Preservation of Mass Achievement
Preservation of Mass20
Break the rules in Test Chamber 07
This can be gained in Chapter 2 – Test Chamber 7.
This one is a lot easier than is seems but you would miss it without knowing you could do it. Play the chamber as normal up until the point of walking through the exit door.
The disintegration field on the door is broken, so head through it and place a portal on the wall on the other side of it. Now head back into the main chamber and put another portal on another passable wall. Now pick the cube up, walk through it and head to the lift for the achievement to unlock.