4.2 The chemistry of water

The microscopic structure of water (ESBMS)

In many ways, water behaves very differently from other compounds. These properties are directly related to the microscopic structure of water, and more specifically to the shape of the molecule and its polar nature, and to the intermolecular forces that hold water molecules together.

In the previous chapter you learnt about molecular shape and polarity. Water has two hydrogen atoms around a central oxygen atom. The central oxygen atom also has two lone pairs of electrons. This gives water a bent (or angular) shape. It also means that water is polar since the two hydrogen atoms are not parallel to each other and so do not cancel out the bond polarity (refer back to the previous chapter on molecular shape). We can see this in the following image:

Water molecules are held together by hydrogen bonds. Hydrogen bonds are a much stronger type of intermolecular force than those found in many other substances, and this affects the properties of water.

The unique properties of water (ESBMT)

We will now look at a few of the properties of water.

1. Specific heat

   Specific heat

   Specific heat is the amount of heat energy that is needed to increase the temperature of a unit mass of a substance by one degree.

   Water has a high specific heat, meaning that a lot of energy must be absorbed by water before its temperature changes.

   You have probably observed this phenomenon if you have boiled water in a pot on the stove. The metal of the pot heats up very quickly, and can burn your fingers if you touch it, while the water may take several minutes before its temperature increases even slightly. How can we explain this in terms of hydrogen bonding? Remember that increasing the temperature of a substance means that its particles will move more quickly. However, before they can move faster, the intermolecular forces between them must be disrupted. In the case of water, these forces are strong hydrogen bonds, and so a lot of energy is needed just to break these, before the particles can start moving further apart.

2. Absorption of infra-red radiation

   Water is able to absorb infra-red radiation (heat) from the sun. As a result of this, the oceans and other water bodies act as heat reservoirs, and are able to help moderate the Earth's climate.

   It is the high specific heat of water and its ability to absorb infra-red radiation that allows water to regulate the climate. Towns close to the sea often have less extreme temperatures than inland towns due to the oceans ability to absorb the heat.

3. Melting point and boiling point

   The melting point of water is \(\text{0}\) \(\text{℃}\) and its boiling point is \(\text{100}\) \(\text{℃}\) (at standard pressure or \(\text{0,987}\) \(\text{atm}\)). This large difference between the melting and boiling point is very important because it means that water can exist as a liquid over a large range of temperatures. (This temperature range is only large in the world around us, if we look at space and the universe then this is a very narrow temperature range.)

   When the boiling point of water is measured at sea level (e.g. towns like Cape Town and Durban), it is often very close to \(\text{100}\) \(\text{℃}\) since the atmospheric pressure is almost the same as the standard pressure. If you measure the boiling point of water in a town at a higher altitudes (e.g. Johannesburg or Polokwane) it will have a slightly lower boiling point.

   In grade \(\text{10}\) you studied the heating and cooling curve of water. This is given below.

   

4. High heat of vaporisation

   Heat of vaporisation

   Heat of vaporisation is the energy that is needed to change a given quantity of a substance into a gas.

   The strength of the hydrogen bonds between water molecules also means that it has a high heat of vaporisation. “Heat of vaporisation” is the heat energy that is needed to change water from the liquid to the gas phase. Because the forces between molecules are strong, water has to be heated to \(\text{100}\) \(\text{℃}\) before it changes phase. At this temperature, the molecules have enough energy to break the intermolecular forces that hold the molecules together. The heat of vaporisation for water is \(\text{40,65}\) \(\text{kJ·mol$^{-1}$}\).

   It is very important for life on earth that water does have a high heat of vaporisation. Can you imagine what a problem it would be if water's heat of vaporisation was much lower? All the water that makes up the cells in our bodies would evaporate and most of the water on earth would no longer be able to exist as a liquid!

5. Less dense solid phase

   Another unusual property of water is that its solid phase (ice) is less dense than its liquid phase. You can observe this if you put ice into a glass of water. The ice doesn't sink to the bottom of the glass, but floats on top of the liquid. This phenomenon is also related to the hydrogen bonds between water molecules. While other materials contract when they solidify, water expands. The ability of ice to float as it solidifies is a very important factor in the environment. If ice sank, then eventually all ponds, lakes, and even the oceans would freeze solid as soon as temperatures dropped below freezing, making life as we know it impossible on Earth. During summer, only the upper few metres of the ocean would thaw. Instead, when a deep body of water cools, the floating ice insulates the liquid water below, preventing it from freezing and allowing life to exist under the frozen surface.

   Antarctica, the “frozen continent”, has one of the world's largest and deepest freshwater lakes. And this lake is hidden beneath \(\text{4}\) \(\text{km}\) of ice! Lake Vostok is \(\text{200}\) \(\text{km}\) long and \(\text{50}\) \(\text{km}\) wide. The thick, glacial blanket of ice acts as an insulator, preventing the water from freezing.

It should be clear now, that water is an amazing compound, and that without its unique properties, life on Earth would definitely not be possible.