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