The chemical element copper has an enormous number of applications in every-day life and in science. I have noticed that there is a bit of a confusion concerning its correct pronunciation. The correct British pronunciation is [ˈkɒpə(r)], whereas the correct American pronunciation is [ˈkɑːpər], see here.
The key difference between these two pronunciations is that the letter o is pronounced differently. In fact, in American English, there are a number of words in which o is pronounced as [ɑː], including hollow (see also this post), follow, sorrow, con, and borrow. Importantly, the [ɑː] sound is a long sound (follow the above links to listen to examples), and is clearly different from the short pronunciation [ʌ] of the letter u in words such as cut, hut, or supper.
In the case of copper, I often hear people use the incorrect pronunciation [ˈkʌpə(r)] (that is, with cop– pronounced like cup, see also cop). I am quite sure that the origin of this mistake is the American pronunciation of copper, [ˈkɑːpər], which sounds similar to the non-native ear but is not at all identical. Just as the word not ([nɑːt] in American English) is not pronounced like nut [nʌt], copper is not pronounced as [ˈkʌpə(r)].
The pronunciation of other chemical elements is discussed here.