Water is Unique

So how does hydrogen bonding cause water’s uniqueness?

The presence of hydrogen bonds means that more energy is required to convert ice to water and water to steam than for other molecules of a similar size (e.g. methane). So water has a higher melting point and boiling point. This is good for us as it means that water (the liquid) is available at most of Earth’s ambient temperatures.

Another effect of hydrogen bonding is the density of ice versus water. We are all familiar with the ice floating on the top of a drink such as water or coke or scotch. This happens because ice is less dense than water. That means there is less weight per unit of volume. In other words, 1 ml of ice weighs less than 1 ml of water. What would happen if, as for many compounds, the solid form of water was denser than its liquid form? Not only would it mean that we would have ice sinking to the bottom of our drinks, we also wouldn’t have ice fishing. In fact, lakes and ponds would freeze solid as the ice would sink to the bottom and the top surface keep freezing. What happens now is that the layer of ice on the lake surface insulates the lower reaches of a lake. The water below is at 4 oC which is when water is densest. This enables fish and other water creatures to stay alive in a cold lake.

So how does hydrogen bonding cause this change in density? As water freezes, it expands as the hydrogen bonds cause each water molecule to be pushed away from each of its neighboring molecules. Or a better way to look at is that the hydrogen bonds pull the liquid water molecules towards each other when the solid ice melts.

Another important property of water is its high surface tension. The molecules in water are tightly attached and attracted to each because of their polar nature. This means that the surface of liquid water acts like a film. One way to experience this is to take a bowl of water and press down on the surface with your hand. You can feel the water resisting your hand. This is important as it allows water skaters to skim across the surface of a pond or allows for the capillary action of water which is important for plants as this is how water and nutrients move up from the roots to the leaves.

Water is also a good solvent, so much so that it is sometimes called the universal solvent. This is more due to its polar nature, but hydrogen bonds play a role here too. The polar nature is important as the partially negative charged oxygen portion of the water molecular surrounds a positive charged ion and vice verse for a negative charged ion. This is shown in the picture for table salt, sodium (Na+) chloride (Cl):

Salt in water


For larger molecules such as glucose, water forms hydrogen bonds with hydrogen of the oxygen and hydrogen (hydroxyl (OH)) side groups. Glucose and related compounds are more soluble in water than would be expected for a molecule of a similar size without hydroxyl groups.

glucose in water

Many reactions, in both biology and chemistry, take place in water – known as the aqueous phase. It is important that compounds are soluble in water so that these reactions can occur. It might mess up biology if a reaction took place and the product wasn’t water soluble.


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