Science Puzzle
Why Does a Glow Stick Fade Faster in Hot Water?
Bend a glow stick until it snaps inside and it starts to glow. Drop it into hot water and it glows much more brightly, but it's worn out within an hour or so. Put a glowing stick in the freezer and it almost goes out, yet the next evening, once it warms up, it glows again.
Why does temperature make such a difference?
The Answer
A glow stick shines because of a chemical reaction, and chemical reactions speed up when they're warmer. In hot water the reaction races along, giving more light every second but using up its chemicals much sooner.
Inside the stick is a thin glass tube of one chemical, surrounded by another. Snapping the glass lets them mix. Their reaction releases energy, which is passed to a dye that gives it out as light instead of heat. The colour depends on the dye.
Molecules can only react when they bump into each other hard enough. Warm them up and they move faster, so they collide more often and more of the collisions are hard enough to count. A common rule of thumb is that many reactions run about twice as fast for every 10 °C rise in temperature. Cool the stick right down and the reaction slows almost to a stop, which saves the glow for later.
That's exactly why a fridge keeps food fresh. Food goes off because of chemical reactions, many of them run by bacteria, and cold slows them all down.
The principle: Reaction rate and temperature. Chemical reactions speed up as temperature rises, because molecules collide more often and harder, so a glow stick shines brighter but runs out sooner when warm.