Water: A Versatile Solvent | Properties | Conservation | ScienceMonk

Water is the only substance that exists on Earth naturally in the form of all the three physical states of matter- gas, liquid and solid. About 70 percent of the Earth’s surface is water-covered, and the oceans hold about 97 percent of it, a necessary molecule for the survival of life.

Introduction

Water is an inorganic, transparent, tasteless, odourless, and nearly colourless chemical substance, which is the main constituent of Earth’s hydrosphere and the fluids of all known living organisms. It is vital for all known forms of life, even though it provides no calories or organic nutrients. Its chemical formula is H2O, meaning that each of its molecules contains one oxygen and two hydrogen atoms, connected by covalent bonds.

Covalent bonding in the water molecule.
Figure 1:  Covalent bonding in the water molecule.

Water is most often taken for granted as a common and ordinary substance especially from the common people point of view, however, if one can look into its unique and unusual properties in deep definitely end up with lots of interesting outcomes. Many people may not know that without its existence of life on the face of the Earth would not exist; this shows the importance of water as a versatile substance.

According to the great philosopher, Thales of Miletus (640-546 B.C.) it was the only natural substances present on the early Earth and from which everything kicked off including complex chemical reactions for the evolution of life. Indeed he was so right because modern research also highlights the importance of water-molecule for the evolution of life. It is the mediator of chemical and biochemical processes.

Water Resources:

Water is the only substance that exists on Earth naturally in the form of all the three physical states of matter- gas, liquid and solid. About 70 percent of the Earth’s surface is water-covered, and the oceans hold about 97 percent of it, a necessary molecule for the survival of life. The total supply of the world is distributed in the five parts of the hydrological cycle. Among these parts, the largest portion is found in the oceans. Another fraction is found as vapour in the atmosphere.

 

A fraction of approximately 2% is found in the solid-state as ice, glacier and pollen ice-caps. Surface water is found in lakes, rivers and streams. Groundwater is locked in the pore spaces of soil and rocks. It is to be noted that only 1% is on the surface and groundwater is found as freshwater on Earth.

Distribution of water content on the Earth
Figure 2:  Distribution of water content on the Earth

The consumption of water by a human is basically in the form of fresh surface and groundwater. But in arid regions, a severe lack is observed. So in order to distribute water supply, a small fraction of it is taken from ocean sources.

It is the most precious resources in the world but unfortunately, this resource is now under major stress around the globe. The total amount of water on Earth is fixed or constant and cannot be increased. It is repeatedly being recycled through the hydrological cycle. However, many regions of the United States and other parts of the world are experiencing severe shortness due to the uneven distribution of rainfall and unnecessary heavy use of water.

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The Unique Properties of Water:

Water covers about 70% of the planet; therefore it is so common to assume that it is a typical liquid, much like any other. But in fact, virtually every physical and chemical property is unusual when compared with other liquids and these differences are essential to life as we know it.

So overall, it is that precious liquid that possesses many unusual and even unique properties also. Some of them are discussed in the below section.

  • Water and Hydrogen Bonding:

Most of its extraordinary properties are due to the presence of hydrogen bonding in a water molecule. Though hydrogen bonding is much weaker compared to ionic or covalent bonding, still it has a pronounced effect on its physical properties in both of its states (liquid and solid).

A single molecule can form four hydrogen bonds. The oxygen atom in the molecule can be bonded to two hydrogen atoms of other water molecules. This is due to the presence of two pairs of unshared electron. Similarly, each hydrogen atom in a water molecule can bond to an oxygen atom of another molecule.

These bonds are directional in nature i.e. they can be formed only when the molecules are appropriately oriented to each other. In the liquid state, the molecules have constant motion leading to form and break the hydrogen bonds continuously. In this state, not all possible hydrogen bonds are formed due to the random arrangement of the molecules.

In the case of a solid form(ice), the picture is totally different. Here, the molecular motion is at a minimum level which led the molecule to be oriented in such a way that a maximum number of hydrogen bonds could be formed.

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Under this circumstance, this results in an ordered, extended, strongly bonded three-dimensional open framework lattice structure. The bond angle in the water molecules determines how close the adjacent molecules can be. The bond angle, in turn, dictates the size of the holes in the lattice. Due to the presence of these holes adjacent molecule is not close enough to each other as they are in the case of the liquid state. This explains why ice is less dense than water and floats above the water surface.

Distribution of water content on the Earth
Figure 3: Three-dimensional structure of the water-molecule
  • Density:

Water is the only common substances that expand when it freezes maximum during in found at 4˚C, which means that for temperatures above and below the point, it continuously becomes lighter and more buoyant. As a result, ice floats. If it did not, ice that would form on the surface of water-bodies would sink to the bottom making it possible for lakes to freeze too solid. The expansion of water as it freezes also contributes to the weathering of rocks by literally breaking them apart when water freezes in the cracks.

  • Melting & Boiling Points:

If water were similar to other substances (such as H2S, H2Se and H2Te) it would boil at normal earth temperature, thus existing mostly as a gas rather than a liquid or solid. It also has an unusually high difference in melting point (0oC) and boiling point (100oC), thus remaining a liquid over most of the globe.

That means if this difference is not present then water would not be a liquid over much of the surface of the Earth where temperatures are relatively high and we would not have an ocean.

It is well known that the heat of fusion of water is around 80 Cal/g. This means that this much energy in the form of heat is needed to break the strongly bounded hydrogen bonds in the three-dimensional structure of ice.

Similarly, the heat of vaporization of water is almost 540 Cal/g, which indicates that this much additional energy is required to break the hydrogen bonds for converting the water to vapour form. So from the above explanation, it is clear that why it possesses an exceptionally high difference in melting and boiling point.

Comparison of boiling points
Figure 4: Comparison of boiling points
  • Specific Heat:

The specific heat of water (4184 J/Kg˚C) is higher than it is for any other known liquids except ammonia. It is five times higher than the specific heat of most common heavy solids, such as rock and concrete. This means it heats and cools slower than almost anything else. This helps moderate climate near large body’s water, and it also serves the important function if protecting life from rapid thermal fluctuations, which are often lethal.

  • The Heat of Vaporization:

The heat required to vaporize water (2258 KJ/kg) is one the highest of all liquids. This high heat of vaporization means that vapour stores an unusually large amount of energy, the energy that is released when the vapour condenses. This property is important in distributing heat from one place on the globe to another and is a major factor affecting the Earth’s climate.

  • Water as a Solvent:

It is an excellent solvent for a variety of substances including many ionic compounds such as acids, bases and salts. Some gases also dissolve well in it. Many biologically important compounds including sugars are also soluble. As a result, it serves as an effective medium for both transporting dissolved nutrients to tissues and organs in living things as well as eliminating their wastes.

 

The solvent property of water can be understood by considering the structure and bonding of the molecule. As we know that in water-molecule dipole moment is arising due to the opposite charges on both hydrogen and oxygen end. So salts like NaCl can easily be dissolved in it through ion-dipole interaction, which is shown in the following figure,

NaCl in water demonstrating Ion-dipole interaction
Figure 5: NaCl in water demonstrating Ion-dipole interaction

Water for Survival of Life on Earth:

From the above discussion of these unusual properties, it is quite clear that the above-said properties add an important contribution to the survival of life on the Earth. Apart from those above properties, some other aspects of this uniqueness, which is necessary for the survival of life on Earth are discussed as follows.

It was proposed that the origin of life proceed through the various chemical reactions in the primaeval sea. The formation of proteins and nucleotides which are the principle building blocks of living cell occurred in the aqueous medium through polymerization reactions.

Not only the formation of these molecules but it also plays an important role in maintaining protein structures by binding to the protein chain at various locations through hydrogen bond which is responsible for holding up the protein chain in their peculiar folded three-dimensional conformations.

Water is vital both as a solvent in which many of the body’s solutes dissolve and as an essential part of many metabolic processes within the body. Metabolism is the sum total of anabolism and catabolism.

In anabolism, water is removed from molecules (through energy requiring enzymatic chemical reactions) in order to grow larger molecules (e.g., starches, triglycerides, and proteins for storage of fuels and information). In catabolism, it is used to break bonds in order to generate smaller molecules (e.g., glucose, fatty acids, and amino acids to be used for fuels for energy use or other purposes). Without water, these particular metabolic processes could not exist.

It is a very well-known fact that plants derive their energy from the reaction of H2O and CO2 to form glucose and oxygen in the photosynthesis process.

Formation of glucose-photosynthesis reaction
Figure 6: Formation of glucose-photosynthesis reaction

With the exception of mercury, water has the highest surface tension of any other liquid. Surface tension is the attractive force exerted by the molecules below the surface on those at the liquid-air interface.

This inward force tends to restrain the liquid from flowing. Polar compounds tend to have much higher surface tension than non-polar compounds. Hydrogen bonding of water molecules is attributed to the exceptionally high surface tension of water.

Due to the larger surface tension (73 dyne/cm at 20 and hydrogen bonding it has the ability to wet surface, is the basis for the capillary action which is responsible for the movement of water through soils, blood through blood vessels, and water carrying nutrients through the roots of the plants to the leaves.

Surface tension and Capillary action
Figure 7: Surface tension and Capillary action

As we have already mentioned earlier that water has a maximum density at 4 and over and below 4, it starts expanding. During the winter season in colder countries outside or atmospheric temperature is very low, it is dropping below freezing point. In these conditions, the upper layer in lakes and ponds starts cooling.

When the temperature of the surface layer falls to 4 the water body acquires maximum density and sinks down. The water that sinks down displaces water below and the lower layer of water simultaneously rises up. This also gets cooled to 4 and again sinks down.

When the temperature finally goes below 4 the density decreases and as a result water does not sink down. The surface finally freezes at 0, while the lower part still remains at 4. The floating ice (insulator) does not allow heat to pass through it easily, so the freezing of the water below is a very slow process and in this way aquatic life is survived.

Survival of aquatic life under extreme cold weather
Figure 8: Survival of aquatic life under extreme cold weather

Water Shortages and Conservation:

About 50 years ago, when the population on the Earth was half of the current population, at that time there was an infinite number of sources of water on Earth. But, today due to increased population the sources are decreasing rapidly and results in severe competition for water sources.

There are mainly two sources of water that can be used for human consumption and other activities. The first source is the surface water that can be available in lakes, rivers, ponds and streams. Another source is groundwater that can be obtained from aquifers. Worldwide, 70% of the water is used for irrigation and 22% is used for industrial purpose and the remaining 8% is for domestic use.

It is observed that the freshwater supply is decreased because many aquifers are over-pumped and the rate of recharge is slow down. The total amount of freshwater is also decreasing due to climate change, which in turn has caused receding glaciers, reduced stream and river flow, and shrinking of lakes. Conservation of water is necessary if serious shortages of water are to be avoided.

There are plenty of ways by which it can be conserved. The popular method of spraying water over the land by lawn sprinklers is not a suitable method because in this method so much water is evaporated before it can enter into the soil and reach plant roots.

From the above discussion, it is clear that a major portion is being used for agricultural purpose throughout the globe. So in order to conserve the water, farmers need to use efficient irrigation methods such as the use of soil moisture monitoring or climate information device to determine when they should start irrigation. Another useful device in this context is a water flow meter that can help to measure and control the amount of water being used in irrigation.

Recycling wastewater is another common method for water conservation. It can be conserved by recycling the wastewater from different home sources such as kitchen sinks, laundry tubs and further using it for purposes such as flushing toilets and watering lawns. Not only it can be recycled, but it can also be stored in the form of rainwater and using it in various other activities that do not require drinking-quality water.

One convenient way to increase the supply of freshwater is to employ the desalination process. This process is nothing but the removal of salts from seawater and brackish water and thereby makes it suitable for human consumption. This technology could be used in the arid region as their people often suffer from a lack of fresh water.

But in practice, this technology is used in only regions where there is no alternative source of freshwater. This is due to the fact that the installation of such kind of desalination plant is very expensive as it requires a huge amount of energy.

This process has another drawback of disposing of mountains of salts which is produced as a byproduct of desalination. The salt could be returned to the ocean near a cost, thereby increasing salt content could impose an adverse effect on wildlife in coastal-water.

Conclusion:

God has gifted this unique solvent for the whole of humanity. The survival of life is impossible without this molecule as described in detail in this article but unfortunately, the importance of this molecule is still not taken as seriously as it should be and that leads to misuse of this solvent at a mass level.

If people are not made aware of this or knowing also if they themselves are not showing interest to conserve this precious solvent and stop misusing or overusing, the day is not so far when human civilization will be completely demolished due to lack of water.

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