The National Academies Press. doi:10.17226/25421
  • 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.

Leave a Reply

This site uses Akismet to reduce spam. Learn how your comment data is processed.