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Name: Vinh
Status: student
Grade: 6-8
Location: N/A



Question:
What happens to coppers properties when heated?


Replies:
Hi Vinh,

When copper is heated until it glows dull red, then cooled (either slowly or by plunging it in water), it becomes very soft and easy to bend or to "work" by other means. This condition is called being "annealed".

Copper cannot be hardened by any method of heat treating. The only way copper can be hardened is by so-called "work hardening". This is accomplished by (for example) rolling thicker sheets of copper and squeezing them into thinner sheets. You can easily demonstrate this yourself by taking a strip of copper and heating it red hot, then cooling it in order to anneal it. Now try to bend the strip sharply 90 degrees or so. It bends very easily. Once bent, try to unbend it straight again. You will notice that the bent area tends to stay bent, and another area nearby gets bent back instead. What has happened is that the first bend has "work hardened" the copper in that area, and it resists bending back again; so the copper bends back elsewhere, in a nearby area that has not been previously bent.

Why does this happen? When copper is heated close to dull-red-hot temperature, the many small crystals of metal grow into each other and form fewer, larger, crystals. This is the soft, or annealed condition. Now, when you bend or stretch the annealed copper, the large crystals of copper are microscopically fractured into many irregularly shaped smaller ones. These small crystals do not easily slip past one another when the metal is stretched or bent, and so they act to strengthen the copper.

The above process of annealing then work hardening again is completely reversible. When sheet copper is made by repeatedly passing a thick piece of copper through successively closer and closer sets of rollers, the copper is heated to anneal it several times during this process, otherwise the copper would get so hard that cracks may develop.

Regards,
Bob Wilson


A lot of things can happen, and it depends on how much you heat it. It can melt, and then vaporize if you heat it extremely (although you would need special equipment). In 'normal' temperature ranges, like those possible with a butane torch, it simply expands slightly. Its thermal conductivity also drops slightly.

Hope this helps,
Burr Zimmerman


Vinh,

If the copper is heated in the presence of oxygen, the heat speeds up oxidation and copper(II) oxide (black in color) is formed. Copper(II) oxide has an entirely different set of properties from metallic copper.

On the other hand, if the copper already has a good layer of copper carbonate (notable for its green color - like in the Statue of Liberty), further corrosion/oxidation may be prevented and heating will not have as strong an effect.

If the copper is heated in such a way as no oxidation can take place (say in an inert environment or in a vacuum), then it would go through the transformations you would expect from any heated solid/metal: softening, melting, sublimation, etc.)

Greg (Roberto Gregorius)



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