In the course of chemistry study lately, I saw in the Merck index that isopropyl alcohol (rubbing alcohol) can be separated from water by adding salt. I've done some experiments and I think that children would enjoy exploring with this system. Its virtues include that it takes common,nontoxic materials that can be discarded without harming waste water treatment.
Here is the basic system.
Materials:
water,
rubbing alcohol (either 99% or70%),
paraffin or candle wax (not bee's wax),
a graduated cylinder,
three containers, one of which is tall and narrow (like olive jars),
table salt without "anti-caking agents" such as silicon dioxide (sand - it clouds solutions). I used sea salt with nothing added.
You will also need liquid food coloring.
Here's what you do. Measure the total volume of your tall container and use a total volume that will allow you to stir the liquid without spilling over.Measure out half your total volume in water and half in alcohol (if you are using 99%). If you have 70% alcohol, use 5 parts alcohol and two parts water. To start, put the alcohol and water in separate containers. Drop a small piece of paraffin in each container. This wax should float in water and sink in the alcohol.
Now mix the water and the alcohol, along with their pieces of wax in a third container. The paraffin should float. Add salt and stir. For my 70 ml total volume, it took a couple of heaping spoonfuls of salt. Add the salt a spoonful or so at a time and watch as you stir. Stop stirring occasionally and see if the paraffin is still on top. As you get enough salt in solution,something happens. The solution separates into two phases and the paraffin drops halfway down and floats there.
What has happened? The alcohol initially mixed completely with the water and formed a solution. Alcohol is a slightly polar molecule that interacts with the very polar water and can be mixed with it in any proportions. As the salt dissolves, the sodium and chloride ions increase in concentration and interact strongly with the water. They displace the alcohol from the water molecules and it separates. Since it is less dense, it rises to the top of the container. The paraffin sinks in alcohol, so it drops to the top of the salt water and floats there.
If you want to explore further, add a drop of green food color to your container. It will sink to the top of the salt water and then, if you don't move or shake the container, it will slowly diffuse through the liquids. You can leave sitting (with a plastic wrap cover to keep the alcohol from evaporating) for several days. Then there is this little mystery. I haven't had time to do the experiments to figure it out. After about a week, the green color is still there in the salt water layer, but the alcohol turns turquoise blue. The yellow part of green food color leaves the alcohol by some means. Other mixtures of food color gave different results so I still don't know what happened to the yellow in the alcohol. I hope you and your students enjoy exploring with this activity. It could stimulate thinking about density, diffusion, and molecules of dye.
- Priscilla Spears, Ph. D.
http://www.bigpicturescience.biz/
I recommend her books. Especially the Plant Manual!
Materials:
water,
rubbing alcohol (either 99% or70%),
paraffin or candle wax (not bee's wax),
a graduated cylinder,
three containers, one of which is tall and narrow (like olive jars),
table salt without "anti-caking agents" such as silicon dioxide (sand - it clouds solutions). I used sea salt with nothing added.
You will also need liquid food coloring.
Here's what you do. Measure the total volume of your tall container and use a total volume that will allow you to stir the liquid without spilling over.Measure out half your total volume in water and half in alcohol (if you are using 99%). If you have 70% alcohol, use 5 parts alcohol and two parts water. To start, put the alcohol and water in separate containers. Drop a small piece of paraffin in each container. This wax should float in water and sink in the alcohol.
Now mix the water and the alcohol, along with their pieces of wax in a third container. The paraffin should float. Add salt and stir. For my 70 ml total volume, it took a couple of heaping spoonfuls of salt. Add the salt a spoonful or so at a time and watch as you stir. Stop stirring occasionally and see if the paraffin is still on top. As you get enough salt in solution,something happens. The solution separates into two phases and the paraffin drops halfway down and floats there.
What has happened? The alcohol initially mixed completely with the water and formed a solution. Alcohol is a slightly polar molecule that interacts with the very polar water and can be mixed with it in any proportions. As the salt dissolves, the sodium and chloride ions increase in concentration and interact strongly with the water. They displace the alcohol from the water molecules and it separates. Since it is less dense, it rises to the top of the container. The paraffin sinks in alcohol, so it drops to the top of the salt water and floats there.
If you want to explore further, add a drop of green food color to your container. It will sink to the top of the salt water and then, if you don't move or shake the container, it will slowly diffuse through the liquids. You can leave sitting (with a plastic wrap cover to keep the alcohol from evaporating) for several days. Then there is this little mystery. I haven't had time to do the experiments to figure it out. After about a week, the green color is still there in the salt water layer, but the alcohol turns turquoise blue. The yellow part of green food color leaves the alcohol by some means. Other mixtures of food color gave different results so I still don't know what happened to the yellow in the alcohol. I hope you and your students enjoy exploring with this activity. It could stimulate thinking about density, diffusion, and molecules of dye.
- Priscilla Spears, Ph. D.
http://www.bigpicturescience.biz/
I recommend her books. Especially the Plant Manual!
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