Cobalt Colors

Cobalt chloride (CoCl₂) has a dramatic color change when combined with water, making it a great water indicator. A concentrated solution of cobalt chloride is red at room temperature, blue when heated, and pale-to-clear when frozen. The cobalt chloride we’re using is actually cobalt chloride hexahydrate, which means that each CoCl₂ molecule also has six water molecules (6H₂O) stuck to it.

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For this experiment you’ll need:

• cobalt chloride
• cotton swab
• goggles
• test tube with stopper
• index card
• distilled water
• hair dryer

Download Student Worksheet & Exercises

Fill your test tube partway with water and add 1 teaspoon of cobalt chloride. Cap and shake until the solids dissolve. Continue to add cobalt chloride, 1 teaspoon at a time, until you cannot dissolve any more into your solution. (You have just made a saturated solution.)

Using your cotton swab like a paintbrush, dip into the solution (your “paint”) and write on the index card. Use a hair dryer to blow across the solution. (Be careful not to scorch the paper!) What happens? Stick it in the freezer. Now what happens? What if you blow dry it after it comes out of the freezer? What else can you come up with? What happens if you spritz it with water?

What’s Going On? The cobalt changes color when hydrated/dehydrated – think of it as an indicator for water. It should be red when you first mix it, but blue when hit with the hair dryer. It doesn’t react to acids and bases the way the anthocyanin (in red cabbage juice) or universal indicator does, but rather with humidity.

Bonus Experiment Idea! You can grow red crystals by cooling off a cup of hot water. Here’s how: into a test tube, add 40 drops of hot water and 2 small spoon measure of cobalt chloride. Suspend a small pebble attached to a thread into the test tube (this is your starter-seed for your crystals to attach to). If after a day or two your crystals aren’t growing, just reheat the solution and add a little bit more of the chemical.

ANOTHER Bonus Experiment Idea! By soaking a strip of tissue or crepe paper (it’s got to be thin) in the cobalt chloride solution, you can create your own weather forecaster! Simply let dry and when it turns blue, you’re in for blue skies and pink means it’s going to rain. (It’s basically a humidity gauge.)

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