Measuring the volume of a gas can be done accurately using the water displacement method, a common technique in chemistry.
Understanding Water Displacement
The core principle behind the water displacement method is based on Archimedes' principle, applied to gases. When a gas is collected over water, it occupies a specific volume and pushes the water level down. This displaced volume of water is equal to the volume of the gas collected.
The Water Displacement Method Setup and Procedure
This method is particularly useful for collecting gases that are insoluble or only slightly soluble in water. A typical setup involves collecting gas generated, often from a chemical reaction, directly into an inverted measuring cylinder or gas collection tube filled with water.
As stated in a common example, "An example of such a system is the water displacement method, in which the volume of gas generated in a chemical reaction is measured simply by tracking the volume of water displaced from an inverted water-filled measuring cylinder."
Here's a step-by-step breakdown of how to perform the measurement:
- Prepare the Water Bath: Fill a trough, basin, or pneumatic trough with water.
- Prepare the Collection Vessel: Completely fill a graduated cylinder, eudiometer, or gas collection tube with water.
- Invert the Vessel: Carefully invert the water-filled collection vessel into the water bath, ensuring no air bubbles enter. The mouth of the cylinder should be submerged in the water.
- Position the Gas Source: Place the tube delivering the gas (e.g., from a chemical reaction or storage container) under the opening of the inverted collection vessel.
- Collect the Gas: Allow the gas to bubble up into the collection vessel. As gas enters, it will displace the water downwards.
- Read the Volume: Once the desired amount of gas is collected or the reaction is complete, raise or lower the collection vessel until the water level inside the cylinder is the same as the water level outside (to equalize pressure, although for simple volume measurement at atmospheric pressure, aligning the levels is key). Read the volume mark on the cylinder at the surface of the water inside.
The volume reading on the cylinder corresponds to the volume of gas collected at the temperature of the water and the ambient atmospheric pressure (adjusted for the vapor pressure of water at that temperature).
Why This Method Works
The gas collected physically occupies the space within the cylinder that was previously filled by water. Because gas molecules are much less dense than water and the gas doesn't dissolve significantly, the space it fills directly corresponds to the volume of water it pushed out.
Practical Applications
This method is widely used in laboratory settings for:
- Measuring the volume of gases produced in chemical reactions (e.g., hydrogen gas from a metal-acid reaction).
- Collecting gas samples for further analysis.
- Demonstrating the concept of gas volume and displacement.
By following these steps and understanding the principle, you can accurately measure the volume of a gas using the straightforward water displacement technique.
