Water

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Introduction

Water is an inorganic compound essential for sustenance of all forms of life. It is the predominant compound present in all living systems. It is present in all living cells in different proportions ranging from 70% to 95%. The human body is made up of 70% water that is distributed differently among different types of cells. For example, our brain cells have 85% water while bone cells have only 20%. Water is not limited to living systems only, as it covers more than 71% of our earth.

The unique physical and chemical properties of water make it the most suitable molecule for all life forms. In this article, we will discuss some important properties of water and their role in supporting different forms of life.

Structure

Structure of Water

All the physical and chemical properties of water depend on its chemical structure. Water is a molecule made up of one oxygen atom and two hydrogen atoms. The two hydrogen atoms are linked to the same oxygen atom via covalent bonds. The term water is used for the liquid state of H2O, the solid-state being called ice or snow and gaseous state vapors. 

The oxygen atom present in a water molecule has high electron affinity and electronegativity. It tends to pull the electrons of hydrogen towards itself. As a result, the water molecule becomes a dipole in which the negative charges are centered around oxygen making it a negative pole and the protons of hydrogen atoms make them positive poles. Oxygen in water molecules have a partial negative charge while hydrogen atoms carry partial positive charges.  

The angle between oxygen and hydrogen in water molecule is 104.5o. The high electronegativity of oxygen and the presence of two lone pairs in water molecule makes it ideal for forming hydrogen bonds. Thus, all H2O molecules in water are bound together via hydrogen bonding. One molecule of water overall carries two lone pairs and two hydrogen atoms. Thus, four hydrogen bonds are formed per water molecule. 

General Physical Properties

Here are some of the important general physical properties of water.

States of Water

As mentioned earlier, the term water is used for the liquid state of H2O. This liquid state of water exists at the range of temperature and pressure that is most suitable for the existence of life. At the normal atmospheric pressure at sea level, the liquid state of water exists between 0 to 100-degree Celsius temperature. 

At or below 0oC, it changes into solid-state called ice as it is the freezing point of water. 

At or above 100oC, it converts into its gaseous state, the water vapors. At normal atmospheric pressure, the boiling point of water is 100oC.

The temperature range changes with the change in external atmospheric pressure. The boiling point decreases with a decrease in pressure and vice versa. This is the reason why water boils earlier on mountains where atmospheric pressure is very low.

Tasteless and Odorless

Water is regarded as a tasteless and odorless compound in its purest form. The taste you feel while drinking water is not of pure water but is due to the substances that have been added to it during mineral processing. The taste one feels while drinking groundwater is also due to the minerals that get added to it from the surrounding environment. 

Although it is an odorless compound, humans and other organisms with a sense of smell can detect whether water is potable or from the smell coming out of it. water with a large amount of biological matter or salts gives a characteristic smell. A foul smell also starts coming from a long stand8ing water due to biological matter in it. 

Color and Appearance

Pure water appears blue when viewed in daylight against a pure white background due to the absorption of red light. However, natural water bodies may appear green due to the presence of algae or other suspended solid particles. 

Solvent Properties

Water is an ideal biologic solvent because of its unique chemical structure. It is an excellent solvent for a wide range of organic as well as inorganic substances. The great solvent properties of water are due to its ability to form dipoles and hydrogen bonds. 

Charged substances in Water

When a charged substance is added to water it immediately breaks up into positive and negative ions. The water molecule tends to attract the individual ions towards their partial positive and partial negative poles. The forces of attraction among the ions in the substance are overcome by water-ion forces, the ions separate and get surrounded by water molecules. Water molecules form a hydration shell around each ion. In a hydration shell around a positive ion, a single ion is surrounded by various water molecules with oxygen atoms, carrying a partial negative charge, directed towards the ion while the hydrogen atoms directed outwards. The opposite is true for negative ions. In this way, ions are kept separate by hydration shells and they remain dissolved in water.

For example, if NaCl crystal is added to water, the ionic forces between Na+ and Cl are immediately overcome by the attraction of water molecules. As a result, Na+ and Cl separate, hydration shells are formed around individual ions and they are kept dissolved. 

Polar substances in Water

Polar substances also carry partial positive and partial negative charges just like water molecules. When a polar substance is added to water, forces of attraction develop between its dipoles and dipoles of water. As a result, water molecules form hydration shells around individual molecules just like they form around the ionic species. The only difference is that ionic compounds are broken down into individual ions while in the case of non-ionic polar substances, hydration shells are formed around the entire molecule keeping its covalent bonds intact. 

Non-Polar Compounds

Unfortunately, water proves to be a poor solvent for non-ionic, non-polar compounds due to its polarity. Such compounds do not dissolve in water. When non-polar substances are added to water, they clump together to form insoluble particles that keep floating in water. 

Amphipathic compounds

These are the compounds having both polar and non-polar parts, for example, phospholipids, cholesterol, etc. when such substances are added to water, the polar part of compound dissolves while the non-polar part remains undissolved. 

Hydrogen bonding also contributes to dissolving various substances in water. The compounds having functional groups with one or more hydrogen atoms bound to an electronegative atom are effectively dissolved in water due to polarity and the ability to form hydrogen bonds. Examples of such compounds include alcohols, carboxylic acids, amines, amino acids, etc. 

Importance

Because of these properties, all the chemical reactions in a cell take place in an aqueous environment. Enzymes that catalyze all the chemical reactions work only in the presence of water. The insolubility of fats, lipids and other non-polar substances in water maintains the membrane structures and is necessary for making compartments within a cell. 

Heat capacity

Heat capacity of a substance is its ability to absorb heat without much increase in its temperature. It is measured in terms of specific heat capacity defined as the amount of heat required in calories to raise the temperature of 1g of a substance by 1 oC. 

Recall that the temperature of a substance is a measure of the ability of its molecules to undergo random motion. As the temperature increases, the kinetic energy of the molecules increases, so does there random movements. Water has high heat capacity owning to the strong hydrogen bonds among its molecules. A tremendous amount of energy is required to break these bonds and render the water molecules free. Thus, water can absorb a large amount of heat with a minimum increase in its temperature. The specific heat capacity of water is 1 cal. or 4.18 joules. This is the maximum specific heat capacity of any liquid known to man. That is why water takes too long to heat.

Importance

It is evident from the above discussion that water can resist external temperature changes without any significant change in its own temperature. This property of water allows it to act as an efficient temperature stabilizer for living organisms as they are mainly made up of water (70 to 95%  body is made of water). It protects living organisms from drastic and rapid changes in the external environment. This is the reason why your body’s temperature does not rise while playing cricket in hot weather. 

The heat capacity of water is 5 times more than that of sand, making the aquatic environment a better habitat than land.

Heat of Vaporization

It is the amount of heat required to convert a liquid into its vapors. It is measured in terms of specific heat of vaporization defined as the amount of energy in calories required to convert one mole of a liquid into its gaseous state while the temperature remains constant. 

A liquid is converted into vapors when the kinetic energy of its molecule becomes greater than the attractive forces causing them to leave from the surface of liquid. Energy from external sources must be provided in the form of heat to serve this purpose. Water has a high heat of vaporization due to strong hydrogen bonds among its molecules. a large amount of energy is needed by water molecules to break the hydrogen bonds and leave from the surface of water. The specific heat of vaporization of water is around 586 calories/mole. 

Importance

This high heat of vaporization allows water to serve as a heat sink in living organisms. It provides a cooling effect when it is evaporated from living bodies in the form of perspiration in animals and transpiration. It is especially important for animals and plants living in hot habitats.

Water as lubricant

It is one of the primary functions of water in living bodies. It acts as an effective lubricant providing production against damage caused by friction. 

  • It is a major component of synovial fluid, a lubricant in synovial membranes surrounding joints. 
  • It is also present in meninges surrounding the brain, peritoneum surrounding the abdominal organs, pericardium around the heart, and pleural membranes guarding the lungs. In all these structures, water acts as a cushion protecting the vital organs from damage caused by trauma.  
  • As a component of saliva and other secretions of the GIT, it acts as a lubricant that helps in forward propulsion of food. 
  • Water in the form of tears forms a protective layer in the eyes protecting the cornea from friction caused by rubbing of eyelids.

Water as a Transport Medium

Being an excellent solvent, water acts as an excellent transport medium for ionic and polar compounds. It is the major transport medium of blood for transporting glucose and other nutrients as well as gases. It is also used as a transport vehicle for both intracellular and extracellular transport of molecules. 

Water as Reaction Medium

Recall that all the enzymes require an aqueous medium for proper functioning. All the metabolic reactions, therefore, require water as a reaction medium in which substances can dissolve and react in the presence of enzymes. 

Summary

Water is a molecule of life as it is essential for all the metabolic processes taking place in our body. 

A molecule of water is made up of one oxygen atom to which two hydrogen atoms are attached via covalent bonds making a bond angle of 104.5o

All water molecules are held together via strong hydrogen bonds among them. One water molecule can make a total of 4 hydrogen bonds. 

In its pure form, water is tasteless and odorless liquid that appears blue against a white background. 

Due to its polarity and its ability to form hydrogen, it acts as an excellent solvent for polar as well as ionic substances. This property makes water a universal solvent although it cannot dissolve non-polar substances. 

Because of strong hydrogen bonding, water has two very important properties;

  • High specific heat capacity
  • High heat of vaporization

Because of high heat capacity, it acts as a temperature stabilizer in living bodies. 

The high heat of vaporization makes it a heat sink for living bodies providing cooling effect upon vaporization. 

Water also acts as an excellent lubricant providing a cushion effect to protect vital body organs. 

Water also acts as an ideal transport medium as well as reaction medium in living organisms. 

References

  1. Harper’s Illustrated Biochemistry, 31st Edition
  2. “Water, the Universal Solvent”USGSArchived from the original on 9 July 2017. Retrieved 27 June 2017.
  3. Reece, Jane B. (31 October 2013). Campbell Biology (10 ed.). Pearson. p. 48. ISBN 9780321775658.
  4. Ball, Philip (2008). “Water: Water—an enduring mystery”. Nature. 452 (7185): 291–2. Bibcode:2008Natur.452..291Bdoi:10.1038/452291aPMID 18354466
  5. Kotz, J.C., Treichel, P., & Weaver, G.C. (2005). Chemistry & Chemical Reactivity. Thomson Brooks/Cole. ISBN 978-0-534-39597-1.