Glowsticks in the freezer

Prior to lockdown here in March, I went to the pound shop to pick up a few things I thought would be useful for entertaining two small children at home for an indefinite period.  One of the items I bought was a pack of glowsticks which hadn't yet been used, and on a dark, wet afternoon with two grumpy boys I decided to get them out.  My sons haven't seen glowsticks before, and my eldest wasn't sure what they were.  We closed the curtains, turned off the light, and I asked him to gently bend one.  He was amazed to see it start to glow brightly, and bent it a little more.  The little one was equally impressed, and I helped him to bend one and did another for myself.  They then happily danced around the room with their glowing sticks for a while.

Two glowsticks before changing temperatures

Once the excitement had worn off, I asked the 4 year old how he thought they worked.  He wasn't sure, but I explained that it was a chemical reaction that gave out light (chemiluminescence).  It's not a simple reaction to explain to a small child, so I simplified it a lot... I said that two things need to react (similar to when we mix vinegar and bicarbonate of soda) and that in the glow stick they are kept apart as one is inside a little container that you snap open when you bend it.  When they start to mix, the reaction happens and light is produced.  It's actually a reaction that happens in several steps, with the intial reaction (between hydrogen peroxide and a diphenyl oxalate) producing a molecule that is unstable and which breaks down, producing carbon dioxide and releasing energy.  The energy is absorbed by separate molecules of a coloured dye, raising the energy level of electrons in the dye.  When the electrons return to their original ('ground') energy level, the energy is released as photons of light.  Different dye molecules in different glowsticks release different wavelengths (colours) of light. 

Glowsticks provide a nice visual way of seeing how fast a chemical reaction is going - brighter means faster!  I decided to show my son how we could slow down the reaction by cooling the glowstick in the fridge.  I put one in the fridge and he kept the other one (of the same colour) to play with, and after a few minutes I got the cold one out to compare.  It was indeed dimmer and when he held it in his warm hand, the end he was holding started to glow a little more brightly. We talked about whether he thought a glowstick would glow for longer if it was kept in the fridge as the reaction was going more slowly but the glowstick contained the same amount of things to react together before they were used up.  He was a bit sad about the fact that the glowstick would stop glowing, but decided that the fridge would keep it glowing longer so we left them all in the fridge overnight.  This is a poor science experiment as there was no control to compare to, but they were still glowing the next morning.

Cold glowstick (bottom) and room temperature (top)

We improved on this experiment another evening with two new glowsticks (identical coloured, because different dyes glow for different lengths of time).  We had a play for a while in the dark, then we put one glowstick in the freezer and left the other at room temperature.  The next evening, my son was disappointed to discover that the glowstick that he'd left in the room was only glowing faintly.  However we went to the freezer, and he took out the glowstick we'd frozen.  He was more disappointed because it wasn't glowing at all, but a few seconds later the part under his warm hand started to glow brightly and in less than a minute the whole stick was glowing brightly again and much excitement ensued.  

Warmth from his hand restored the glow

He wanted to know if freezing the room temperature stick would make it glow again, so we talked about how chemical reactions stop when they run out of the things that are reacting (like our pumpkin stopped frothing when it ran out of vinegar or bicarbonate of soda).  We put the previously frozen glowstick back in the freezer again for another evening after about half an hour to prolong the fun (and make me feel less guilty about the single-use plastic)!