Collecting a Gas by Water Displacement
The drawing on the right represents a general chemistry lab apparatus used for generating and collecting gases. A gas is generated by a chemical reaction in the flask, travels through the delivery tube, and rises into the collection tube.
The collection tube has been filled with water, then inverted into the pan of water. Atmospheric pressure pushing down on the water in the pan will keep the water in the tube from coming out.
As the gas rises in the collection tube, its pressure pushes the water out (displacement).
This apparatus allows the experiment to provide three important quantitative measurements of the gas in the collecting tube: volume, temperature, and pressure.
- Gas volume can be directly measured if there are graduations on the collecting tube. If there are no marks, make a pencil mark on the tube where the gas stops pushing the water out. After the experiment, fill the tube to this line with water, then transfer the water into a graduated cylinder.
- The assumption can be made that the temperature of the gas is the same as the water through which it bubbled.
- Pressure is a little more complicated. When the water levels inside and outside the collection tube are the same, the pressure of the gas inside the tube is exactly that of atmospheric pressure. Atmospheric pressure can be obtained from a class barometer or from a local weather Internet site.
The complication is this: any gas collected "over water" contains water vapor (another gas). Dalton's Law of Partial Pressures states that; the total pressure of any mixture of gases is equal to the sum of the individual pressures exerted by each gas.
To get the pressure of the collected gas, the water vapor pressure must be subtracted from the total. Chemical reference books provide charts for the vapor pressure at all temperatures. A basic water vapor pressure table is provided here.