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Ask A Scientist

Why is it when two gas elements such as hydrogen and oxygen are combined, they create a liquid?

Asked by: Chris Field
School: Sidney High School
Grade: 11
Teacher: David Pysnik
Hobbies/Interests: Football, hunting, fishing, camping, hiking, traveling, history
Career Interest: History teacher in middle or high school

Answer from James A. Dix

Associate professor of chemistry, Binghamton Unive

Research area: Biophysical chemistry; transport through biological membranes; educational technology

Ph.D. school: University of California, Los Angeles

Educational background: NIH Postdoctoral Fellow, Harvard Medical School; Research Fellow, Cardiovascular Research Institute, University of California, San Francisco; Visiting Scientist, Theoretical Biology and Biophysics, Los Alamos National Laboratory.

Family: Married; two children in college

Interests/hobbies: Old English sports cars and motorcycles; model trains; computer programming; socializing

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Oxygen gas and hydrogen gas do indeed react to form liquid water when the reaction is done on a warm day. However, if you were to do the reaction outside on a night like we've been having (below freezing), then oxygen and hydrogen gases would form ice. The key question is why at a given temperature, some things are liquids, some things are gases, and some things are solids? The way I like to look at it is a competition between the forces that are trying to keep molecules moving and the forces that are trying to keep molecules stuck together. The forces trying to keep molecules moving are related to temperature; the higher the temperature, the greater the force trying to keep molecules moving and in the gas phase. The forces trying to keep molecules stuck together in a liquid or solid phase depend on the molecule's structure. For oxygen and hydrogen molecules, there's not much of an attraction between molecules. At most temperatures we are likely to encounter, the oxygen and hydrogen are gases. The water molecule is different. The water molecule has a structure in which one part of the molecule is positively charged and the other part negatively charged. The positive part of one water molecule is attracted to the negative part of another water molecule, and the water molecules like to stick together. Between the freezing temperature and the boiling temperature of water, this attractive force is enough to overcome the force trying to force molecules apart, and water is a liquid. When the temperature drops below freezing, the force trying to keep water molecules apart is even less, and the water is solid ice. Ask a Scientist appears Sundays. Questions are answered by faculty at Binghamton University. If you have a question, write to Ask A Scientist, c/o Binghamton University, Office of Communications and Marketing, PO Box 6000, Binghamton, NY 13902-6000 or e-mail Check out the Ask a Scientist website at

Last Updated: 9/18/13