Few will argue the statement that water is the most important substance to exist on planet Earth, as we know it today and to the life residing on it. Even fewer know that water is actually one of the most fascinating substances in the world with a chemical complexity that enables the existence of life. Only a few know that water has very special characteristics, some of which are very strange and surprising.
Water is the most common chemical compound on Earth and covers approximately 70.8 percent of the planet’s surface. A water molecule contains two hydrogen atoms and one oxygen atom, together creating the molecular formula H2O. Water allows the existence of life in this world and serves as a basis for all plant and animal life on Earth. Water makes up 55-78 percent of the human body, and about 90 percent of the blood flowing in our arteries.
Water is so important to our survival - its unique features allow for life, as we know it. The importance is so great that the search for life on other planets throughout space is based on looking for water. The assumption is that water in liquid state allows life, so if water exists on another planet, there is a greater chance something is living there.
Water dripping at very slow motion | Video produced by Strainoff
The states of matter of water
First and foremost, water is the only material found in all three major states within Earth’s natural temperature range. Most of the time it is in the liquid state, but its solid state is no stranger to us in the form of snow in cold regions, a cube of ice in soft drinks or as giant icebergs in the polar regions. Water as a gas is found in steam and clouds as water vapor. The existence of the different physical states and the transition between the states of matter forms the water cycle that shapes Earth and all the life it contains.
When we are dealing with physical states of water, we come across some strange and unique phenomena. The first revelation is the number of states of matter. If you ask almost anyone about water’s several states of matter, the answer is quick and obvious - three states: gas, liquid and solid. The truth is somewhat different; in actual fact water has five different liquid states and at least 14 different solid states, as discovered by scientists to date. If you add the one gaseous state, water has no less than twenty states of matter.
Even if we're simplifying the issue and considering only the three main states of matter - gas, liquid and solid - we will still encounter strange behavior. Two phenomena are particularly interesting in this context.
The first phenomenon is the boiling and melting temperatures of water. Water changes from solid to liquid to gaseous states at relatively high temperatures compared to other similar molecules. If we compare two similar molecules - water (H2O) and hydrogen sulfide (H2S) - water melts into a liquid state at 0°C, while hydrogen sulfide melts at -82°C. Boiling water turns into gas at 100°C, while hydrogen sulfide boils at -60°C. The reason for these large differences will be clarified later.
In addition to the high temperatures that are required for the transition between different states of matter, water as liquid and ice differ from most other substances in regards to another feature - most substances in the liquid state are characterized by a greater freedom of movement with molecules that are relatively far away from each other. With the transition to the solid state the molecules lose their freedom of movement and line up in a more dense and closed structure, resulting in a higher density for their solid state compared to their liquid state.
The story for water is a little different. When cooling water in the liquid form, its density is increased, but only up to 4°C. Continuing to cool the water below this temperature actually decreases its density. So ice is less dense than liquid water, and therefore floats in it.
(Almost) everything dissolves in water
Water is known as the “universal solvent” because a huge variety of substances can be dissolved in them. This dissolving action is witnessed in our every day lives - sugar and caffeine dissolve in water when we make coffee, menthol is dissolved in water when we brew tea with mint, and cooking salt or sodium glutamate is dissolved in water when cooking or preparing soup.
Many other substances also dissolve in water, and many of them are necessary for the survival of both animals and plants. In fact, all materials within our body are transported in a water environment, so it is essential to our daily survival that important substances dissolve in water.
Enzymes, hormones and proteins also dissolve in water and allow for the majority of cellular activities in our body. DNA dissolves in water and allows it to duplicate and be translated into proteins. Many gases, including oxygen, dissolve in water and this dissolved oxygen allows for the process of our breathing as well as the respiration of many sea creatures that live under the water. Even the transfer of electrical signals in the body is done with the help of ions – mostly metals – that are dissolved in water.
Water’s high heat capacity
Simply put, when we talk about heat capacity it means a large amount of energy is required to heat water. This feature ensures the world’s oceans, rivers and lakes remain at much the same temperature throughout the year, despite their exposure to the energy that comes from the sun. In general, water is a major factor in climate balance. Heat capacity of water also allows the human body to easily maintain a fairly constant temperature.
Water molecules “like” to stay together
Water molecules always strive to be next to other water molecules. This desire manifests itself in two very interesting features for water - high surface tension and capillary action (capillarity).
When water is exposed to another medium, such as air, there is a layer of water molecules that are in the area of contact between water and air. This area, called water’s surface, is where water molecules are exposed to air. However, because the water molecules are seeking to be exposed to other water molecules only, those exposed to the air prefer to be exposed to water so they attempt to decrease their exposure to the other material. Thus the surface of the water behaves more as a stretched sheet of cloth determined not to tear.
The high surface tension of water is clearly visible in the video above where a drop of water encounters a sheet of “stretched” water. The drop of water meets the sheet of water below and some of its water molecules are transferred to the water surface. As the molecules from the droplet are transferred to the surface, energy is released, which forces the rest of the droplet to bounce up in the air. In the air, the surface tension forces the water molecules to form a sphere, in which they have the smallest contact area with air. This creates a smaller droplet each time, until all the water molecules are transferred to the sheet of water.
The high surface tension of water allows many insects and amphibians to “walk” across water that should really be sinking them. In the video below you will again see the surface tension of water, but this time with the effect of zero gravity at the space station.
What happens when you wring out a towel in space | Video by NASA
Capillary forces are created in water because their molecules are seeking to stay together. A thin tube placed in water will cause some of water to climb up its sides. With the molecules sticking to each other, they will continue to climb up until the weight of the water is too great. At this point it will stop climbing because the system has reached equilibrium. The capillary is one of the biology world's most important forces – using it, plants are able to “draw” water from the soil and transfer them to all parts of the plant.
So why do these features occur and why are they unique to water?
To understand the source of all these features we need to look at the water molecule.
Water molecules | Illustration from Wikipedia; Created by Qwerter
Oxygen is a relatively large compared to the hydrogen atom, and electrons tend to concentrate around it, so it has a partial negative charge. Hydrogen is a small atom and tends to create a partial positive charge. A hydrogen bond can form between an oxygen molecule and a hydrogen molecule from another water molecule, binding the molecules together.
Hydrogen bonds between molecules help explain the high surface tension and capillary forces based on the “desire” of water molecules to stick together. The numerous hydrogen bonds form a dynamic and branched network of hydrogen bonds among the water molecules. This dynamic network allows it to absorb a lot of energy and explains the high heat capacity of water. The amount of bonds to be broken to free the water molecules explains the high temperatures required for the transition between the different states of matter.
Hydrogen bonds between water molecules also help dissolve organic materials including, among others, those consisting of oxygen or nitrogen atoms. These materials include sugars, proteins, hormones and enzymes. In addition, the presence of a partial negative charge on oxygen and partial positive charge on hydrogen helps dissolve ionic substances in water, such as cooking salt. These properties explain why water is indeed the universal solvent.
We use water constantly – it is an integral part of us and our environment. The culmination of all its rare features and it being such a small and simple molecule has enabled the complex biological diversity on Earth. From now on, whenever you are drinking water, remember above all – water is an incredible chemical substance.