Flexi Says: Gases that are produced in laboratory experiments are often collected by a technique called water displacement (see Figure below). This technique is relatively simple and does not require expensive equipment. A bottle is filled with water and placed upside-down in a pan of water. The reaction flask is fitted with rubber tubing which is then fed under the bottle of water. As the gas is produced in the reaction flask, it exits through the rubber tubing and displaces the water in the bottle. When the bottle is full of the gas, it can be sealed with a lid.