Water Is Wet

Hydrogen sulfide is a gas at standard temperature and pressure. It boils at a chilly –59.55°C. Selenium is a heavier analogue of sulfur, and hydrogen selenide boils at a still really brisk –41.25°C. Tellurium is a heavier analogue of selenium, and hydrogen telluride's boiling point is –2.2°C. Clearly, the heavier the molecule, the hotter it boils. Oxygen is a lighter analogue of sulfur, but the boiling point of dihydrogen monoxide (water) is 100°C! What's going on?

Recalling the electronegativity table from earlier, sulfur's electronegativity of 2.58 is kind of close to hydrogen's 2.20. So the S-H bond has only a slight variation in how well it holds onto electrons. This means the bond is only weakly polar. Selenium and tellurium also have low electronegativities near that of hydrogen, so the Se-H and Te-H bonds are also practically nonpolar. But oxygen's electronegativity of 3.44 is considerably higher than hydrogen's. It is enough of a difference that the O-H bond is very polar; though the bond is covalent and therefore the two atoms share a pair of electrons, they do not share equally, and the electrons spend more time orbiting the oxygen nucleus than they do on the hydrogen. This produces a partial charge on each atom; positive for the hydrogen and negative for the oxygen.

Now we know that opposite charges attract, so what happens when opposite partial charges meet? They also attract, and the result is called a hydrogen bond. Hydrogen bonds are fairly strong, up to around a tenth as strong as some of the covalent bonds. A hydrogen bond even has some character of the hydrogen atom being shared between the two heavy atoms. In other words, if a hydroxy group is hydrogen bonded to a nitrogen atom, then while the atoms are mostly in the configuration of OH···N, they do spend a little bit of time in the configuration O···HN.

Hydrogen bonds between water molecules are what keeps water liquid at standard temperature and pressure. They are also the reason why methanol through butanol and methanoic through butyric acids are all liquids when methane through butane are gases. Hydrogen bonds between nitrogen atoms are not as strong as between oxygens, since nitrogen is less electronegative than oxygen, but hydrogen bonding ensures ammonia's boiling point is –33.34°C compared to its heavier analogue phosphine (PH₃) with a –87.7°C boiling point.

Recent studies show that even C-H bonds are slightly polar and can form weak hydrogen bonds. However, it's not enough to have noticeable effects under ordinary conditions.

Molecules themselves can be polar or nonpolar, even if their bonds are polar. The C=O bond is polar, but carbon dioxide is nonpolar because the two oxygens are on opposite sides of the carbon. The molecule doesn't have a positive end and a negative end. Water is polar because its hydrogen atoms are less than 180° apart, forming a positive side between them and leaving a negative side where they're absent. Methane is nonpolar because its hydrogens are evenly spaced. Sulfur dioxide is polar because, unlike carbon dioxide, there's a kink in the molecule and the oxygens are less than 180° apart.

Polar molecules are more likely to be soluble in water. Alkanes are insoluble, as are benzene, hydrogen sulfide, and longer chain alcohols and acids. Methanol, ethanol, isopropanol, and t-butanol are all soluble. Carbon dioxide is slightly soluble, but it reversibly reacts to form carbonic acid, H₂CO₃ or HO-C(=O)-OH. Sulfur dioxide is very soluble, but it reacts to form sulfurous acid, H₂SO₃. Oil cannot mix with water, because water molecules will always exert stronger attractive forces on each other than the oil molecules can overcome, so the water stays together and excludes the oil from being able to disperse. Equally, water cannot dissolve in oil because it sticks too strongly to itself and can't disperse into the oil. Substances that prefer to associate with water are called hydrophilic, while those that prefer oils are called hydrophobic.

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