PAUL ROBERTS: Hi, my name is Paul Roberts. I'm a chemistry teacher here at Andover College.
I've been teaching chemistry for eight years. And today, I'm going to talk you through how
to do the core practical of distillation. So there are some important safety considerations
in this practical. First of all, we're using a Bunsen burner, so it's very important the
students know how to safely light a Bunsen burner and how to control it. Also, we're
going to be heating the water in an elevated position, so it's important to clamp the conical
flask.
The first part of the practical setup, you're going to need to prepare the bung thermometer
delivery tube apparatus. An important safety consideration in this delivery tube setup
is that there are no blockages in the delivery tube. Any blockages could result in the overpressure
of the heating system and could result in injury. So first of all, just check that there
are no blockages. This can be done by carefully blowing through. And you can feel air coming
through out at the other end.
So now, we have our delivery tube apparatus set up. Now, I'm going to add some ink to
the conical flask here. And I've got some black ink. I've prepared this by putting a
few drops of just fountain pen ink in 200 ml of water. Need to add approximately 50
centimeters cubed. The volume isn't critical. We're going to have some left over at the
end anyway. Now to ensure a nice, smooth boil, we need to add some anti-bumping granules.
The amount of these anti-bumping granules isn't critical. Just a spatula full in the
flask is fine.
So now, we have our ink and anti-bumping granules in our conical flask. We can now put in the
thermometer, bung delivery tube apparatus, and push it in nice and snugly. Notice at
the other end here, we have the delivery tube going into a test tube and the test tube is
in an ice bath. That way when the water boils, it will condense in the tube and will be continued
to be condensed in the test tube.
OK, so I'm going to light the Bunsen now. Always light a Bunsen with the hole either
closed or partially closed. And I'm going to slowly increase the temperature of the
Bunsen now. Now, the Bunsen is going to be heating fairly vigorously. And I want it to
bring the water or the ink, which is mostly water, up to a boil. Now I can keep an eye
on the thermometer there, but the thermometer should start rising as I heat it up. And it's
going to take a couple of minutes for the ink to come to a boil. So whilst you're heating
the ink, it's very important to keep one hand on the Bunsen burner. That way if the ink
starts to boil quite violently, you can quickly move the Bunsen out of position.
I can just see the ink just starting to boil now, bringing it to boil. And I'm being mindful
not to leave the Bunsen there. The temperature's still rising. And at this point, I can just
start to see condensation appearing in the delivery tube. So now, this is the kind of
boil we're looking for-- a nice, steady, rolling boil. I don't want to heat it too violently.
If I heat it too much, then the ink will actually boil all the way to the top of the bung and
we'll get carryover into our receiver flask.
I can now see liquid slowly coming down the tube. It's condensing here in this tube. Now
bear in mind, this tube is going to be very hot. This tube is going to be 100 degrees,
so don't touch it. And I'm continuing this in and out type process, where I'm heating
the ink, keeping it on a nice rolling boil.
Now after a couple of minutes of doing this, you should be able to see through the receiver--
through the ice bath and the receiver-- that we have collected some water. And at this
point, I can turn the Bunsen burner off.
Now, I'm not going to do anything to the apparatus until the boiling has stopped and until there's
no more steam coming out of the delivery tube. Bear in mind you can see the thermometer now,
the thermometer is at 100 degrees and so is the whole of this apparatus. However, my ice
bath and my receiver is actually quite cool now. This is actually cool to touch.
Now, I can carefully remove the ice bath. It doesn't matter about any water dripping
here. And I can see now, when I take out my test tube-- my receiver test tube-- that I've
got pure water remaining. And there's no ink in the original sample.
So it's very important now to keep an eye on the temperature and not to start dissembling
the apparatus until the temperature drops to 50 degrees. Everything here will still
be very hot. The rest of this can go down the sink, as there's no hazards in pure water.
So there are a few common problems that can occur in this practical. First of all, you
get black ink in your test tube-- in your collection vessel. This can be caused essentially
by the ink boiling too vigorously and the ink carrying over into the tube. A simple
remedy to this is not to heat the conical flask as strongly and constantly be moving
the Bunsen burner in and out. Another problem can be if you're using a small conical flask,
then it boils too quickly and there isn't the volume for the boiling to calm down. Again,
I suggest using a 250 ml conical flask.
Now, the other problem might be you get no water at all in your receiver. Now, this can
be due to there being a hole, or a not a snug fit in the thermometer and delivery tube,
or there not being a snug fit in the bung into the conical flask.
So in summary for the distillation practical, first of all, you prepared the delivery tube
apparatus and made sure there were no blockages. We've also looked at how to safely heat the
apparatus and make sure there's no carryover. And last of all, we've looked into some troubleshooting
of how to get over some problems that might occur.