Why Does a Glow Stick Light Up When You Snap It?

What Does Snapping a Glow Stick Actually Do?

A glow stick is a plastic tube with a smaller, fragile glass tube sealed inside it. Each space holds different chemicals, and they are kept apart until you want light. When you bend the stick, the glass breaks, and shaking mixes the liquids [1]. One of the liquids contains hydrogen peroxide (the same chemical, much weaker, that is sold as a bleaching or cleaning liquid). The other contains an oxalate ester, a kind of organic compound, plus a fluorescent dye [2].

Which liquid sits in the glass and which sits in the plastic is not told the same way everywhere. One university demonstration page says the glass vial holds the diphenyl oxalate and the plastic tube holds the hydrogen peroxide, while a different part of the same page names another oxalate, called TCPO, as the vial chemical [1]. The picture on Wikipedia’s glow stick page shows the opposite arrangement, with the peroxide in the glass capsule [3]. The sources differ, and I cannot tell whether that is a difference between brands, so this note only claims what all the sources agree on: one liquid has the peroxide, and the other has the oxalate ester and the dye.

The snap is a switch that lets the two meet. And once they meet, the stick cannot be turned off or reused [3]. So what happens when they meet?

What Happens When the Two Liquids Meet?

The university demonstration describes three steps [1].

1. The hydrogen peroxide reacts with the oxalate ester. This makes a phenol-type by-product and a very unstable substance called a peroxyacid ester.
2. That unstable substance quickly turns into another one, 1,2-dioxetanedione. This one is also short-lived, and it falls apart into carbon dioxide, the same gas you breathe out. Falling apart releases energy [2].
3. The released energy is passed to the dye, and the dye gives off light.

Now comes the surprising part. The reaction itself does not make light. Wikipedia’s article on this kind of reaction says that without a fluorescent dye, only heat is released, and light appears only when a dye is there to take the energy [2]. A simple way to picture it is a thrown ball. The unstable substance releases energy, the dye takes some of it in, and then the dye gives off light. In reality no ball moves, only energy, but the picture shows why the dye matters so much.

Chemists call this kind of glow chemiluminescence, which just means light made by a chemical reaction. Because most of the energy comes out as light and very little as heat, it is often called “cold light” [3]. Exactly how the energy jumps from the intermediate to the dye is still described as a proposed mechanism. The best-known idea involves electrons moving between the two, and it has a long name, chemically induced electron exchange luminescence, or CIEEL [2]. I could not check the original research papers behind it, so treat it as the standard explanation rather than a closed case.

Who Decides the Color of the Light?

The dye does. The chemicals that react can stay the same, and a different dye gives a different color [1]. The demonstration page lists 9,10-diphenylanthracene for blue, and other dyes for green, yellow, orange and red [1]. Wikipedia adds that the dye rubrene gives an orange-yellow light, and that mixing several dyes can make other colors, such as white [3].

This is why a bag of glow sticks in many colors can be built from one basic recipe. The energy from the reaction is the same; each dye turns it into its own color. If you want to see the same idea somewhere else, think of a highlighter pen under a black light: the ink absorbs invisible light and gives off visible light in its own color. A glow stick dye does the same, except the energy comes from a reaction instead of a lamp.

Why Does a Warm Glow Stick Shine Brighter but Not for as Long?

Heat speeds up most chemical reactions, and this one is no exception. A warm stick reacts quickly, so it is brighter, but it uses up its chemicals sooner and fades earlier. A cool stick reacts slowly, so it is dimmer but lasts longer [1][3]. The amount of chemicals inside is limited, so a bright start tends to mean an earlier fade.

Wikipedia adds two more details. Cooling a stick in a refrigerator before you snap it makes the first burst of brightness more even, and a stick that has been chilled or frozen will start glowing again once it warms up [3]. So cold slows the reaction instead of stopping it. When a stick finally goes dark, it is not tired. The reaction has slowed down until the glow is too faint to see.

Who Worked Out This Kind of Glowing Chemistry?

Wikipedia credits Michael Rauhut and his team with first reporting this oxalate-and-peroxide type of light in 1967, working with diphenyl oxalate [2]. The same site says a compound that made the light much brighter was invented in 1971 by Rauhut at a company called American Cyanamid, building on work from Bell Labs, and that several patents for glow-stick-style devices followed in 1973 and 1974 [3]. These dates come from Wikipedia only. I did not read the 1967 paper itself, so use them as a rough guide to when the idea took shape, and check the original if you need exact details.

Another kind of cold light is made by living things, such as fireflies. Both are chemiluminescence, but a firefly uses substances made in its own body, while a glow stick uses chemicals made by people [3].

Is the Liquid Inside a Glow Stick Safe to Touch?

Not really. The mixture contains hydrogen peroxide, and phenol is made during the reaction. The demonstration page says to keep it away from skin and eyes, and not to swallow it if a stick cracks or splits. Spills on skin can cause irritation [1]. Wikipedia adds that one of the ingredients can sting and burn the eyes [3]. The liquid can also dissolve some plastics. So never bite, cut or open a glow stick. If the liquid gets on skin, wash it off, and if it gets in eyes, ask an adult for help right away.

How Can You Look Into This Yourself?

• With an adult’s help, compare two unused glow sticks of the same size. Keep one at room temperature and put the other in a sealed plastic bag in a bowl of cool water, after snapping both. Which one is brighter at first? Which one lasts longer? Do not open them, and do not use hot water.
• Look at the colors in a pack. If the reaction is the same in each, what must be different?
• Read the university page [1] and the Wikipedia pages [2][3], then find where they disagree about which liquid sits in the glass.

Sources

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