Polarity and water

Water (18 g/mol) boils at 100 °C. Methane (16 g/mol), almost as heavy, boils at about −161.5 °C.

Rub a plastic comb on your hair and hold it next to a thin stream of tap water. The stream bends towards the comb. Try the same with a stream of cooking oil and very little happens. Water molecules have a positive end and a negative end, and the charged comb pulls on them. That lopsided charge is called polarity, and it explains much of why water behaves so differently from other small molecules.

Polar bonds

In a covalent bond between two different atoms, the more electronegative atom pulls the shared pair closer. Oxygen's electronegativity is 3.44 and hydrogen's is 2.20, so in each O-H bond the electrons spend more time near oxygen. Oxygen carries a small negative charge, written δ−, and each hydrogen a small positive charge, δ+. The bond is polar. Bonds between identical atoms, as in H₂, O₂ or Cl₂, have a difference of zero and are nonpolar. A C-H bond, with a difference of 0.35, is usually treated as nearly nonpolar.

Shape decides whether a molecule is polar

A molecule can have polar bonds and still be nonpolar overall. Carbon dioxide has two very polar C=O bonds, but the molecule is a straight line, so the two pulls point in exactly opposite directions and cancel, like two equal teams in a tug of war. Methane's four slightly polar bonds point to the corners of a tetrahedron and cancel too. Water is bent. Its two O-H pulls both point partly towards oxygen and do not cancel, so the whole molecule has a negative side at oxygen and a positive side between the hydrogens. Ammonia, pyramidal, is polar for the same reason.

Hydrogen bonds

The δ+ hydrogen of one water molecule is attracted to a lone pair on the oxygen of a neighbour. This attraction between molecules is called a hydrogen bond. It is much weaker than the covalent O-H bond inside the molecule, but each water molecule can make several, and together they hold liquid water together firmly. To boil water you have to break this network, which is why water boils at 100 °C while methane, of nearly the same mass but nonpolar, boils at about −161.5 °C.

Hydrogen bonds also explain why ice floats. When water freezes, each molecule locks into four hydrogen bonds arranged around the tetrahedral directions of oxygen, which builds an open framework with gaps. Ice is therefore less dense than liquid water, about 0.92 g/cm³ against about 1.00 g/cm³. Lakes freeze from the top down, and the ice layer insulates the water below.

Polarity also decides what dissolves in what. Polar water dissolves salt and sugar, whose ions and polar groups are attracted to water molecules. Oil is made of long chains of carbon and hydrogen, which are nearly nonpolar, so water molecules stick to each other rather than to the oil, and the two separate. Chemists sum this up as "like dissolves like".

Electronegativity differences give a rough ladder from one kind of bond to the next. A difference of zero, as in Cl-Cl, is a pure covalent bond. Small differences, like the 0.35 of C-H, are nearly nonpolar. Medium differences, such as 1.24 for O-H, make polar covalent bonds. At about 2 and above, as with 2.23 for Na-Cl, the electron effectively changes hands and the bond is called ionic. Bonds slide along this ladder rather than jumping between boxes, so labels near the boundaries are partly a matter of convention.