Sometimes when I do a cold open with a loud noise, people complain down in the comments
so headphone users, you are duly warned that today is going to be going off with a bang.
This demonstration is a nice way to wrap up a lesson on squeaky pop tests.
If you've just been doing a lesson where students have been collecting and testing for hydrogen gas,
they may have noticed that some of the
tubes might have gone a little bit quieter than some of the others. For example,
or by comparison,
Now a lot more satisfying and, of course, this is all down to the differences between the
fuel air mixtures that students will have collected when doing this demonstration.
If they're lucky enough to be in a school where there's a hydrogen cylinder kicking around
maybe the teacher might demonstrate for them a whole hydrogen balloon going off,
but in the environment of a school laboratory it's actually not recommended to play around
with hydrogen–oxygen mixtures in party size balloons. This is due to the fact that the smaller
scale of the room can lead to damaging levels of sound and this is where dynamite soap comes in.
So the max recommended quantity of hydrogen–oxygen stoichiometric mixtures
to ignite in the school laboratory setting is 10 cm3 so what you can do is load up
a couple of cylinders or syringes of hydrogen and oxygen and use these to put in the correct
mixture into a smaller syringe like this, ready to ignite at the end of the lesson.
Now if you are lucky enough to be in a school where you have a hydrogen and an oxygen cylinder, of course, you can
load these directly from the cylinder. However, I'm a big fan actually of the Bruce Mattson technique.
We've used it a couple of times previously in the Lighting up oxygen demonstration, links down below as always,
and the Avogadro in action demo where we wanted to create some a small amount of hydrogen
again for use on the fly. Now if you're down pat with these, of course, you can skip on ahead, but if
you haven't seen this before do check out those previous videos. We'll just quickly go over it now.
To make a syringe of hydrogen you can place around 0.05 to 0.06 g of
magnesium in the sample vial lid. Lower to the base of the 60 ml syringe and then draw up between
3 and 5 ml of 1 M HCl. You can seal the end with the syringe cap or with sticky tack.
The oxygen syringe uses the same technique, but this time 3ml of 20 vol peroxide, if fresh,
should give you about 60 ml of oxygen. I'm using about 0.05 g of potassium iodide
to catalyse the decomposition here. Optionally you can transfer to a dry syringe for easier handling.
Do check out Bruce's book for loads more tips and tricks.
By pushing and pulling between the syringes
you can load up about 6 cm3 of hydrogen into the smaller syringe like this.
And I'm applying a little bit of pressure on the table here to assist me. It's harder
for me to film this with one hand on the ground while I'm operating the camera at the same time.
Okay there's 6 ml, that's nice. And then we do the oxygen next.
So now you've got your dynamite soap mixture ready to go. Your audience will want to be about
two metres away, wearing eye protection. And you might want to demonstrate the hydrogen on its own
to begin with for a number of reasons. So I've got some hydrogen here left over from before.
I'm going to blow in about 10 cm3 of the hydrogen gas.
Again we just want to have this comparison.
Now one of the benefits of doing this,
is that we can just check, is our bubble mixture stable enough to stick around long enough for us to light this?
Not a very satisfying reaction, but crucially we did get the bubble mixture to hang around.
If you're finding that your bubbles are popping a little bit too quickly I can
highly recommend swapping out store-bought bubble mixture for the CLEAPSS recipe.
CLEAPSS' recipe is absolutely fantastic for making stable bubbles and as always the recipe and instructions are down in the links underneath.
A couple of other things I might point out. A little extra tip
is that if you have something a little bit smaller – this is also plastic again but being a little bit thinner diameter
just allows a little bit of extra support to the bubbles if they're not hanging around long
enough and if you're struggling to actually get a smooth flow to get like smaller bubbles
then cutting the end off of a dropping pipette like this and sticking it onto the end of
your syringe can help you to deliver some smaller bubbles. So for comparison, again, just the hydrogen.
We've got like a nice neat little holder here that we paint up
like that. Again we're not expecting anything too exciting just yet.
Okay pure hydrogen not looking great. So now is our time perhaps to actually investigate the
difference with our dynamite soap. When ready to blow your bubbles, you can show your students how
to properly cup their hands over their ears, not sticking their fingers directly into their ears, of course,
and because your hands won't be available for that, you're going to need to have some ear protection
either some defenders like this or earplugs. You can pop them on, blow your bubbles,
pushing in slowly, getting a nice bubble raft. Warn your students that there's going to be a loud noise,
which should be obvious by now.
I'm going to give myself a little bit of extra distance.
Now wasn't that a lot more satisfying. There are lots of fuel oxygen demonstrations that you can do,
but one of the things that I really like about this one is that essentially if I've just finished
a practical and I don't want to be spending time doing something that's going to involve
lots of set-up, having a couple of syringes set up like this, one of these and a little
bit of bubble mixture in a Petri dish or some similar small plastic container
and I'm good to go. So do give it a try. Let us know how you get on. We love hearing about
how people are using our demonstrations and see you in a couple of months.