December, 2015:
Kids, how can you trap colors inside ice? In this melting ice experiment you'll make a colorful ice sculpture while learning about freezing point depression and erosion. You can use many types of salt for this project. Table salt (sodium chloride, NaCl) is fine, as are coarse salts, such as rock salt or sea salt. And other types of salt, for example, Epsom salt (magnesium sulphate heptahydrate, MgSO4·7H2O), works well too. It's a lot of fun to use food coloring, water colors, or any water-based paint like tempera; you can use liquids or powders, whichever you have handy.
First, make some ice. You can use ice cubes for this project, but it's nice to have larger pieces of ice. Freeze water in shallow plastic containers, such as disposable storage containers for sandwiches or leftovers. Fill the containers only part way in order to make relatively thin pieces of ice.
Keep the ice in the freezer until you are ready to experiment, then remove the pieces of ice and place them on a cookie sheet or in a shallow pan. If the ice doesn't want to come out you can use warm water around the bottom of the dish to loosen it.
Sprinkle salt onto the ice or make little salt piles on top of the pieces of ice. The salt can melt holes all the way through thin pieces, making interesting ice tunnels. Experiment! Then dot the surface with coloring. The coloring doesn't color the frozen ice, but it follows the melting pattern. You'll be able to see channels, holes, and tunnels in the ice, plus it's eye-catching. You can then refreeze your project to enjoy again at a later time.
This is a messy project best performed outdoors or in a kitchen or bathroom. The coloring will stain hands and clothes and surfaces so have an adult partner around to help keep surfaces clean.
Why Does Salt Melt Ice?
You know that you can sprinkle salt on an icy road or sidewalk to help keep it from becoming icy. Take a look at freezing point depression to understand how it works. Salt melts ice essentially because adding salt lowers the freezing point of the water, that is, the salt water needs a temperature lower than 32°F (0°C) to stay frozen. How does this melt ice? Well, it doesn't, unless there is a little water available with the ice to dissolve the salt and make salt water. You don't need a lot of water to start the process. Ice typically is coated with a thin film of liquid water, which is all it takes.
Pure water freezes at 32°F (0°C). Water with salt will freeze at some lower temperature. Just how low this temperature will be depends on the salt used and how much salt is dissolved in the water. If you put table salt on ice at 15°F, the salt will be able to prevent melting ice from re-freezing. Magnesium chloride works down to 5°F while calcium chloride works down to -20°F. If the temperature gets down to where the salt water can freeze, energy will be released when the liquid changes state and becomes a solid. This energy may be enough to melt a small amount of the pure ice, keeping the process going.
As a bonus effect, the ice exposed to the salty water melts faster than other ice, so holes and channels form. Think about erosion in geology and the geological shapes formed by running water.
-------------
References:
http://chemistry.about.com/od/chemistryactivities/a/Melting-Ice-Science-Experiment.htm
Editor, Dr. Kathleen Carrado Gregar, Argonne National Laboratory
