Collection of Gases over Water


Related Posts:

This figure shows a diagram of equipment used for collecting a gas over water. To the left is an Erlenmeyer flask. It is approximately two thirds full of a lavender colored liquid. Bubbles are evident in the liquid. The label “Reaction Producing Gas” appears below the flask. A line segment connects this label to the liquid in the flask. The flask has a stopper in it through which a single glass tube extends from the open region above the liquid in the flask up, through the stopper, to the right, then angles down into a pan that is nearly full of light blue water. This tube again extends right once it is well beneath the water’s surface. It then bends up into an inverted flask which is labeled “Collection Flask.” This collection flask is positioned with its mouth beneath the surface of the light blue water and appears approximately half full. Bubbles are evident in the water in the inverted flask. The open space above the water in the inverted flask is labeled “collected gas.”
Figure 1. When a reaction produces a gas that is collected above water, the trapped gas is a mixture of the gas produced by the reaction and water vapor. If the collection flask is appropriately positioned to equalize the water levels both within and outside the flask, the pressure of the trapped gas mixture will equal the atmospheric pressure outside the flask (see the earlier discussion of manometers). Source: OpenStax Chemistry 2e

Collection of Gases over Water (OpenStax Chemistry 2e)

A simple way to collect gases that do not react with water is to capture them in a bottle that has been filled with water and inverted into a dish filled with water. The pressure of the gas inside the bottle can be made equal to the air pressure outside by raising or lowering the bottle. When the water level is the same both inside and outside the bottle (Figure 1), the pressure of the gas is equal to the atmospheric pressure, which can be measured with a barometer.

However, there is another factor we must consider when we measure the pressure of the gas by this method. Water evaporates and there is always gaseous water (water vapor) above a sample of liquid water. As a gas is collected over water, it becomes saturated with water vapor and the total pressure of the mixture equals the partial pressure of the gas plus the partial pressure of the water vapor. The pressure of the pure gas is therefore equal to the total pressure minus the pressure of the water vapor—this is referred to as the “dry” gas pressure, that is, the pressure of the gas only, without water vapor. The vapor pressure of water, which is the pressure exerted by water vapor in equilibrium with liquid water in a closed container, depends on the temperature (Figure 2); more detailed information on the temperature dependence of water vapor can be found in Table 2.

A graph is shown. The horizontal axis is labeled “Temperature ( degrees C )” with markings and labels provided for multiples of 20 beginning at 0 and ending at 100. The vertical axis is labeled “Vapor pressure ( torr )” with marking and labels provided for multiples of 200, beginning at 0 and ending at 800. A smooth solid black curve extends from the origin up and to the right across the graph. The graph shows a positive trend with an increasing rate of change. On the vertical axis is ( 7 60) and an arrow pointing to it. The arrow is labeled, “Vapor pressure at ( 100 degrees C ).”
Figure 2. This graph shows the vapor pressure of water at sea level as a function of temperature. Source: OpenStax Chemistry 2e

Vapor Pressure of Ice and Water in Various Temperatures at Sea Level

Temperature (°C)Pressure (torr)Temperature (°C)Pressure (torr)Temperature (°C)Pressure (torr)
Table 2


Flowers, P., Theopold, K., Langley, R., & Robinson, W. R. (2019, February 14). Chemistry 2e. Houston, Texas: OpenStax. Access for free at:


Related Research

Research Article: Chemotaxonomic Study of Citrus, Poncirus and Fortunella Genotypes Based on Peel Oil Volatile Compounds – Deciphering the Genetic Origin of Mangshanyegan (Citrus nobilis Lauriro)

Date Published: March 13, 2013 Publisher: Public Library of Science Author(s): Cuihua Liu, Dong Jiang, Yunjiang Cheng, Xiuxin Deng, Feng Chen, Liu Fang, Zhaocheng Ma, Juan Xu, Mingliang Xu. Abstract: Volatile profiles yielded from gas chromatography-mass spectrometry (GC-MS) analysis provide abundant information not only for metabolism-related research, but also for chemotaxonomy. To study the chemotaxonomy … Continue reading

Research Article: Ethical Implications of Modifying Lethal Injection Protocols

Date Published: June 10, 2008 Publisher: Public Library of Science Author(s): Leonidas G Koniaris, Kenneth W Goodman, Jeremy Sugarman, Uzoezi Ozomaro, Jonathan Sheldon, Teresa A Zimmers Abstract: Teresa Zimmers and colleagues argue that it is difficult to conceive how lethal injection research activities could be carried out in a fashion consistent with ethical norms. Partial … Continue reading

Research Article: Development and deployment of a field-portable soil O2 and CO2 gas analyzer and sampler

Date Published: August 28, 2019 Publisher: Public Library of Science Author(s): Zachary S. Brecheisen, Charles W. Cook, Paul R. Heine, Junmo Ryang, Daniel deB. Richter, Débora Regina Roberti. Abstract: Here we present novel method development and instruction in the construction and use of Field Portable Gas Analyzers study of belowground aerobic respiration dynamics of … Continue reading

Research Article: Prediction model for the water jet falling point in fire extinguishing based on a GA-BP neural network

Date Published: September 4, 2019 Publisher: Public Library of Science Author(s): ChaoYi Zhang, Ruirui Zhang, ZhiHui Dai, BingYang He, Yan Yao, Jie Zhang. Abstract: Past research on the process of extinguishing a fire typically used a traditional linear water jet falling point model and the results ignored external factors, such as environmental conditions and … Continue reading

Research Article: Collaborative Emission Reduction Model Based on Multi-Objective Optimization for Greenhouse Gases and Air Pollutants

Date Published: March 24, 2016 Publisher: Public Library of Science Author(s): Qing-chun Meng, Xiao-xia Rong, Yi-min Zhang, Xiao-le Wan, Yuan-yuan Liu, Yu-zhi Wang, Yongtang Shi. Abstract: CO2 emission influences not only global climate change but also international economic and political situations. Thus, reducing the emission of CO2, a major greenhouse gas, has become a … Continue reading

Research Article: A spherical falling film gas-liquid equilibrator for rapid and continuous measurements of CO2 and other trace gases

Date Published: September 25, 2019 Publisher: Public Library of Science Author(s): A. Whitman Miller, Amanda C. Reynolds, Mark S. Minton, Long Chen. Abstract: Use of gas-liquid equilibrators to measure trace gases such as CO2, methane, and radon in water bodies is widespread. Such measurements are critical for understanding a variety of water quality issues … Continue reading