NOTES ON
“Copper Into Gold: The Alchemist’s Dream”
This zinc dust is pure zinc metal, and pure
metals are normally shiny. Explain.
Normally, pure metals have an organized
surface with a low surface area. This means
that the light reflects off them mostly in the
same direction, so pure metals are normally
shiny. When metals are ground into a ‘dust,’
they have very high surface areas. Now light
can reflect off them in all different directions,
so they no longer appear bright.
The lab handout said to use a concentrated
solution of potassium hydroxide (KOH) .
However, we used sodium hydroxide (NaOH)
instead, because it didn’t matter which one we
used. Explain.
Both of these compounds are alkali metal
hydroxides, i.e., they have the same anion,
(the hydroxide) and their metals are both in
the same family on the periodic table. This is
why they have similar properties.
When you heat the zinc in the concentrated
solution of NaOH, does it form a
heterogeneous or homogeneous mixture?
The mixture is heterogeneous; you can tell
because there are two distinct phases.
What is the charge on the pure zinc metal?
The charge on pure elements is always zero.
When a penny is heated in a mixture of zinc
dust and a concentrated solution of sodium
hydroxide, what happens?
A silver-colored, smooth and shiny metallic
coating forms on the penny.
What is this coating made of?
Zinc
Explain the process which produced this
coating.
Although most of the zinc remained
unchanged in this process, a very small
amount at the surface of the zinc underwent
this half reaction:
Zn0(s) → Zn2+(aq) + 2eThis is the zinc
dust you
weighed out.
The (s) states
that it is in a
solid state.
This is zinc that lost
These are the
lost 2 electrons, so it 2 electrons
became a cation,
that the zinc
which made it
lost. They
possible for that
went into the
zinc to dissolve into
solution.
the NaOH solution (aq)*
aq is short for aqueous. Something that is
aqueous is dissolved in water. (aq) means that
the Zn2+ ion is dissolved in the solution (which
is mostly water + NaOH)
How do you know that this half reaction must
have happened?
Pure metals cannot dissolve in water; only ions
of a metal may be able to dissolve. The zinc
ions travelled through the solution to get to all
surfaces of the penny. The zinc attaches itself
to the penny.
Is the zinc attached to the penny zinc ions or
pure zinc metal, and how do you know?
The layer of zinc is shiny and metallic-looking.
This suggests that it has become pure zinc
metal again, and that the surface area of the
zinc is no longer high. The zinc ions have
plated onto the penny.
What half reaction occurs when the zinc ions
regain their 2 lost electrons and become pure
zinc metal again?
2e- + Zn2+ → Zn0
When you place the zinc-coated penny on a
hot hotplate, what happens?
The color of the penny quickly changes from a
bright silver-color to a bright gold-color.
Have we fulfilled the alchemists’ dream to
change a base metal into gold?
No. In a chemical reaction or a physical
change, the identity of an element cannot
change. (Only in nuclear reactions can this be
done.)
So - what causes the penny to become
permanently gold-colored?
The zinc layer melts into the copper penny.
This is a physical change. They melt together
to form a homogeneous mixture – i.e., a
solution of zinc and copper.
What is another name for a homogeneous
mixture?
A solution
What is a homogeneous mixture with metallic
properties called?
An alloy.
What is the alloy of zinc and copper called?
BRASS! You have made a brass penny!
You now have a brass penny!