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Salty Science: How to Separate Soluble Solutions

A fun chemistry challenge from Science Buddies



George Retseck

Key concepts
Chemistry
Solubility
Water
Temperature

Introduction
Have you ever mixed sand and salt together and wondered how you could separate them again? If you had to separate them, would you have nightmares of tiny tweezers, a magnifying glass and hours spent picking grains of salt and sand apart? Don't be afraid, there is another way! Using the differences in solubility between salt and sand, you can find the simple "solution" to this problem.

Background
Chemistry for the most part is the study of matter and how it behaves and interacts with other kinds of matter. Everything around us is made of matter. One important property that matter has is solubility. We think about this when we dissolve something in water. If a chemical is soluble in water, then when you add it to water it will dissolve, or disappear. If it is not soluble, then the chemical will not dissolve and you can see it, either suspended in the water or at the bottom of the container.

When a chemical is dissolved in a liquid, such as water, it creates a solution. In a solution, the liquid is the solvent, and the soluble chemical that is added to and dissolves in the liquid is the solute. Even though a solvent dissolves a solute, the latter blends in but is still there. If you evaporated all of the liquid from the solution, you would be left with the dry solute again. In fact, this is how salt is processed in giant salt flats where seawater is slowly evaporated, leaving behind huge amounts of sea salt.

Materials
•    Strainer (optional)
•    Salt
•    Sand
•    A napkin
•    Magnifying glass
•    Two glass jars with lids
•    Measuring cup
•    Water
•    Teakettle or pot
•    Two spoons
•    Stove and oven
•    Funnel
•    Coffee filter
•    Spoon
•    Oven mitts

Preparation
•    If the sand has a lot of debris in it, use a strainer to strain out the large debris and purify the sand.
•    Place some salt and sand separately on a napkin and, using the magnifying glass, closely examine the salt and sand. What do you notice? How does the size, shape and color of the grains of sand compare with the grains of salt?
•    Be careful when using the stove and oven, and when handling the boiling water. An adult should help you with these steps.

Procedure
•    In a glass jar add one quarter cup of salt and one quarter cup of sand. Put the lid on the jar and shake until the salt and sand are completely mixed together.*
•    Using the magnifying lens, closely examine the mixture. What do you notice? Can you still see the individual grains of salt and sand?
•    Fill the teakettle or pot with at least one cup of water. Heat the water on the stove until it is boiling. Be careful when using the stove and handling the boiling water. An adult should help you with this.
•    Very carefully pour one half cup of boiling water into the jar and stir the mixture with a spoon. Be careful when handling the boiling water, which will make the jar become very hot! (Caution: You should pour the water very slowly, so the glass jar does not shatter from a rapid change in temperature.)
•    Using the magnifying lens, closely examine the solution. What do you notice? Can you still see the individual grains of salt and sand?
•    Place the coffee filter in the funnel and place the funnel on top of the second glass jar. Slowly pour the entire solution over the filter. As the solution seeps through the filter, let it collect in the second jar.
•    Looking at the coffee filter, what do you see? Carefully scrape off any particles from the coffee filter with a spoon and place them in the first jar.
•    Turn the oven on to 325 degrees Fahrenheit. Be careful when using the oven and ask an adult to help you with this. Place both jars on to a cookie sheet and bake them in the oven until all of the water has evaporated from them. This may take over an hour. When you check on the jars to see how much water has evaporated, what do you notice?
•    Turn off the oven and let it cool down. (When glass changes temperature very quickly, it can shatter.) Then, using oven mitts, carefully remove the jars and allow them to cool to room temperature. They will probably still be very hot!
•    After the jars are cool, closely examine their contents using the magnifying glass. What do you notice? Can you still see the individual grains of salt and sand? Are they mixed together or separated?
•    Extra : At the end of this activity, you can carefully use a measuring cup to measure the amount of salt and sand you ended up with. Do these amounts match the amounts you started with? Why do you think this happened?
•    Extra: Many different chemicals have different degrees of solubility. By adding different amounts of salt, sugar or baking soda to water you can see how soluble each chemical is. Just add each chemical, one teaspoon at a time, to a half-full glass of water until you notice that it no longer dissolves when you stir it. Which chemical is the most soluble (dissolves the most into the same amount of water); which chemical is the least soluble?
•    Extra: How might temperature affect the solubility of a chemical? Try dissolving the same amount of sugar in hot water, room-temperature water and ice-cold water, using the same amount of water each time. What happens? Can you think of other variables that might affect solubility?


*Correction (6/11/12): The sentence was edited after posting to remove reference to liquids; at this stage in the activity, only solids —sand and salt— have been mixed.

Observations and results
When you added the boiling water, did the salt dissolve or disappear? After drying in the oven, did the salt appear in the second jar and mostly the sand was in the first jar?

Salt is soluble in water whereas sand is insoluble (not dissolvable ) in water. If sand were soluble in water, we would not have beautiful sandy beaches! Because of this, when the boiling water was added to the mixture of salt and sand, the salt should have dissolved, or disappeared, whereas the sand stayed visible, creating a dark brown solution with possibly some sand particles stuck on the walls of the jar. Temperature can affect the solubility of a chemical, and in the case of salt in water the hot temperature of the boiling water improved the salt's ability to dissolve in it. The dissolved salt should have easily made its way through the coffee filter and into the second jar whereas the muddy, undissolved sand particles became stuck on the coffee filter (possibly mixed with some salt as well). After the liquid in the two jars was evaporated in the oven, the salt in the second jar should have become apparent again, mostly as a crusty white substance along the sides and the bottom of the second jar.

More to explore
Matter from Chem4Kids.com
Solutions from Chem4Kids.com
A Soluble Separation Solution from Science Buddies

 

This activity brought to you in partnership with Science Buddies
ScienceBuddies

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