Key concepts
Chemistry
Solutions
Miscibility
Polarity
Solubility
Introduction
You probably know some liquids, such as oil and water, do not mix together. If you pour them into the same container, they will form separate liquid layers, one on top of the other. Other liquids, for example rubbing alcohol and water, can be mixed with each other. But did you know that once both of these liquids have mixed you can separate them again into two different layers? How can you do that? The answer might surprise you—with salt! In this activity you will find out how this works.
Background
When two liquids can be mixed together, they are “miscible”—they form something called a homogeneous solution, which means that you cannot distinguish the two liquids anymore. In contrast, when they cannot be mixed, they are “immiscible”—they will form two separate layers, called a heterogeneous solution. To be able to mix, the molecules of both liquids have to be able to attract one another. Molecules that are polar (meaning their electric charge is distributed unevenly so they have a more positive side and a more negative side) tend to form hydrogen bonds whereas nonpolar molecules (which have an equal charge balance) do not tend to form such bonds. Because water molecules are polar, any liquid that does not have polar molecules—such as oil—is usually immiscible with water.
Rubbing alcohol molecules have a polar and nonpolar part, which means they are able to form hydrogen bonds with water and therefore able to mix with it. But how can you break these bonds in order to separate both liquids once they are mixed? You have to add something to the mixture that competes with the alcohol in binding to the water molecules. One substance that can do that is salt. Salt is an ionic compound, meaning it is a substance made up of electrically charged molecules called ions. When ionic compounds dissolve in water, the individual ions separate and get surrounded by water molecules—a process called solvation. Because the salt ions are charged, they dissolve much better in a polar solvent, which is also slightly more charged than a nonpolar solvent. For this reason, salt ions attract the water molecules much more strongly than alcohol molecules do because alcohol is less polar than water. This means that when there is a lot of salt, all the water molecules will bond to the salt ions, leaving none to form hydrogen bonds with the alcohol molecules. As a result, the alcohol becomes immiscible with water and starts to form a separate layer. This process is called “salting out,” or “salt-induced phase separation.”
Historically this method has been used in the soap-making process to remove ingredients that should not be in the final soap product. Salting out is also commonly used in biochemistry laboratories to purify proteins, because different protein molecules become immiscible at different concentrations of salt solutions. Chemists use this technique to extract liquids out of a solution, which is what you are going to do in this activity: You will separate a rubbing alcohol and water mixture using just a teaspoon of table salt!
Materials
- Four transparent mini cups (two ounces) with lids
- Permanent marker
- Tap water
- Rubbing alcohol (70 percent isopropyl alcohol)
- Table salt
- Set of measuring spoons
- Work area that can tolerate spills
- Ethanol or acetone (can be found in hardware stores) (optional)
- Salt substitute such as potassium chloride or Epsom salt (optional)
Preparation
- With the permanent marker label the mini cups 1, 2, 3 and 4.
- Add one and a half tablespoons of water to cups 1 and 3.
- Add one and a half tablespoons of rubbing alcohol to cups 2 and 4.
Procedure
- Add one teaspoon of salt to the water in cup 1. What happens to the salt? Does it dissolve in the water?
- Put on the lid and shake the cup for about 20 to 30 seconds. What does the mixture look like?
- Repeat the previous two steps using cup 2 (with rubbing alcohol). What happens to the salt this time? Does the mixture look different from the water–salt mixture?
- Take the cap off the permanent marker and swirl its tip in the water in cup 3 for about 10 seconds. Put the lid on the cup and shake it for five seconds. Does the ink dissolve in the water? What does the solution look like after shaking?
- Repeat the previous step with cup 4 (rubbing alcohol). Does the resulting mixture look different? If so, what is different? Can you explain the differences?
- Next, pour the alcohol from cup 4 into the water in cup 3. Put the lid back on and swirl the mixture for five seconds. Does the rubbing alcohol mix with the water? What happens to the color of the mixture? Do you see separate layers forming?
- Now, add one teaspoon of salt to the mixture in cup 3. Put the lid on the cup and shake it for 20 to 30 seconds. What happens when you add the salt to the mixture? Does the mixture look different before and after shaking? If so, how does it look different? Can you explain your results? What color is the mixture?
- Extra: Can you separate other liquid mixtures using salt? What about ethanol and water or acetone and water? Try different liquid mixtures to find out!
- Extra: Are there any other salts—for example potassium chloride, a salt substitute, or Epsom salt—that you could use to separate liquids? Repeat the test, but this time use a different salt than table salt. Do you still see the same results? If not—how are your results different?
- Extra: How much salt do you need to separate the rubbing alcohol and the water? Find out by varying the amounts of salt that you add to the rubbing alcohol and water mixture.
Observations and results
You should have seen that the salt easily dissolved in the water in cup 1. (After shaking it the salt seemed to disappear.) Remember that this occurs because the ionic salt molecules easily bond to the polar water molecules. The salt, however, did not dissolve as easily in the rubbing alcohol in cup 2. (Even after shaking it you will still be able to see the salt.) This occurs because the alcohol molecules are less polar than water is, so the salt ions do not bond with them as easily.
With the permanent marker ink you should have observed the exact opposite phenomenon. The ink does not dissolve well in water but it does easily in the alcohol, giving the latter much more color. This is due to the fact rubbing alcohol also has a portion of its molecule that has no charges, and is nonpolar. This portion is more compatible with nonpolar molecules such as the marker ink.
When you mix the rubbing alcohol with water, the latter’s molecules make hydrogen bonds with the water molecules. The alcohol dissolves in the water to form a homogenous solution, so you cannot distinguish the alcohol and the water anymore. If you add salt to the mixture, however, the salt wants to dissolve in the water and competes with the alcohol for the water molecules. Because there are fewer water molecules available to make hydrogen bonds with the alcohol molecules, the alcohol becomes less soluble in the water–alcohol mixture, eventually forming a separate layer on top of the water. Both layers should have a different color, with the water mostly clear and the alcohol more colored. This occurs because the marker ink is more soluble in the rubbing alcohol.
Cleanup
Flush all your mixtures down the sink with plenty of cold water. Wash your hands and clean your work area.
More to explore
Chemistry for Kids: Solutions and Dissolving, from Ducksters
Create Underwater Fireworks with Chemistry, from Scientific American
Science Activity for All Ages!, from Science Buddies
This activity brought to you in partnership with Science Buddies
