[jeangenie]

How can you distinguish between two chiral molecules and two identical molecules one of which has been rotated 180 degrees with respect to the other?

[refurb]

That's the definition of chirality. If you have a tetrahedral orientation of four different atoms around a center atom (e.g. carbon), there will exist a "right handed" and "left handed" version.

You can rotate the "left handed" version any way you want, but you won't be able to superimposed it on the "right handed" version.

[cousin_it]

There's no way to rotate a clockwise screw that makes it a counter-clockwise screw. Try it.