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  • welcome to this lecture on chemical curiosities

  • I'm gonna start with the liquid in this container and I just pour some into this cylinder

  • since it's a nice bright red color

  • let's see what happens if I keep pouring

  • I think you can see every time I pour out the liquid I seem to get a different color

  • so, in the dictionary the word curious

  • is defined to mean something which is puzzling or surprising or unexpected

  • and this demonstration might seem rather puzzling at first

  • until we realize that the cylinders were not empty at the start

  • each of them had a little speck of chemical which reacted with the liquid

  • in this container

  • and it produced a color change and we'll look at the chemistry of that in just a moment

  • let's have a look at the liquid in these two beakers they are both colorless

  • let's see what happens when I pour the liquid from this beaker

  • into this one

  • so again we see a color change it's turning blue, the blue is getting darker

  • as i keep pouring goes away again

  • thats also rather odd it seems as if

  • a chemical reaction began and produced the color change

  • and then it sort of changed its mind and went backwards so did it go backwards did that chemical

  • reaction go backwards

  • so the chemistry of these demonstrations is based on a simple idea which is that

  • every substance can be thought of is either an acid

  • or an alkali and if it's neither if it's sort of in the middle

  • we say that it's neutral now we can use certain substances to tell us whether a

  • material is acid or alkali

  • and probably one of the most famous of these is called litmus

  • so litmus is a material which is red in acid conditions

  • and its blue in alkali conditions and there are lots of other

  • indicators. I normally used in this experiment was called universal

  • indicator this has a range of different colors it's red when things are strongly

  • acidic

  • in the middle where things are neutral its green and in strongly alkaline conditions its

  • purple

  • and this experiment is based on an indicator called thymolphthalein.

  • which is colorless in acid and its blue

  • in alkaline conditions so these

  • cylinders had different amounts of acid and alkaline in them producing the various

  • different colors

  • in this experiment the first beaker had a mixture of thymolphthalein

  • and some acid and the second Beaker had some alkali

  • and the key to this is that when acid

  • mixes with alkali they react to produce a salt

  • plus water so they are sort of opposites they kind of cancel each other out

  • so as I started to pour the liquid the

  • acid and final fehling from here went into the alkali

  • the alkali quickly cancelled out the acid so the final fehling is now

  • an alkaline solution that turns blue

  • but as I keep on pouring I'm adding more and more acid

  • its neutralizing the alkali and eventually this beaker becomes acid

  • as well

  • and the final fehling goes back to being colorless so this reaction was not going

  • backwards it was just the same reaction

  • all along we could ask is there a chemical reaction that goes backwards

  • can chemical reactions go backwards at all well it turns out to be a really

  • interesting question and it's a question

  • that we gonna come back to several times during the course of this lecture

  • but let me just show you now an other example

  • of a reaction involving universal indicator and it's this column of water

  • which has universal indicator and also a little bit of sodium hydroxide which is

  • alkaline

  • and so it's turned it this sort of bluey purple color I'm gonna add some acid

  • we should see it go through a sequence of colors rather like these

  • now the particular acid that i'm gonna use is acid that's going to be made

  • in the water from carbon dioxide

  • so, in this beaker I have carbon dioxide but it's frozen its at

  • -79 degrees centigrade it's become a solid we call this dry ice

  • because when it warms up it doesn't melt to a liquid it goes straight to the gas its

  • always dry

  • so when I add the dry ice to the water

  • it will react with the water to form an acid called carbonic acid

  • thats the same stuff that's in fizzy drinks thats what gives the the fizzy drinks that fizz

  • so let's see what happens when I add this now

  • watch for the color changes

  • you should see that sequence of different colors

  • okay

  • so in all the reactions we've seen so far we mix two things together

  • it produced a chemical reaction which gave rise to a color change

  • so let's have a look at this flask. This flask has

  • a a colorless liquid in, but if I shake the flask

  • it turns blue

  • that's a bit surprising because i didn't seem to be mixing two things together I was

  • i was just shaking up a single liquid

  • here is another flask it's a similar idea this is a yellow liquid

  • if I shake it it turns red

  • if we gave it a really good shake it turns green

  • there is something else rather surprising about this as well. If we keep watching

  • the green is turning back to red

  • and if we look here the blue is turning back to being colorless

  • infact the red will go back to being yellow so it's going back through that

  • sequence of colors again

  • what's more I can even repeat it so If I shake it again goes back to being blue

  • shake this again goes back to being red and so on

  • if I wait it will go back so again it looks as if we have a chemical reaction

  • that's going backwards but the first mystery

  • is why do we have a color change at all I didn't seem to be mixing two things

  • together

  • what we have to remember of course is that this flask not only contains

  • water but it contains a gas in fact the gas is just air

  • and air of course is a chemical and when I shake

  • the flask I'm mixing oxygen from the air

  • with the liquid and that's producing the chemical reaction

  • so the next question is did this chemical reaction then go backwards

  • as it fades from blue to colorless is it a chemical reaction that"s going backwards

  • well unfortunately it is'nt because what's happening is there's a second

  • chemical reaction taking place

  • this flask contains a dye called methylene blue

  • when it reacts with oxygen it goes from colorless to blue

  • but also in the flask there is some glucose and that glucose slowly turns methylene

  • blue

  • from blue-color back to being colorless this is the same idea

  • but with the different material called indigo carmine

  • so again we didn't have a chemical reaction going backwards

  • but we're gonna keep on looking for such a reaction as we go through the lecture

  • so in the reactions we've seen so far then we mix to things together

  • and we got a color change so let's have a look at what happens when i mix

  • these two colorless liquid together so first all this machine is just called a

  • magnetic stirrer

  • it just spins these little magnets and keeps the liquid stirring it's just because i'm too

  • lazy to stand here stir them by hand

  • so if a colorless liquid being stirred i'm gonna add a second colorless liquid

  • and watch closely and see if you can detect a color change

  • so keep watching

  • (Audience surprised)

  • okay (Laughing)

  • very strange, very strange indeed, we mixed these two chemicals and it seemed as if no

  • reaction took place

  • we just sat there for 10 seconds and then suddenly it reacted

  • now that seems very odd very surprising but what was really going on

  • what's really going on is that there are actually two different chemical

  • reactions taking place inside this beaker

  • the first reaction was quite a slow reaction

  • it was a reaction between two chemicals that produced Iodine

  • so imagine this reaction taking place and slowly releasing iodine into the

  • solution

  • now the iodine would appear as a sort of brown color

  • you can't see the iodine because there's a second

  • chemical reaction taking place there's a material in the solution

  • which is reacting very quickly with the iodine and it's absorbing the iodine as soon as it's produced

  • and the secret to this is to arrange that second material

  • is in short supply so the iodine is being produced slowly

  • is being mugged up by the second material as soon as it's produced

  • when that second material runs out after about 10 seconds or so

  • the next little piece of iodine to be produced remains in solution

  • because the iodine is a bit hard to see from the back of the room we've added

  • some starch

  • the iodine reacts with the starch and produces a very dark blue color that appears to be

  • almost black okay so thats called a clock reaction

  • so now you understand how that one works. Have a look at this one

  • this involves three color solutions so I poured out one

  • into there and I pour this into here again watch closely

  • okay

  • so thats a sort of two-stage clock so i leave you to think about how

  • that one might be working so in the reactions we've seen so far then

  • we mix chemicals together and we know that a reaction has taken place because we had

  • a change of color

  • but there are lots of other ways that a chemical reaction can show up

  • and one way is called a change of state so the state of something

  • just means whether it's a solid or liquid or gas so something turns to a solid

  • to liquid or from a

  • gas to the solid then it's changed state so we show you an example

  • of a chemical reaction that involves a change of state so

  • we will use these two liquids i have a red liquid and a colorless liquid

  • what I'm going to do is to pour the

  • colorless liquid onto the red liquid very carefully

  • and try to make two layers

  • so what I want to happen is for the colorless liquid to be floating

  • on top of the red liquid in<