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