What Makes A Piece Sound Classical?

Question is in the title. For example, if you hear a piece by Haydn (which you don't know at the time) it has a particular sound to it that would make you (well me anyway) think it sounds like classical music. Same would go for Baroque, Romantic, and others. What I would like to know is what makes your brain assume what period the piece was written in and how would you imitate that?

Sorry if the question is confusing/vague.

It's not confusing or vague. In fact, it's simple, but may take some time until you become able to distinguish the styles of the different periods and the different composers.

If you have never studied History of Music, it'll be even more difficult. By studying it, from Middle Age to the present era, you'll pass by the evolutions on texture, harmony, form, style, genres etc, and the most significant composers of each period.

Each period of music has some general characteristics that make it "distinguishable" amongst the others. Besides studying these characteristics, you should also listen to a lot of music of that era and spot these features.

You'll be able to recognize these styles by becoming familiar with this kind of music. For example, I really like Ravel and listen to many compositions of his constantly, so that I may be able to recognize a work of his just by listening to it, without having heared it before. I might also be able to spot some "ravelian" characteristics on other composer's works, by saying "well, this passage sounds a lot like Ravel...".

There are no easy rule to know these things. Harmony is a good beginning, as it has evolved a lot through the ages. Instrumentation is another good tipo for this. If I hear a piece with the harpsichord, obviously depending on other aspects too, it has a big change of being a baroque work. If the orchestra is full, with many brass instruments, it should probably be from Romantism on.

As I said before, get familiar to classical music and you'll be able to recognize some of these features naturally^^