media.laits.utexas.edu:8080/.../Demo_17_zinc-plus-hcl.mp4

There are a variety of different
kinds of chemical reactions

that we learn about.

We know about acid-base reactions.

You know about solubility and
about how salts fall to the bottom

and form precipitates.

There's a category of reaction
involving metals that's less familiar

to folks, but actually is pretty darn
important.

It's the simple question of what
happens when you throw

metals into water.

There are actually four different
categories of this thing

called a displacement reaction.

A displacement reaction in which
you throw metal into water

will have the following results:

The metal will be turned into a
cation, the water will be turned

into a base, a hydroxide,
and then hydrogen gas will be evolved.

And the most famous of those
examples,

throwing sodium and lithium into water
makes a big explosion

even in cold water and the
solution turns pink from

phenoltheline being in there
because of the hydroxide ions

and that explosion is hydrogen gas.

As you move across the periodic
table, you move from the most

active metals like lithium and sodium
to a second category of metal

which is the alkali earth metals
and those are the kinds of things

than can react even in hot water,
and then there's a whole suite

of metals in the periodic table,
transition metals,

which dissolve in acid.

You're all familiar with this.

If you throw iron into water
that is acidified you end up dissolving

the iron away.

Well we're going to do that
experiment here,

only we're going to use zinc
as a transition metal and

we're going to use zinc in particular
because zinc finds a

substantial amount of use in
coins and I'm going to be

showing you another demonstration
in which you get to watch

what happens when you pour hydrochloric
acid onto a penny.

It's kind of interesting.

So let me go ahead and pour the acid
in and you'll notice

the foaming occurring.

This foaming is from the evolution
of hydrogen gas while

the zinc metal is being oxidized
to zinc ion and the solution

is turning basic.

Now, if I was to capture the
evolving gas by putting a balloon

over the top of it and then tie it off,
I could light off that balloon

and I would see a pop coming
from the hydrogen gas

that had evolved.

So this is an examle of a
displacement reaction and

this is an example of a metal which
is not so reactive

that it dissolves in water,
it is not so reactive that

it dissolves in hot water, but is
in that category where if you

dump it into acid, in particular
hydrochloric acid here,

you'll be able to get it
to dissolve away.