Gas pressure
Dry ice is placed in the water rocket.
 Dry ice is solid carbon dioxide. The solid particles do not move around much. As water is added the dry ice turns directly into gas (sublimes). The gas molecules travel at very high speeds and collide against the walls of the container. High pressure is generated as a result of the frequent and violent collisions with the wall.
The pressure generated by the carbon dioxide gas molecules colliding with the wall of the container is enough to launch water rockets to great heights. The water is expelled from the bottle with great force generating thrust.
Water is added to the dry ice. As the dry ice sublimes pressure is built up int he bottle. Click to see a 120kb movie of a water rocket launch
Continue with a detailed look at gas pressure (senior school Chemistry)