Sound and Unsound Sacrifices

I am slightly confused what makes a sacrifice qualify as sound or unsound. I see this in tactics and analysis of top level games and become a bit confused. What is the general theory which qualifies a sacrifice as sound or unsound? Is there a definition? Can you show some examples to show the difference?

Thanks! All help is appreciated!!!!!!

If it's a sacrifice that falls down in a forest, and no ones around to hear it, it's unsound.

It's simple: if a sacrifice benefits your position in the long term then it's sound, if it hurts your position in the long term then it's unsound. There have been a lot of examples of beginner's games posted in the Game Analysis forum on this site that have unsound sacrifices. I'll post links and/or games here if I happen across them.

Unsound sacrifices by beginners often include a Nxf7 or Bxf7+ by White, which usually merely lose two material points (one pawn gained minus one minor piece lost = 1 - 3 = -2 material points behind) for no good reason. The sacrificing player gets a strong attack for a while, but with proper defense, that attack eventually fizzles out, then the attacked player enters into a won endgame.

To my knowledge, there are only about two conditions in which you know a sacrifice is sound: (1) It's part of the opening book, like the Muzio Gambit of the King's Gambit; (2) One player has made mistakes serious enough that it's clear to his opponent that forcing moves suddenly become more important than material. Still, the attacker can't just make sacrifices haphazardly and win: it's just that the situation typically runs into situations where the best moves turn out to be sacrifices. Type (1) sacrifices occur in the opening and are gambits, type (2) sacrifices occur just before one player gets a sudden winning advantage, usually toward the end of the recorded game, as a result of an accumulation of errors.

P.S.--Here are two games with unsound sacrifices from the Game Analysis section I mentioned:

http://www.chess.com/forum/view/game-analysis/my-second-game-in-11-years
http://www.chess.com/forum/view/game-analysis/rooks-trapped-by-diagonal-pawns-bishop-sacrifice-always-look-for-check

Unsound sacrifices are sacrifices which are not correct. So with correct play your opponent gets some advantage after that. But it isn't always clear whether a sacrifice is correct or not, or how to refute it if it isn't. 

Learning how to refute unsound sacrifices is a skill in its own right.  Ask yourself if it's okay to accept it, if yes then do so (people oftentimes incorrectly accept sacrifices!) via a rigorous calculation process, if no then find the optimal way of refusing. 

Thanks! Helped me understand a lot.Yeah those sacrifices don't work. Second one cuz no pieces are helping the queen

yup calculating it out should be effective in deciding whether or not to accept. Thanks!

Here's something in a book I came across today that was on this topic. This was one of 100 common chess mistakes listed in the book.

----------

(p. 255)
93
AILMENT:
Sacrificing without good reason.

There are two types of sacrifices: sham ones and
true ones. In a sham or pseudo sacrifice--the most
common kind--a favorable outcome can be fore-
seen, so it's no sacrifice at all. With a true sacrifice,
the consequences are not entirely clear, so there is
an element of risk. You might get into the habit of
making the latter type. Though you'll startle most
players at first, you'll probably lose in the end, for
if there's no obvious justification for a sacrifice, the
chances are good that the defender will find a refu-
tation. After all, he has no choice but to defend
himself.

Rx
1. Sacrifice only with a definite reason.
2. Consider a sacrifice if your opponent has played
badly and you have a powerful position.
3. Calculate the sacrifice carefully and deeply.
4. If you can't see a positive and sure result to the
offering, don't take the risk.
5. Don't take unnecessary chances just because
you're facing a weaker player.
6. The only times you should consider sacrificing
without certainty are when you're losing anyway
or when you're in a must-win situation.
7. If your opponent plays a suspicious-looking sacri-
ice, look extra hard to find a refutation.

Pandolfini, Bruce. 1995. The Chess Doctor. New York: Simon & Schuster.