Saturday Morning Science Experiment: Traffic lights

This fun, partially-reversible chemical reaction brought to you via reader Aaron Rowe. Great video suggestion, Aaron!

Thumbnail image courtesy Flickr user grendelkhan, via CC


  1. Great demo… but… what’s lacking is an explanation of exactly what chemical reactions are taking place, and when. I assume that at the end, where they show oxygen making the solution turn red, which then goes back to amber, it means that the oxygen is leaving the solution. But what causes the green color? What causes the yellow? Is the green transitional state caused by temperature differences? Can you get back to green after it initially turns green?

    1. Googled “glucose ‘sodium hydroxide’ experiment” and found lots of useful info. Here are a couple of links:

      From those, it seems (briefly) that changing the pH of the solution using sodium hydroxide causes the glucose to be oxidized, a process the pigment in the solution participates in by being reduced itself. Adding oxygen reverses the reaction, oxidizing the pigment back to its original state and changing the color. But left to sit, the original reaction occurs again.

      I haven’t read enough of the descriptions to understand whether the glucose is eventually consumed (completely oxidized) by the repeated reactions or if it is also being reversed back to its non-oxidized state each time. From the description of “partially-reversible” I suspect the former.

      And my usual rant: I have never really had an intuitive understanding of the whole “oxidation/reduction” thing, in no small part because they use the word “reduction” to describe the reaction where an electron is _added_. That just never made sense to me! Grrr…

      1. Think of it this way: lets say you have a cation, like Copper 3+. When you add an electron, the oxidation number goes down, to Copper 2+. In other words, Cu(III) goes to Cu(II). Number goes down, it’s reduced. You could also use the OIL-RIG (oxidation is losing, reduction is gaining) or LEO the lion says “GER” (losing electrons, oxidizing)(gain electrons, reduced).

  2. As a chemistry teacher I have to say that they explained it using an expression that is at the least not precise. The reaction is not reversible at all. Glucose reduces the dye to a different form, with a different color. Oxygen oxidizes the dye back to its original color. Then the glucose reduces it again, and is oxidized in the process. So the dye is just a catalyst to the reaction, being regenerated in the end. Glucose is being spent, the reaction goes only in one direction, and the cycle stops after a while. Beautiful demo!

  3. You might be interested to know that this video was made by the Chemistry faculty at Nottingham University in the UK

  4. Whenever a chemist sees pictures or video of people doing chemistry one of the first things they notice is the person’s protective equipment.

    This woman is not wearing any sort of protective gloves (latex or nitrile), but she should be, especially when working with sodium hydroxide.

    It sounds completely nitpicky, but if a science magazine or documentary (or even a short video like this) shows someone doing chemistry, they should always be properly protected. That way anyone watching the video or reading the article will see what the proper amount of PPE is for that type of reaction.

    It’s good that she was wearing safety glasses and a lab jacket, but she should have also been wearing gloves.

  5. Sodium hydroxide == lye. We’ve been making soap with lye for ages without gloves.

    That said, it’s still corrosive. So yes, gloves should of been used. Even those cheap dollar store dish wash gloves would be fine.

  6. Pointless! Without explaining WHY this is happening she might just as well be playing with food coloring.

  7. I love the demo, even if it is light on explanation. It might work to get kids interested in science at a very young age (in the sense that it suggests that science can do things which are contrary to your expectations… learn more about it), however there’s no real element of doing science in the demo.

    It would, however, make an excellent demo for middle school kids. Instead of simply writing down the chemical equation, I would note that pouring the liquid makes it change color. Ask students to formulate hypotheses about what it is that makes it change. The obvious answers are 1. kinetic energy or 2. interaction with something in the air. These can be easily tested in a classroom (the first by mixing it in a sealed container with no air, the second by bubbling air through the mixture as was done). Once students see it is air that makes the change, ask what component of the air; if the science lab is well equipped, a supply of oxygen or nitrogen can be used.

    Telling them the chemical reaction is not really that important (maybe as a final note in the class). Students will never have to know precisely the chemical reaction taking place here, but they will be expected to know how to rationally problem-solve using scientific information.

  8. Speaking as a research chemist, the last thing I want people using weak acids and bases to do is wear gloves. Gloves encourage sloppy behaviour in students and mean that you don’t notice minor spills, causing chemicals to get spread further by far. Gloves should be worn only when dealing with genuinely nasty materials or when trying to avoid a stench; anything else invites complacency.

    That said, the video was very pretty but useless except aimed at primary school children. Without any explanation of the chemistry involved, it’s just an empty scene of coloured water. Even a brief mention that it’s a pH senstive, redox-active dye would have helped.

  9. The experiment seems to be using sodium hydroxide solid, which is highly corrosive. If it were a 1M solution or something, then I’d agree that gloves are unnecessary.
    Handling the solid definitely requires gloves. In fact, under EU law and COSHH regulations, using gloves should be mandatory. I know it’s only a small amount, but you should used disposable gloves.
    I’ve had a near miss because I used some sodium hydroxide (20%, I think) without gloves and got a tiny splash on my finger. I didn’t notice it on my finger, but later I rubbed my eye which was quite badly irritated. Fixed by sticking my head under a tap for 10 minutes, but it still looked like I’d been in a fight!

    The accent is pretty generic English. It’s a bit like middle-class estuary, but more carefully pronounced. She could be from Nottingham (where the film was made) or pretty much anywhere in the middle-to-south-east of England.

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