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For example, why does for example oxygen turn into gas at a much lower temperature than water?

Does it have anything to do with the molecular structure? A water molecule does have a more complex structure than oxygen, though the R-410A (a mixture of two gases commonly used in heating pumps) is much more complex than water, and it boils at -48.5 degrees Celsius.

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The boiling point of a liquid depends on the intermolecular forces present between the atoms or molecules in the liquid since you must disrupt those forces to change from a liquid to a gas. The stronger the intermolecular forces, the higher the boiling point.

Two oxygen molecules are attracted to each other through London dispersion forces (induced temporary dipoles between the molecules) while water molecules are attracted to each other by hydrogen bonding (attraction of the + dipole on H in one molecule to the – dipole on an oxygen in an adjacent molecule) that is relatively strong. (Hydrogen bonding is an important intermolecular force for molecules where H is directly covalently bonded to F, O or N, which are quite electronegative and thus form bond with H with a relatively strong dipole.) London dispersion forces become more important for atoms and molecules with more electrons. Dipole–dipole attractions are also important in some molecules.

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This is a good, to-the-point answer, so this comment is just supplemental for anyone wanting to explore the question further. This link provides a "dancing molecules" analogy that explains relationship between intermolecular charge bonding, gases, surfaces, and evaporation. Further down, it also addresses why crystals form at lower temperatures. It's heavy on analogy and requires no specific science background, but keeps the analogy scientifically accurate. –  Terry Bollinger May 11 '12 at 3:25
I would like to add that molecular mass may have something to do with boiling point. It generally takes more energy to move more massive objects, and so a higher mass may lead to a higher boiling point. (It seems difficult/impossible to separate out the effects intermolecular interaction from mass.) –  Eric Brown Jun 19 '13 at 14:25
@Eric - Boiling point does increase with molecular mass - but not because of the increase in mass. It's a coincidental increase. The reason is that temperature is already a measure of kinetic energy - two systems at the same temperature have the same average molecular kinetic energy. The reason that boiling poin increases has to do with the increase in induced polarizability of larger atoms, and the increase in the number of contact points for larger molecules. –  thomij Aug 13 '14 at 18:00

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