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Name:  Jamison R
Status: educator
Grade: 6-8
Location:MD
Country: N/A
Date: 1/28/2005


Question:
What is the reaction of vinegar and copper at boiling temperature?


Replies:
Copper does not always do a predictable single-step reaction in acids. You will have to use your eyes, do some experimentation, and make your own mental model of the surface condition and its evolution.

What is the starting condition of the surface? As indicated by its appearance: is it dull, dark, tarnished? That would be an oxidized surface condition. Copper oxides, oxy-chlorides, and sulfides are dark-colored films on the surface. Some of them can be greenish.

If it is a shiny clean copper-color, the oxides are very thin or not present. A poignant orangish copper color can happen when oxide films have an intermediate thickness. Copper which is clean but microscopically rough tends to be less shiny and more pinkish.

Now to your question. Distilled vinegar is 95% water + 5% acetic acid (CH3CO-O-H).

The first thing the acetic acid does is dissolve the oxides off the surface of the copper. Dark-colored copper will get brighter and possibly somewhat shiny if the roughness under the oxide was not too great. This is a clean, almost naked copper metal surface.

What happens second depends on how much background oxygen happens to be available in the solution. Any oxygen from air dissolved in the solution (or any hydrogen peroxide you add) will oxidize some of the copper on the bare metal surface in the solution. And this oxide will be promptly dissolved in this acidic solution. The solution will be "etching" the copper, but probably not very fast. Stirring or bubbling air into the liquid would tend to increase the rate of this etching. In a little while, the liquid may take on some coloring from the copper ions it has dissolved.

Thirdly, the acetic acid will be evaporating away faster than the water. Given long enough boiling, this will discontinue the acid's tendency to dissolve dark oxides. In fact, all the extra copper ions now dissolved in the solution will gradually start to precipitate, becoming solid copper oxides on the copper surface. These oxides may be uniform black, or mottled multiple colors; I do not know from experience. Either way, it could have uses for art or technology.

This whole sequence might be summarized as "musical chairs" plus a little additional oxidation.

By the way, boiling vinegar has an extremely strong sharp stench. It is a bit dangerous. What do you do for ventilation, personal protection, and maybe containment?

Jim Swenson



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