Growing a Metal Crystalline Deposit
To your right is a movie of
copper suflate electrodeposition viewed through a microscope.
So that we can explain this experiment to you, we begin with a very
short review of kitchen table chemistry:
To grow the zinc frost in the picture (which has false background color)
a negative post attached to a battery, or power supply, was inserted
into a solution of zinc sulfate. The small negative post attracted the
positive zinc ions in the solution. When they arrived at the post,
they gained a negative charge from the battery (or power supply), and
attached to previous zinc ions already present. In this fashion an
"aggregate" of zinc ions accumulated. Because of the way that zinc
ions bond, at a microscopic level the ions form a crystal. But
because of the environment for this particular growth, a branching, wavy
structure grows. This is similar to the growth of wavy ice patterns on
a window or windshield, as opposed to the more highly ordered growth of
- Positive charges attract negative charges.
- Table salt is NaCl, a combination of sodium and chloride
ions. (You know that there is sodium in table salt; people on reduced
sodium diets use salt substitutes for this reason.) Sodium ions are
positive, and chloride ions are negative, that's why they stick
together as salt.
- Table salt dissolves in water. When it does, the Na and Cl ions
separate somewhat so that there are positive and negative charges in the
solution. That's why salty water is a good conductor of
- There are many other salts in nature. Examples include potassium
iodide, added sometimes to salt to provide iodine (a liquid) in the
diet; and, silver chloride, a combination of silver metal and chlorine
gas used to make photographic film. The property of all salts is that
when dissolved in water they separate into positively and negatively
charged ions with water molecules attached.
- Zinc is a metal (necessary in small quantities in your diet) and
copper is a metal (you have seen it in the electrical wires in your
house). Both zinc and copper can combine with sulfate molecules to
produce a salt. When zinc sulfate dissolves in water it breaks up into
positive zinc ions and negative sulfate groups. Copper sulfate
dissolves in water the same way.
The experiment works like this: A solution of zinc sulfate is sandwiched
between two plates of glass. At the center of the plates is an exposed
negative wire. The positive zinc ions in the solution are attracted to
this wire, and attach to it. Subsequently, more zinc ions attach
themselves to the deposit which is kept negative. With time the
branched aggregate develops.
The pattern which develops can be controlled by changing the applied
voltage. This is what sets the negative charge on the central wire.
A careful inspection of the zinc branches shows that they are dendrites
similar to the the branches of a snowflake.
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