Separate Liquids with Salt!

Introduction

You probably know that some liquids, such as oil and water, do not mix together. If you pour them into the same container, they will form two separate liquid layers on top of each 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 two liquids cannot be mixed together (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 each other. Molecules that are polar (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 hydrogen bonds. Since water molecules are polar, any liquid that does not have polar molecules - such as oil - is usually immiscible with water.

The rubbing alcohol molecule has a polar and non-polar part, which means it is able to form hydrogen bonds with water, and is 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 charged than a nonpolar solvent. For the same reason, salt ions attract the water molecules much more strongly than the alcohol molecules do, as rubbing 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, and there are no water molecules left 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, scientists have used this method 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, as 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/water mixture using just a teaspoon of table salt!

Materials

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. However, the salt 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 rubbing alcohol molecules are less polar than water, so the salt ions do not bond with them as easily.

With the permanent marker ink, you should have observed the exact opposite. The ink does not dissolve well in water, but dissolves easily in rubbing alcohol. This makes the rubbing alcohol look much more colored than the water. This is due to the fact that rubbing alcohol also has a portion of its molecule that has no charges, and is non-polar. This portion is more compatible with non-polar molecules such as the marker ink.

When you mix the rubbing alcohol with water, the alcohol 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. However, if you add salt to the mixture, the salt wants to dissolve in the water and competes with the alcohol for the water molecules. Since 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, it forms 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.

Additional Resources

• Chemistry for Kids: Solutions and Dissolving, from Ducksters
• Salting-out Soap Scraps, from Classic Bells
• Naming Ionic Compounds, from ChemTalk