Red Paper Turns Blue in a Flash! A Hair Dryer Reveals the Secret of “Invisible Moisture”
Ken Kuwako, Science Trainer, here. Every day is an experiment!
Ever noticed that the cobalt chloride paper sitting on a shelf in the lab has quietly turned pink or pale and washed out? Here’s the fun part: just hit it with a hair dryer, and it flips to a vivid blue in no time. And you can repeat the trick as many times as you like. This is the story of a magical little paper that makes invisible humidity show up in color.
Cobalt chloride paper is a “water detector”
Cobalt chloride paper is a test paper used to check whether water is present. It turns from blue to pink (a reddish color) when it absorbs water, so in middle school science class it’s often used to confirm that water was produced when something is burned or heated.
But the cobalt chloride paper sitting around in a lab, even if nobody has used it, has often soaked up moisture from the air and turned pink or pale. When the room is humid, the paper slowly takes in water from the air.
Blast it with a hair dryer, and red turns to blue
So before the experiment, try blowing a hair dryer over the paper. A red piece turns blue almost instantly. That’s because the water in the paper has been driven off and the paper has dried out.
The humidity we normally can’t see was actually clinging firmly to the paper in the form of tiny water particles (water molecules), and the color change shows it clearly. Every time, I get a kick out of thinking, “Wow, there was that much water hiding in the air!”
So why does the color change?
The secret lies in how many water molecules are clinging around the cobalt. When the paper is holding water, lots of water molecules crowd around the cobalt, and that arrangement looks pink. When the hair dryer blows those water molecules away, chloride ions gather around the cobalt instead, forming a different arrangement, and that one looks blue.
In other words, the color depends on who’s hanging out around the cobalt: water molecules or chloride ions. Let it absorb water and it goes back to pink; dry it out and it turns blue again. That’s why you can use it over and over.
It has relatives all around you
This property has been put to work in all sorts of places for a long time. For example, some of the silica gel packets tucked into snacks and seaweed used to change from blue to pink, so you could tell at a glance whether they had absorbed moisture.
The same mechanism has also been used in “weather gadgets” that change color with the humidity.
That said, cobalt chloride has been flagged as harmful to human health, and these days more and more products are switching to other indicators that don’t contain cobalt.
Tips for the experiment
When using cobalt chloride paper, keep these points in mind:
- Dry it lightly with a hair dryer before the experiment so it turns blue, then use it.
- Hold the hair dryer a little way off so you don’t scorch the paper.
Invisible humidity, made visible through a shift between blue and pink. Cobalt chloride paper is a tiny lab tool that shows us just how amazing that is. No matter how many times I see a red paper turn blue in the lab, it gets me excited every time. I hope you’ll try it too and feel for yourself that there really is water in the air, all through the magic of color!
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