Abstract: A reaction vessel and method of preventing evaporation employing a vessel body, a cap closing an open end of the vessel body and a piston head movable within the volume defined by the vessel body. A fluid to be reacted is introduced into the vessel body. An inner passage gradually converges toward the closed end from the open end so that the piston head, which is compressible, increasingly deforms under compression as it displaces further away from the open end. The piston head is moved tip a relative position relative to the closed end so that it sealingly engages the wall of the vessel and is positioned adjacent the upper surface of the fluid. In this position, the fluid may be heated to promote the reaction and evaporation is prevented by the piston head being adjacent the upper surface of the fluid and sealingly engaging the vessel wall.