What are the Properties of Water?
Water is everywhere. Around 70% of the Earth’s surface is covered by water. The human body is composed of 65% water. Water is such a constant presence in our daily lives that, odds are, you probably don’t think about it all that often.
Despite its mundane appearance, water is in fact incredibly unique in the chemical world, and its uncommon features are exactly what makes it so essential to living organisms like us!
Water’s special properties are primarily due to its highly polar makeup (it has a positive and negative side) and the fact that it readily forms hydrogen bonds with other water molecules. For example, water has a very high boiling point for its molecular weight. This is because all the hydrogen bonds linking water molecules together means that more energy, or heat, is required to break them apart.
Hydrogen bonds make water relatively slow to boil, freeze, or change temperature in general. This is important to organisms that live in and around water, which could be killed by sudden dramatic swings in temperature. Hydrogen bonds are also what allow us to cool ourselves down by sweating; when sweat evaporates from our skin it releases the energy held in the hydrogen bonds in the form of heat.
Water Strider. (Analogicus, 2018)
Water has very strong adhesion (attraction to different molecules) to other polar molecules as well as strong cohesion (attraction to the same molecule) to fellow water molecules. Have you ever seen a water strider moving across the surface of a pond without sinking? This is possible because of something called “surface tension”, which is caused by the strong cohesion of water molecules to each other.
In order for an object to sink, the water molecules would have to move away from each other and the hydrogen bonds between them would have to be broken. Small, light organisms like water striders aren’t heavy enough to break those hydrogen bonds. They hydrogen bonds between water molecules are the reason that water has the highest surface tension of any liquid substance except mercury.
All substances become denser when they are cooled, which means that for the vast majority of substances the solid state sinks in the liquid state. Yet ice floats on top of liquid water, seemingly breaking the laws of physics.
This is because, as water molecules cool and slow down, it becomes easier for hydrogen bonds to form between them. As hydrogen bonds form between all the water molecules, they eventually arrange themselves into a crystalline structure. The arrangement of bonded water molecules end to end holds them apart from each other, so the volume of the water increases and the resulting ice becomes less dense.
Without water’s rare properties, and the hydrogen bonds that produce them, life on earth would not be able to exist.
Analogicus (photographer). (2018). Nature Water Strider Gerridae [photography]. Retrieved from https://pixabay.com/photos/nature-water-striders-gerridae-3339980/