Originally posted by AThousandYoung
If the King cannot be captured, how do you trade for it?
One could argue that checkmate is the inevitable capture of the king on the next move.
Of course, typically a trade is done by one traded piece capturing the other only to be recaptured, and this is impossible between the king and enemy queen. The king cannot remain in check, and if the queen is protected, then the king cannot capture her.
If what is meant by trading a queen for a king is allowing her capture in order to checkmate, then that is better described as a sacrifice..