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:

  • 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 electricity.

  • 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.

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 snowflake.

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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