Under construction and in draft form (will also be making pdfs and providing slides for instructors/teachers)
EXPERIMENTAL DETAILS: Materials, set-up, and procedure. (details taken from here)
The methodology, itself, can be found in various places on the net, but this here below is a really nicely done YouTube video on the matter, which we’ve used as the basic template.
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Here are the basic supplies needed:
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And here is what you can put out on the bench from the very get go.
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There’s a couple key things that we can point out here. You will need isopropanol, which is actually quite easy to get (look for the 99% rubbing alcohol in your local pharmacy, although make sure it is isopropanol or isoamyl alocohol). Note that you do need to be careful around isopropanol (link). Dry ice on the other hand is sometimes tricky to get (in Canada for instance, there are official rules for its transportation). Also note that dry ice, if not handled properly can cause mild frost burn, so definitely warn the class to try avoid touching the dry ice.
The tin foil pie plate is essentially the base that will be made cold (with the dry ice), so as to create a temperature gradient, which in turn is responsible for producing an isopropanol cloud. As such, there’s a few important things it needs:
1. The base needs to be of a colour that allows you to see an isopropanol droplet cloud easily. Most videos seem to suggest something with a black surface, but this isn’t always easy to find, and possibly expensive if you need 12 of them. We’ve tried pie plates that were silver/grey (usually the most common) and red in colour (red was really difficult to observe), but the tin foil variety actually worked really well. This seems partly because it’s able to reflect the incoming flashlight, so that you can control the angle of light (just so) and in a way to best see this cloud.
2. The base needs to be deep enough to encase a sufficient enough amount of dry ice, so as to more effectively maintain that cold temperature gradient. Here, we tested plates that were about 1/2 inch deep versus 1 inch deep, and the 1 inch variety worked much better. Presumably, it would also work if you just sat a thin sheet of metal right on top of a dry ice block (we were using pellets). Note that the flat dry ice block works really well, but tends to result in needing a lot more amounts of dry ice – you can try to break the block into smaller pieces that happen to fit within the pie plate (foil or otherwise).
3. The base needs to be of a material that best transfers the coldness of the dry ice to the rest of the chamber. This is why metal is often suggested, but the tin foil was just about perfect here. It’s metal, but it’s also very thin. I noted that the chamber got cold very quickly and reliably.
Another piece of equipment that needs mentioning, is the plastic cup. You can use glass, but the plastic cup works just as beautifully. Don’t forget that it has to be small enough to create a supersaturation situation, and it’s true (as the video suggested), that the smaller it is, the quicker you can see results. If possible, try to get cups without ridges so that there’s no obstructions to the observations.
Anyway, when you put it together, it’ll look a little like this:
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There’s a few key points here as well:
1. Having enough isopropanol in the system seems to be important. In this set up (with a 250ml cup) there seemed to be a big different in what we saw when there was 10ml versus 15ml of isopropanol in the system: in a nutshell, the 15ml generated a lot more visible activity. Currently, we actually use 20ml.
2. Concurrently, this also means you need something capable of absorbing a decent about of the alcohol, so that it’s not drippy. The video mentioned using a piece of felt, but a clipping from a “super absorbent auto sponge” worked much better (it was also a lot easier to anchor to the roof of the cup).
3. This part is IMPORTANT. The seal between the cup (after the sponge and isopropanol steps are done) and the tin foil pie plate must be completely airtight. This is why plasticine is such a great idea! As well, you should do this before any of the cooling steps, since the cooling will simply condensate water onto anything and everything, making surfaces wet and difficult to work with. I suggest putting together the cloud chamber in the following order:
(i) Construct cup + sponge + plasticine + isopropanol + tin foil pie plate” contraption. NOTE: add the isopropanol into the cup (directly on the sponge) immediately before sealing the system with plasticine. This minimizes the amount of isopropanol fumes hitting the air. In the same vein, direct handling of isopropanol is best done by an adult, and in our case, we’ll get the kids to do the sealing but will supply them with gloves (partly as a precaution, and partly because kids just like wearing gloves in a science lab) – full safety details can be found here and here (MSDS) (Thanks Dave).
(ii) Move these contraptions to your dark room (if you’re not already in it – we’ll be using a windowless lecture hall for instance).
(iii) Flip the contraptions upside down, so that the empty pie plate is now on top, AND THEN load the dry ice. (Here, of course, you’ll need to flip the whole thing upright again, so that the sponge is back on top with the dry ice at the bottom encased in the upside down pie plate – we’re going to do this with the base of an ice bucket but some sort of cold resistant matt should also work well). NOTE that dry ice should also be handled by an adult as prolonged contact can cause frostbite – see here for MSDS.
(iv) Then turn the lights off, and use your flashlight to shine a beam of light in such a way as to see that droplet cloud (looks a little like a miniature snow storm), and then, well…, then you wait. You should see something within a few minutes, but it definitely helps to be patient here.
ACTIVITY/LECTURE NOTES PART 1: What is Science? What is in the jar? (30 minutes)
The first part of the session begins with the kids thinking about what science is. It’s usually best to do this as a group activity, so that kids can discuss with their immediate neighbours. It might even be good to do this exercise where they have to write out their answers.
In this context, you’ll probably get a bunch of different answers that will range from a who’s who of disciplines/subjects (i.e. it’s about chemicals, it’s about animals, it’s about medicine…), or sometimes about science culture (it’s about being in a lab, wearing a lab coat, doing experiments, making technology…), and occasionally, you might even get answers that essentially delve into the epistemology of science (it’s about asking questions, it’s about understanding the world). All are technically correct, and make sure you emphasize that point, but for the purposes of our session, we really want the kids to focus on this important outcome of what science is all about.
This statement can actually be a little tricky to grasp, so it’s useful to guide the kids through an exercise that makes them think about what might be real or not real. For instance, what has worked well in the past, is to ask whether they think you (the instructor) are real (i.e. you would simply say, “How do you know that I am real?”) The most common responses tend to revolve around vague statements like , “Because you’re right there!” and if so, do your best to bring up alternative answers that would refute that general response. Examples might include statements like, “How do you know that I am actually there? What if I am a ghost? What if I am a hologram? Could I be a ghost? Could I be a hologram? How would you would be able to tell a real me, from a ghost or a hologram?”
The hologram example seems to work well, especially since the lab space clearly has a projector shining a light in the general vicinity of where you might be standing (i.e. you could ask, what would a hologram need – a projector? Well then, how could you test for whether I am a hologram, etc). All to say, that hopefully, this goofy discussion will get the kids to think that sometimes figuring out what is real or not isn’t always so simple. As well, introduce the idea of “evidence” here, or perhaps use the word “proof” which seems to be a word they have better grasp of.
With that in mind, then you introduce the next slide, which simply shows the below picture with the statement in big bold letter: WHAT IS IN THIS JAR? (note that for full effect, you can even show them the jar, which happens to be in the lab).
Again, get the kids to discuss with their neighbours for a few minutes, and then query them on what answers they have come up with. You’ll find that there are some common answers (such as “nothing” or “air”), and that some classes will also come up with fancier answers like “molecules/gas/oxygen” or even “germs.” Occasionally, you’ll get a joker who will refer to “something invisible” – this is actually a great response. In fact, if it doesn’t come up, suggest it anyway – maybe even in a funny way (we’ve used invisible miniature unicorns for example!) Essentially, highlight the fact that it is a perfectly good suggestion, but that one shouldn’t forget that eventually, you’ll need to come up evidence or proof around the idea.
Then, see if you can coerce them to explore some more subtle answers. There are two great non-obvious possibilities. One would be to imagine the jar wrapped in a blanket and to imagine being inside that jar – what is missing that is obviously there without the blanket (that’s right – light!). As well, ask them what would happen if you let go of a coin at the top of the (open) jar – that’s right, it would fall! What would be responsible for that? And if they say “gravity,” then ask them does that mean that “gravity is in the jar?”
Overall, you want a list of possible things in the jar, which again, will hopefully reinforce the idea that figuring out a real answer often takes a bit of effort, and that this is what scientists are trained to do – they basically look at the world in very careful ways, and they also ponder the question of how would you know whether any of these options are real.
Now that we’ve established a few obvious and not so obvious options, we can then move to the “scientific process” proper. Which is to say that we want to established how “science” might go about figuring out whether something is real is or not. As well, this is a good time for the instructor to announce that the experiment we’re going to do today will explore what might be in the jar. In fact, the instructor might even say to them that the jar contains special particles that are actually traveling super fast, and possibly come from very far away. However, they are so small that they are, to all intents and purposes, invisible. Basically, as an instructor, you’ve just announced a “claim.”
And here is how we want the kids to treat that claim, or any of the claims that happen to be on the whiteboard:
You can walk the kids through this (but don’t take too long – at this point, they’ll be starting to get a little restless). If anything, two elements that were emphasized were to (1) really get the kids to appreciate the “QUESTION IT” point (i.e. what does it mean to be skeptical. How a scientist tends to always respond to any idea with a simple “really?” – get the kids to go “Really?” in unison); and (2) that when you get “OTHERS” to look at your evidence, the identity of the OTHER is very very important.
Anyway, at this point, the kids are usually ready to do something with their hands. And so on to PART 2.
ACTIVITY/LECTURE NOTES PART 2: Starting the DIY Cloud Chamber and a bit about the craziness of atoms.
Start off by saying that we’re going to make a cloud chamber, and the reason being that if we can create a “cloud” in the jar, we can maybe see how different things affect it. In other words, even though the things are invisible, we can indirectly see their effect by seeing how the cloud behaves.
Overall, in this section, the primary goals are to guide the kids through the initial stages of the cloud chamber construction, but also to introduce the notion of what types of things we are hoping to detect (sub-atomic particles like muons, electrons, cosmic rays), as well as show a video that nicely encapsulates some of the crazy scientific facts around atomic structure. Finally, we want to give them a heads up on what they’re hoping to see.
Anyway, for the construction part, basically we want the kids to get to a point where they make something that looks like this:
What we have here, is a lining of the top of the plastic cup with a ring of plasticine. The kids can do this using gloves (so their hands don’t get oily), and also on matts provided on the bench tops (so the benches don’t get oily). Then, ask the kids to insert their piece of sponge into the cup in such a way, that they feel pretty confident that the sponge is nicely wedged in. In general, this step takes longer than you would expect (~15 minutes), but at in the end, you want all groups to have done this and whereby someone has done a visual check of the finished product.
ACTIVITY/LECTURE NOTES PART 3: Finishing the DIY Cloud Chamber and looking for subatomic particles.