Learning about energy transfer with color changing straws

The phenomenon of Thermochromism

When I was a child, I would stand in front of fair booths and try on all sorts of mood rings and adore them. The change of color fascinated me. I really wanted to purchase one and my parents didn’t let me. Oh how unfair the world seemed to me. I didn’t care that these rings weren’t worth the money, I just loved watching the colors turn.
Today, I know about the phenomenon of thermochromism and it still fascinates me. But I’ve also studied a little bit of physics and now understand the science behind the phenomenon so much better than when I was a kid. In this blog post, I’ll show you what thermochromism is, why it occurs and what we can learn about energy from it.

A mood ring changes colors, depending on the temperature of the finger.

The thermomic element of the ring usually contains something called liquid crystal. When touched, it changes colors due to the temperature of the finger of the wearer. And there are many examples more of such thermomic elements. Thermochromic T-Shirts for example change colors depending on the body temperature. My favorite presentation of the phenomenon includes straws that change color depending on the temperature of the liquid they are set into.

The pink straw changes to violet color in ice water and back to pink.

These presentations are obviously for fun and giggles. But thermochromism can also be used for medical or industrial reasons. Liquid cristals for examples have an accurately defined range of colors and are applied for thermometers for rooms, refrigerators, aquarium, and medical use, and in indicators of level of propane in tanks.

Learning about energy

What exactly does this have to do with energy, you ask? I want to let you in on a little secret. As you might know, temperature is a measure of how hot or cold something is. More accurately, it is the measure of the average movement of the particles in the matter. Heat is associated with this movement, because it is the result of the movement and is a form of energy.

So by putting the straw in the container with warm water, we transfer energy from the water to the straw. The straw will get warmer, the temperature rises. And the thermochromic material of the straw changes to a lighter color, in the video it is pink.

By putting the straw into the ice water, the energy now gets transferred from the warm straw into the cold water. If you do it more than once, you will notice how the ice will melt. You can compare it to ice water without putting warm straws into it and see that the ice melts faster when we put the straws into the cold water.

What’s the take-home message?

We can learn different things from this experiment. First, we learned about thermochromic materials and that they change color with changes in temperature. But we can also learn about the energy transfer in form of changes in temperature. Temperature is a measure of heat, which is a form of energy. By changing the temperature, energy gets transferred. This phenomenon can be used as a great starting point to talk about energy transfer and thermodynamics.

And then there are always more phenomenon of chromism. Have you ever seen beads lighting up and changing color when illuminated with ultraviolet light? The idea of energy transfer is the same, only that with ultraviolet light, the energy does not come from the heat of the object but from the energy of the rays.

Ultraviolet light is one part of the electromagnetic spectrum, just like visible light and infrared is, too. Find out more about the infrared spectrum in my post about Thermal Imaging Cameras.

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