♪ [upbeat music]
(Sarah) Hi, my name is Sarah Chisnell.
I'm the staff scientist here on the third floor
at Science North in the Discovery Theatre.
This year is the International Year of Chemistry,
and as such we've been celebrating it every month,
which brings us to thermodynamics.
Thermodynamics is a term that refers to the heat of reaction.
So some reactions gain heat, while others lose heat.
And we see this in all kinds of different chemical reactions,
but we actually use them quite often.
So, if you're familiar with one of these,
an instant cold pack,
this is a chemical reaction that is endothermic.
It used up the heat so it feels cold.
This --
not sure if everybody's familiar with them but these are heat packs.
So chemical reversible heat packs.
What happens is, in here, you have a chemical reaction
that actually gives off heat while it's reacting.
And these things stay hot for about 45 minutes or so.
Now, these instant cold packs are not reusable.
As soon as you use them once, once the cold is gone, they're gone.
The heat packs, however, are reusable.
Just because we're able to reverse that chemical reaction.
So, the first one we're going to look at is the instant cold pack.
A lot of people wonder what's in here.
There's some sound. [rattling]
It jiggles around, kind of like sand.
And you have to break whatever's inside
in order for the chemical reaction to happen.
So what we're going to do is we're going to cut this open
and give you a little bit of a glimpse inside.
So inside, we have a baggie. [rattling]
This is the thing that you pop.
And this is just water.
All that is is water.
And we have these little white beads.
Let's take a look at this.
Now, typically, these are going to be ammonium nitrate,
which, when mixed with water, takes away heat.
It uses up heat for that chemical reaction to proceed.
So we'll just cut this baggie of water open,
pour it in.
[water trickles]
OK.
And we're going to give it a little stir.
Now, if you can see,
it just looks like those white beats are floating around in the water.
But a chemical reaction is taking place.
It's just very slow.
Eventually, this will become like a slurry or a gel kind of thing
when all of that ammonium nitrate is dissolved.
But for now, when you put the thermometer inside,
the temperature drops very rapidly.
It'll drop to a certain point,
and stay at that point for about an hour or so,
typically, of these cold packs rahter.
So we'll just let that sit off to the side.
And in a few hours, you'll get to see what the slurry looks like.
On the other hand,
we are going to take a look at these instant heat packs.
So we're going to cut this open now and see what's going on with this solution.
I'm just going to attempt to pour this into this little beaker here.
OK.
In this bag here, the inside is really, really smooth.
The solution is really, really good.
Out here, there's dust, the atmosphere can get to it.
There's already a little bit of crystals around the top of the beaker.
So this one, as soon as it cools, it'll crystallize.
Especially if you just bang on the table or scratch the glass whatever.
This one, however, they have to put these metal disk initiators into the bags.
And that's because the crystal growth can start from here,
but you have to pop that thing.
You have to kind of snap it back.
So that there's a little shock.
The crystals grow in the little grooves that are there.
And then it just propagates throughout the solution.
As soon as you have one crystal to start with,
the whole solution can crystalize, if it's at room temperature.
In here, you eliminate any chance of any kind of crystal growth
other than on this little disk, because it's a closed system.
Out here, most anything can start this crystal growth.
So, a little bit more controlled inside these bags.
And that's why these reusable heat packs work quite so well.
So, I hope that gives you a little bit of information about thermodynamics
and the chemistry involved with releasing heat
or using up heat during reactions.
The next web post will be about reversible chemical reactions.