Hydrogen bonding is crucial to life on Earth and without it DNA wouldn’t form a double helix, ice would sink and water would boil below 0o C. What is the science behind this strange phenomenon and what other properties of water does it influence?
What is a hydrogen bond?
Hydrogen bonds are a type of intermolecular force (attraction between molecules) which occurs in polar molecules containing water and either oxygen, fluorine or nitrogen.
The bond is produced due to the difference in electronegativity between hydrogen and O, F or N. Electronegativity is the ability of an atom to attract an electron in a bonded pair, if it is highly electronegative, it will gain a slight negative charge and the other bonded atom becomes slightly positively charged.
Hydrogen bonds are the strongest intermolecular force and occur so strongly because hydrogen is much less electronegative.
Influence on water properties
Density of Ice:
Water is one of the only chemicals where its solid form is less dense than the liquid form, meaning ice floats on water. This happens because when in solid form, the water molecules arrange into a ordered lattice structure, with hydrogen bonds forming between molecules. When it melts, the hydrogen bonds collapse and the molecules are able to move closer together.
Ice floating may not seem important until you consider how this means water freezes. If ice was more dense, lakes and rivers would freeze from the bottom up to the surface, killing all animals living in the water. A layer of ice forming above the water keeps the rest more insulated and therefore helps water-based life to stay alive during winter. Also, if ice sank in water the sea levels would be much higher.
High melting and boiling points:
Hydrogen bonds hold molecules together very strongly, requiring lots of energy to overcome. This means that in order to overcome these forces, higher temperatures are required. Other molecules of similar size, for example carbon dioxide, have much lower boiling points so are gas at room temperature. Without hydrogen bonding, the oceans would all evaporate, the boiling point of water would be about -80o C.
Water has high surface tension, a property arising due to hydrogen bonding. This means the surface of the water doesn’t break as easily, meaning that a glass of water can actually be overfilled slightly. It occurs because the water molecules on the surface are attracted more strongly to one another – producing a stronger top layer.
Small insects are therefore able to walk on the surface of water, because their small weight is not enough to penetrate the top layer of the water. Surface tension is also the reason for water producing spherical droplets.