There's no chemical reaction taking place here,
so why is it that the liquid in this flask appears to light up when I place it
in front of this filter?
Well, I have a backlight,
in this case an OHP, and I'm blocking all of the light that isn't passing through this filter
with a piece of cardboard here. This is a polarizing filter I've borrowed from the Physics Department.
There's a second polarising filter mounted to the front of the lens
so in theory there shouldn't be any light coming from the OHP
getting through to the back of the camera.
Physics departments are probably going to have some of these filters lying around.
They're used for showing things like stress patterns in plastic.
You could either mount another large filter directly in front of the light source
or you could pass out smaller filters for your viewers to hold.
This is a solution of sucrose. It's a chiral molecule
and it's dextrorotatory, i.e. it rotates plane polarised light
to the right, so it's now able to pass through the second filter.
Other sugars like glucose and fructose are chiral too.
Glucose rotates light to the right.
Hence, also being known as dextrose.
Fructose, which is sold in supermarkets as fruit sugar
rotates light strongly to the left and is also lesser known as levulose.
But have I just added
a glucose solution or a fructose solution? Both look the same.
However if you turn one of the filters back and forth
you can see our solutions are rotating in opposite directions.
The one on your left is fructose.
Now here's a cool extension.
This solution here started off exactly the same as the sucrose solution on your right.
I added a few drops of lemon juice
and then boiled gently for about five minutes.
Sucrose is a disaccharide of glucose and fructose.
What I've done is to begin hydrolysing it.
We now have a mixture of sucrose, rotating right,
glucose, rotating right, and fructose, which rotates strongly left.
Watch as I rotate.
The solution in the centre now rotates somewhere between the other two.
Left hydrolysing long enough,
the solution will begin to rotate left. Hence, being known as invert sugar syrup.
Invert sugar syrup is highly prized by the food industry
because compared to sucrose it's sweeter, it has a lower water activity,
which means it has a longer shelf life
and also it's less prone to crystallisation. The reaction in industry is actually followed by polarimetry.
I've had these flasks for a few weeks
so they'll keep long enough for a whole year group to see.
You can however see that the sucrose has begun to crystallise
whilst the fructose and the partially hydrolysed sucrose have not.
One of the best reasons to show this demonstration
is that it draws out the point that chirality is a property of the whole molecule,
not just a sum of the individual carbon atoms within it.
Because glucose rotates to the right and fructose more heavily to the left
you would expect sucrose, which is made up of the two subunits, to rotate to the left.
As we have seen, it does not.