Which physical method can be used for obtaining a sample of salt from a small beaker of salt water?

Salt, a mineral with the main component being sodium chloride, or NaCl, has been used for millennia to help preserve food (and make it tasty!). In ancient times, salt was used as a form of currency in some cultures. During the period when the ancient Phoenicians controlled the salt trade in the Mediterranean, salt was as expensive as gold! In this lab, we will learn how to separate salt from water through a solar process—you can use the sun to evaporate water, leaving behind the salt.

Evaporate water to form salt crystals.

• Baking sheet
• Black paper
• Salt
• Water
• Cooking pot

1. If you do not have a source of salt water (such as water from the ocean), then mix salt and water in a cooking pot until all the salt has dissolved. Your goal is to make a fairy concentrated solution of salt and water.
2. Lay the black paper on the baking sheet. Why is it important to use black paper?
3. Pour the salt water into the baking sheet, making sure to cover the black paper.
4. Set the baking sheet somewhere warm, like outside in the sun or on a table by a window.
5. Record your observations over a number of days until all the water is gone.

Salt crystals will be left over once all the water has evaporated.

The color black absorbs all frequencies of visible light, which helps the light energize the water molecules which evaporate when they get hot. It is important that the pan is somewhere warm, because the water will evaporate into the air more quickly. As the water evaporates, the salt doesn’t leave with it! Therefore, the concentration of salt in the water left behind increases. Eventually, the concentration gets so high that the water becomes supersaturated, and the salt will begin to recrystallize into a solid. When all of the water is gone, you will have salt!

The same process is done on a very large scale, worldwide. A common method involves allowing seawater to flow into shallow flat beds through channels that are then sealed off. The sun heats the water until all of it evaporates, leaving mountains of salt behind. The salt is then collected and cleaned. Salt’s a particularly important mineral because it isn’t just used for cooking—it’s an essential ingredient in agriculture, science, and countless industrial products.

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One practical application of chemistry is that it can be used to help separate one substance from another. The reasons materials may be separated from each other is because there is some difference between them, such as size (separating rocks from sand), state of matter (separating water from ice), solubility, electrical charge, or melting point.

• Students are often asked to separate salt and sand to learn about mixtures and to explore the differences between forms of matter that can be used to separate mixture components.
• Three methods used to separate salt and sand are physical separation (picking out pieces or using density to shake sand to the top), dissolving the salt in water, or melting the salt.
• Probably the easiest method to separate the two substances is to dissolve salt in water, pour the liquid away from the sand, and then evaporate the water to recover the salt.

Since both salt and sand are solids, you could get a magnifying glass and tweezers and eventually pick out particles of salt and sand.

Another physical separation method is based on the different densities of salt and sand. The density of salt is 2.16 g/cm³ while the density of sand is 2.65 g/cm³. In other words, sand is slightly heavier than salt. If you shake a pan of salt and sand, the salt will eventually rise to the top. A similar method is used to pan for gold, since gold has a higher density than most other substances and sinks in a mixture.

One method of separating salt and sand is based on solubility. If a substance is soluble, it means it dissolves in a solvent. Salt (sodium chloride or NaCl) is an ionic compound that is soluble in water. Sand (mostly silicon dioxide) is not.

1. Pour the salt and sand mixture into a pan.
2. Add water. You don't need to add a lot of water. Solubility is a property that is affected by temperature, so more salt dissolves in hot water than cold water. It's okay if the salt doesn't dissolve at this point.
3. Heat the water until the salt dissolves. If you get to where the water is boiling and there is still solid salt, you can add a bit more water.
4. Remove the pan from heat and allow it to cool until it's safe to handle.
5. Pour the salt water into a separate container.
6. Now collect the sand.
7. Pour the salt water back into the empty pan.
8. Heat the salt water until the water boils. Continue boiling it until the water is gone and you're left with the salt.

Another way you can separate the salt water and sand is to stir up the sand/salt water and pour it through a coffee filter to capture the sand.

Another method to separate components of a mixture is based on melting point. The melting point of salt is 1474°F (801°C), while that of sand is 3110°F (1710°C). Salt becomes molten at a lower temperature than sand. To separate the components, a mixture of salt and sand is heated above 801°C, yet below 1710°C. The molten salt may be poured off, leaving the sand. Usually, this is not the most practical method of separation because both temperatures are very high. While the collected salt would be pure, some liquid salt would contaminate the sand, like trying to separate sand from water by pouring off water.

Note, you could have simply let the water evaporate from the pan until you were left with the salt. If you had chosen to evaporate the water, one way you could have sped up the process would have been to pour the salt water into a large, shallow container. The increased surface area would have exchanged the rate at which water vapor could have entered air.

The salt did not boil away with the water. This is because the boiling point of salt is much higher than that of water. The difference between boiling points can be used to purify water via distillation. In distillation, the water is boiled, but is then cooled so it will condense from vapor back into water and can be collected. Boiling water separates it from salt and other compounds, like sugar, but it has to be carefully controlled to separate it from chemicals that have lower or similar boiling points.

While this technique can be used to separate salt and water or sugar and water, it would not separate the salt and sugar from a mixture of salt, sugar, and water. Can you think of a way to separate sugar and salt?

Ready for something more challenging? Try purifying salt from rock salt.