So this month we're going to be looking at a
supersaturated solution of sodium acetate. I've brought Phil along with me.
All I'm going to get you to do is to drip some of this solution onto these Petri dishes.
Now if you could just point out to the viewers what we've got
on these Petri dishes that you can see.
- Well there's some sort of solution on that one at the front there.
And these two are both empty. - Completely empty?
- That one has some stuff on.
- Okay and there's a little thing on that one as well. We've got these two sort of crystals on there.
All I'm going to do is ask you to drip some of this liquid
onto the Petri dish and I would do this in a way so that everyone can see what's happening.
So what's going on?
- The solution's crystallizing
kind of round the thing that was in there already.
- Do you want to keep going? You can basically
make a nice big stalagmite with that. In fact I'm going to get going with one of the other ones
because these are great fun. Has it frozen in the end?
When I say frozen, and of course that is one of the problems that we get if you dip in
a pipette into your solution
then it will start going through there. Luckily I've got some spare stuff.
Now what I'm going to do is actually with this one
just sort of show that there's a number of different ways of doing this. You can pop over our little
solid piece there. And we can see it immediately
crystallizing there. Now this is something I would do normally
a little bit lower down the school as opposed to with somebody your age and
I would probably ask them to think about
what's happening in terms of the particles.
So this is something I would use as a model for freezing
and with that in mind when something freezes are you visualizing that
as being a process where heat is going to be taken in
or there's no change in heat or heat's going to be released?
- Heat is taken in.
- So if heat's taken in, that means that this has got
colder or warmer or the same? - It's got colder.
- So if you want to just feel the bottom there. - That's warm.
- Now a little bit lower down the school in year 11
when you were introduced to the idea of energy to do with bonds
you would have been told that there was an energy change
when you break a bond and when you make a bond, and of course here
we're forming bonds going from a liquid
to a solid. What kind of energy change is associated when you form bonds?
- An exothermic change. - Energy is being released as you
form bonds between particles. And this is one thing that we're trying to show.
Because you might expect of course we think of ice as being
cold but when we form a solid when we're sort of forming bonds between particles
then actually that is a process that releases energy
and that's why I would show this. So thanks very much Phil.
Now we just need to look at
how would we set up this solution and get going with it. I'll say bye Phil.
So here's my original supersaturated solution of sodium acetate,
or sodium ethanoate if you prefer, and
I made this by getting 125 grams of the sodium acetate
and then heating that gently over a Bunsen with 12.5 grams of water.
Now with that ratio you can obviously multiply that up or down with
whatever you've got ten times by mass of sodium acetate. You'll find that
it's a little bit light on the water side so as you begin to
cool that down if you've lost any water by
evaporation whilst you've been heating you might find that it crystallizes a little bit early.
You can reduce that effect by heating
with a watch glass over the top
and just removing that to stir occasionally.
And if you really struggle to stop it from crystallizing, a little bit of extra water,
it will still work just fine and the demo's very simple.
Start off with a crystal of your sodium ethanoate
on the watch glass and
you can begin to build
some stalagmites.
Now if you find that the end of your
pipette begins to crystallize that's a good opportunity to have some other ones
nearby because you don't want to then
get any solid anywhere near
your original stock solution. I find it's quite handy
to actually
split down my stock solution so that
if something goes wrong then I've always got something else to fall back on.
And I find that I get asked for this quite a lot
because students really enjoy watching these things form
and the key learning point here is of course that
this freezing,
of course it's not freezing, it's crystallising out of the supersaturated solution, but it's
a good model for freezing,
this freezing is actually an exothermic process.
Heat is being released whereas most students will go into this thinking that
freezing of course when you freeze things they get cold don't they,
and so therefore heat is going to be taken in.
So, supersaturated sodium acetate.
Go ahead, form some stalagmites, and of course make sure that you've got somebody
nearby who can feel the bottom of the watchglass and confirm that in fact
heat has been released in this process.