Measuring the volume of a gas produced in a laboratory setting often utilizes a technique called water displacement. This method allows for the collection and subsequent determination of the gas volume based on how much water it pushes out of a container.
Understanding the Water Displacement Method
The core principle behind this technique is Archimedes' principle, applied to gases. When a gas is collected over water, it displaces an equal volume of water. By measuring the volume of water that was displaced, one can determine the volume of the gas collected.
The Process of Collecting Gas by Water Displacement
As described in laboratory procedures, a bottle filled with water is placed upside-down in a pan of water and the gas produced in the reaction flask displaces the water in the bottle. This setup ensures that the gas bubbles up into the inverted bottle, gradually filling it and pushing the water level down.
Here's a breakdown of the typical steps involved:
- Preparation: A container (like a bottle or graduated cylinder) is completely filled with water.
- Inversion: The water-filled container is carefully inverted and placed into a larger trough or pan also containing water, ensuring no air bubbles enter the container during inversion. The opening of the container remains submerged.
- Gas Delivery: A tube from the reaction producing the gas is directed towards the opening of the inverted container.
- Displacement: As the gas is produced, it travels up the tube and into the inverted container, forming bubbles that rise to the top. The gas collects at the top, pushing the water level down.
- Collection: Gas continues to collect in the container, displacing more water, until the desired amount is collected or the reaction stops.
How the Measurement is Taken
Once the gas has been collected, the measurement of the gas volume is made by observing the volume marking on the container where the water level now rests. The volume of gas collected in the container is equal to the volume of water that was originally in that space and has now been displaced by the gas.
- If using a graduated cylinder, the volume can be read directly from the scale.
- If using a non-graduated bottle, the bottle can be marked at the water level after collection, and the volume of water needed to fill the bottle up to that mark can be measured separately using a graduated cylinder.
This technique is particularly useful for collecting gases that are not highly soluble in water, such as oxygen, hydrogen, and nitrogen. Gases that dissolve readily in water (like carbon dioxide or ammonia) are typically not collected this way, as a significant amount would dissolve instead of being collected, leading to an inaccurate volume measurement.
In essence, the volume of the gas collected is measured indirectly by determining the volume of water it displaces.
