Unwrapping the chemistry of Christmas

We find out how best to impress at the dinner table this year with some super science experiments...
16 December 2014

Interview with 

Dr Mark Lorch, University of Hull


How many chemicals do you think your body interacts with at Christmas? Loads! They're everywhere, in the Christmas jumpers you're wearing, the wrapping paper you've ripped to shreds and even in the preservative on your Christmas tree that stops the needles falling off. Mark Lorch is a biochemist from the University of Hull and he unwrapped the chemistry of Christmas for Chris Smith.

Chris -  You mentioned that you were joining us earlier and you've been vigorously peeling an orange, but not for personal consumption I hope, not in the chemistry laboratory.

Mark -  No, of course not.  The thing about orange peel,  and one of the reasons why it smells so nice, is there's a chemical in this peel called limonene.  Limonene is incredibly flammable.  So flammable in fact that you can turn a bit of orange peel into a flame thrower.

Chris -  Flame for the Christmas table?

Mark -  Yeah.  If you've got candles around it, just follow normal safety.

Chris -  So, we have one of my beautiful crystal candlesticks here and there is a candle sticking at the top of it and you have peeled the peel off of a very large orange and you bent the peel over.  So, you're just going to squeeze the peel onto the candle flame.

Mark -  Exactly and watch this.

Chris -  Woh!

Mark -  Isn't that fabulous?

Chris -  That was about a foot high.

Mark -  That was a really good jet of flame there.

Chris -  Is that a trick stunt orange or is this for real?

Mark -  No, this is just a normal navel orange.  Shall I get another so we can do it again?

Chris -  But there was an absolutely huge fireball that came off like a fruit fireball that came off of this orange.  And it was a bright whitey yellow flame  - burnt very fast.  Why is this happening and what's coming out of the peels?

Mark -  So, the stuff in the peel is this chemical called - is an oil called limonene and it's what smells so lovely when you squeeze an orange or a lemon in fact or any citrus fruit.  As I said, it's incredibly flammable so you can get these nice jet of flame.

Chris -  What is it doing in the orange?

Mark -  I believe it has insecticidal causes actually.

Chris -  As well as taste and other properties.

Mark -  It's smelling and tasting really nice.  That's part of the reason why we put rind in things into cakes, etc to get that lovely aroma.

Chris -  Because people used to dangle oranges in various places  - on Christmas trees they used to put them in their wardrobes to ward off insects.  It repels moths very well I'm told.

Mark -  So, I believe.

Chris -  So, as you squeeze the peel, you're literally putting jet to the stuff into the flame and off you go.

Mark -  Exactly.  And actually, there's another interesting thing about the limonene.  It's the same compound in lemons and oranges, but they smell different.  How did this something that's essentially the same smell different?  Well it's because in the orange and the lemons, the compounds are mirror images of each other.  Now, our body interacts with different mirror images of compounds in different ways.

Chris -  So, what you're saying is, you've got a molecule that's like a left hand version and a molecule that's like a right hand version because you have two different gloves, one for each hand.  They're both hands.  They both look the same.  Both got 5 digits but they're mirror images.

Mark -  Exactly.

Chris -  And the molecules smell different because they bind differently in our noses.

Mark -  Exactly.  So, they bind to different proteins in our noses and generate a different sensation for us.  And that happens actually - a lot of other things we might come across at Christmas is, caraway is a mirror image of the compound that gives the taste of spearmint.  So, spearmint and caraway, same compounds...

Chris -  Yeah, you wouldn't want to muddle it up in your loaf though...

Mark -  No.  but actually, now you've mentioned it, sometimes if you taste caraway, you can almost get a vague hint of spearmint in there as well.

Chris -  Well, there you go, fireballs for your Christmas table.  What about making some baubles for my tree though?

Mark -  Right.  Traditionally Christmas baubles are mirrors for example, they were silvered.  The silvering reaction is actually really quite a simple bit of chemistry and I'm going to try and make now, here at the table a Christmas bauble out of something you wouldn't normally hang on a Christmas, although they do hang on mine, round bottom flasks.

Chris -  Beautiful piece of laboratory glassware.  Do you really hang those on your Christmas tree because that is lovely?

Mark -  I really have one of these, a round bottom flask, that I've silvered and yeah, it does hang in my...

Chris -  Okay, so we're going to make the silver on the outside or the inside?

Mark -  On the inside.  So, we're going to put the silver on the inside of the bauble.  I'm going to start off with a silver nitrate.  So, this is a colourless liquid.  Now, this is a totally colourless liquid, put that silver in there.  Now, the next thing I have to add and this bits a bit stinky, I've got to add some ammonia.

Chris -  This also is a colourless liquid, squirting that into...

Mark -  It's liquid and a bit smelly and it goes brown.

Chris -  It smells absolutely disgusting like we've gone into a gentlemen's toilet.

Mark -  The solution has gone brown.  That's silver oxide and that doesn't dissolve in this solution, so it goes - so, we lose it but as I add more ammonia, it then goes clear again.

Chris -  So, we started off with a clear solution, you added the ammonia and it went to dark colour.  Now, it's gone back to clear.  So why has that gone clear?

Mark -  That's because it now got something called diamine silver that dissolves in this bottle.  Now, what I've got to add is a little bit of potassium hydroxide and this makes it alkaline.  It goes dark again and that's ready now for the final step, which is just to add some sugar.  So actually, this is dextrose I'm using here.  Now, it goes dark slowly.  You can see it's got a brown colour and that is actually sort of black colour.  Now, that is a silver coming out in solutions.  That's particles of silver and it will start to coat the inside of the bauble.

Chris -  What's actually going on in there to make the silver come out and stick to the glass?

Mark -  So, this is a reaction called toluene's reagent.  We use it actually in chemistry labs for figuring out whether compounds have aldehydes, a particular type of chemical in it because this reaction only works with aldehydes.  When we react the ammonium silver complex with an aldehyde which you get in these sugars, it causes the silver to come out of solution.

Chris -  It's just convenient that it does that and sticks to the glass.

Mark -  Yeah.  So, we use it as an analytical tool in chemistry.  It's also long been used to make silvered mirrors and silvered baubles.

Chris -  Fantastic!  What else have you got for us?

Mark -  What's Christmas dinner without crackers?

Chris -  And I did this time.  So, what did I get?

Mark -  That bang!

Chris -  I've got a notebook.  It's a really good gift that.

Mark -  That crack from the cracker and also party poppers, but there we go.  That is a small explosion from a very, very unstable compound called silver fulminate.  It's so unstable that if you make more than a few hundred milligrams of this stuff, it actually explodes under its own weight.

Chris -  Goodness!  So, how do they make it then?

Mark -  Really quite easy to make, but you just don't ever make much of it.  You get the same stuff in - you know, the bang snaps you throw at the ground, etc.  It's incredibly unstable.

Chris -  But what's actually happening in the party popper when I pull the string or in the cracker when you pull that piece of brown card and the two ends come away?  What's happening?

Mark -  So, you've got about 50 milligrams of silver fulminate in a cracker like this.  So, tiny amount and it's painted on one side of the strip.  On the other side, you have a bit of abrasive.

Chris -  Like sandpaper.

Mark -  Sandpaper basically and when you pull them apart, that friction is enough to set off the silver fulminate.  The other neat thing you can do with these is if you put it in a candle flame...

Chris -  You're holding the snap a bit out of the cracker in your flame.

Mark -  Yeah, in the candle flame.  When it goes off - let's see if that works - it puts out the flame.  Did you see that?

Chris -  Yes, it has now gone out.

Mark -  It has because the explosion is actually that vigorous that it essentially blows out the flame.  Of course, the other reason for that is the explosion produces nitrogen and carbon dioxide. 

Chris -  ...Which doesn't burn.... 

Mark -  ...Which doesn't burn. So, there you go.

Chris -  So, there's your answer.  If you set fire to your house doing your flame throw experiment, you can put it out by throwing in a huge box of crackers.

Mark -  Yeah.  I'm not sure that's going to work.  If you have enough silver fulminate to put out the fire, it would also take down your house quite easily.

Chris -  Well obviously, no Christmas table is complete without the cake.

Mark -  No.

Chris -  So, what can you do with Christmas cakes?

Mark -  Well, there's a bit of chemistry that of course goes all the time when you're baking.  There's loads of chemistry that goes on in baking and cooking.  One of the simplest ones to show is a quick demo with sodium bicarbonate and you've probably have tried this sort of thing at school when making volcanoes.  The bicarbonate in your cakes is there so when it heats up, it liberates carbon dioxide, which is what the bubbles are when you bake.

Chris -  Until the bicarbonate breaks down and make some carbon dioxide.  That literally inflates the cake.

Mark -  Exactly, yes, blows up your cake but a lot slower than the fulminate, yeah.  You can do the same thing if you just add vinegar to bicarb.

Chris -  So, we've got a spoon full of bicarb in a glass jar here on the table and it goes from white wine vinegar and it's popping up.

Mark -  Exactly, yeah. You could use any vinegar you like.

Chris -  So, you could use heat but you're using vinegar.  What's going on?

Mark -  So again, the bicarbonate - the same sort of thing is going on in your cakes except in this case, the acid is reacting with the bicarbonate to produce carbon dioxide gas as well.

Chris -  Mark, it's been a huge pleasure.  Thank you very much.  I noticed I'm holding our homemade bauble and it's fantastic!  It has completely silvered and covered the inside of the flask.  It does look really rather good.  Can I have this for my Christmas tree?

Mark -  You may of course.  They're quite heavy, but yeah.  Stick a bit of thread through the clip on the top there and hang that on your Christmas tree.

Chris -  Proper chemist's Christmas tree.  I have a reaction vessel to hang on my tree.

Mark -  In fact, it will be then a 'chemist's - tree!'


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