Name: Chemical Curiosities: Surprising Science and Dramatic Demonstrations
Uploaded: 2017-08-21T04:58:18.000Z
Duration: 1 h 9 min 43 s
Description: Thousands of YouTube videos with English-Chinese subtitles! Now you can learn to understand native speakers, expand your vocabulary, and improve your pronunciation...

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

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

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

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

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

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

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

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

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

and this experiment is based on an
indicator called thymolphthalein.

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

in this experiment the first beaker
had a mixture of thymolphthalein

and some acid and the second
Beaker had some alkali

mixes with alkali they
react to produce a salt

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

acid and final fehling from
here went into the alkali

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

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

its neutralizing the alkali and
eventually this beaker becomes acid

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

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

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

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

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

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

you should see that sequence
of different colors

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

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

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

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

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

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

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

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Chemical Curiosities: Surprising Science and Dramatic Demonstrations

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