Have you ever made rainbow ice? Neither had I until recently.
This was a fun experiment that can teach chemistry or chemistry terms on all different levels, such as how sodium (Na) from salt (NaCl) reacts to ice by heating it up and melting it, how salt diffuses in water – quicker if the water is warm, how salt is an ionic compound, and even on a geological level of how rivers and ponds can form over time. There was even a number of times when the flowing of the watercolor through the rivulets resembled veins and arteries of the body.
Have fun with this, and get enthused… it’s contagious.
- salt (a lot)
- liquid watercolor or food dye
- blocks or cubes of ice (frozen the night before) – these can come in all different shapes and sizes
- tray (large, with edges so the water doesn’t pour out)
- napkins (for drips and clean-up)
- containers for each color (jars, cups, etc)
The night before, freeze water in different size containers. I used small plastic cups, ice cubes, and a plastic disposable hummus container.
The next day, prepare your materials.
Use a good bit of liquid watercolor or food coloring to make each color dark. The darker it is, the better you will see the results inside of the ice. We found that yellow and orange were very hard to see.
Pour a few tablespoons of fine salt into the colored water. You want the mixture to be highly concentrated since the sodium (Na) will be melting the ice. We eyeballed this a bit, and even added more salt as we were doing the experiment. Some of the kids got a little impatient with the process, so they added more salt to speed it up – and it worked.
For all that salt that was just added, now you have to mix it up like crazy. It helps if the water is slightly warm so that the salt breaks down/dissolves easier. If you have a student who has a lot of energy or is OCD this is a good job for him!
This is where my instructions wander off into more of an editorial – the rest is up to you on how you conduct your experiment, but I am giving you some pointers with pictures to include. Ready? Get your tray and pipettes ready because you’re going to need them… oh, and don’t forget your patience.
Take your pipette and fill it with the color of your choice. This student chose green. He covered the top of his ice block and let it sit for a few. It took a few minutes for the Sodium (Na) to start “eating” away the ice.
After a number of attacks from the pipette and salty watercolor, you can start to see the rivulets that have been formed in the ice block. They started to get way more excited at this point.
You can also start to see the salt building up in different spots. This was pretty cool because eventually these little tiny”cities” of ice started to form on the inside of the ice cube.
Finally, the ice gave way on this block, and the student was able to use the pipette more internally. I just let the students go with it, since there was nothing to be hurt and it was all completely, 100% experimental.
The colors started to blend together on the outside, but on the inside something was happening…
These cool little colorful ice bubbles started to form inside of the block… this really got their attention, and they wanted to make them bigger…
This was just amazing to see. But they also loved it when they held the ice block up to the sunlight…
Resembling a stained glass window of sorts.
So we tried one from the cup mold, let it sit with the concentrated salt solution for a few minutes, dripped a few more pipette-filled watercolors on top, then flipped it over to view through the sunlight…
Fascinating stuff. We could have lost ourselves in finding all of the rivulets, bubbles, and castles inside of the cubes but it gets cold on the hands very, very quickly.