two molecules or moieties combine to form one single molecule, together with the loss of a small molecule. When this small molecule is water, it is known as a dehydration reaction; other possible small molecules lost are hydrogen chloride, methanol, or acetic acid.
We group artworks with similar characteristics into periods or styles because it is easiest to do it that way - this way we can find common characteristics in these literary/artistic eras, and name them. For example, the Renaissance had a distinct style, which differs greatly from that of Romanticism.
Hiya
I don't see how this is a question, can you please elaborate a bit? Thanks!