The Hydrogen Bond

Water’s chemical formula is H₂O. In the water molecule, the oxygen atom forms one bond with each of the two hydrogen atoms. Chemists call these bonds covalent bonds, which consist of a pair of shared electrons. The oxygen nucleus is bigger and pulls on those electrons harder than the hydrogen nucleus, so those electrons shift closer to the oxygen nucleus. This makes the oxygen side of the bond slightly negative and the hydrogen side of the bond slightly positive, like a little magnet. When two water molecules come together, the negative side of one molecule’s H-O bond is attracted to the positive side of another molecule's H-O bond. This intermolecular attractive force is the hydrogen bond. It is responsible for the unique chemical and physical properties of water.

In liquid water, the angle between the two H-O bonds is 104.5 degrees. As temperature drops, the water molecules become less restless and form more hydrogen bonds with each other. When water freezes, the molecules settle into a crystalline structure and snap into a grid, where the angle between the two H-O bonds widens to 109 degrees. Each water molecule occupies more space in ice than in liquid water. That’s why water expands when it freezes.

A polar covalent bond doesn’t form every time a hydrogen atom shows up. For instance, the C-H bond prevalent in organic compounds is not considered polar; therefore, fat molecules don’t form hydrogen bonds with each other or with water. That’s why oil does not dissolve in water.

But water is actually a very powerful solvent. For a solute (the substance to be dissolved) to dissolve in a solvent, the solvent molecules have to attach to the solute molecules, wrap them up, and pull them away from other molecules of the solute. In the case of salt, because the bond between the sodium and chloride atoms is an ionic bond (they don’t share a pair of electrons; the sodium atom has completely given up an electron to the chloride atom), the little magnets in water molecules tear the sodium ion away from the chloride ion. They exist as separate ions in the solution. In the case of sugar, the sucrose molecules don’t break apart, but they do form intermolecular bonds with the water molecules. Individual sucrose molecules are pulled away from each other by the water molecules that swarm them and dissolve into water.

It takes energy to break the existing bonds among sucrose molecules and the hydrogen bonds among water molecules. As temperature rises, more energy means more sugar dissolves in water. Sugar solutions of different concentrations, when cooled, result in different candy textures. The candy thermometer is a convenient tool to indirectly measure the sugar concentration by measuring the temperature of the syrup.