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Chemical principles: Properties of water

Importance of water

 

Water (H2O) is one of the most abundant chemical compounds on earth, covering 70% of the earth's surface, and is essential to all life. After carbon, water is the most crucial substance to sustaining life. For example humans are made up of over 70% water, with the brain being composed of over 80% water. There are a number of properties of water which are particularly important to biological life. The properties are largely the result of the extensive hydrogen bonds between molecules in water. These bonds are strong enough to produce the characteristic properties but are also easily broken to allow water molecules to move around or change between states. 

 

Three forms of water

 

In nature water can naturally occur in three states; it can exist as a liquid (water), a gas (water vapour) and as a solid (ice). At standard temperatures and pressures, water exists in a state of  dynamic equilibrium between it's liquid and gaseous states. Ice, the solid form of water, only exists at relatively low temperatures (below water's freezing point of zero degrees Celsius). 

 

Although under standard conditions water in its liquid state is the most commonly recognised form, it is not, theoretically the predicted state for water to exist in. If water's similarity to other analogous hydrides is considered then water should exist as a gas under standard conditions, and not as a liquid. For example hydrogen sulphide, which is chemically very similar to water, and is also a hydride, does not exist as a liquid under standard conditions, but as a gas. This property is a result of the polarity of water, described below, which leads to hydrogen bonding and strong interactions between water molecules.

 

Solid, liquid and gaseous forms of water

The arrangement of water molecules in solid ice, liquid water and water vapour

Structure of a water molecule

 

A water molecule is composed of a central oxygen atom that has two hydrogen atoms bound to it by covalent bonds. The covalent bonds are the result of a sharing of a pair of electrons between the oxygen and hydrogen atoms. In forming these bonds only two of the six outer electrons on oxygen are used, this leaves the other four electrons free to form covalent bonds with other water molecules. The four electrons that do not bond with the hydrogen atoms on the same water molecule pair up and then position themselves as far apart from each other as possible.

 

Normally this would result in a tetrahedral geometry in which the angle between the two electron pairs and therefore between the oxygen and two hydrogen atoms, would be 109 degree's, however because the two non-bonding free electron pairs stay closer to the oxygen atom they exert a repulsion against the covalently bonded pairs of electrons. This pushes the hydrogen atoms closer together than would be expected and results in the characteristic 105 degree angle seen between the oxygen and hydrogen atoms in water. 

 

Electronegativity

 

Each water molecule is composed of one oxygen atom that is covalently bonded to two hydrogen atoms. The oxygen atom is strongly electronegative as a result of a strong attraction between electrons and the oxygen atom. This attraction is greater than the attraction between electrons and the hydrogen atoms. As a result there is a net negative charge on the oxygen atom and a net positive charge on the hydrogen atom. This difference in charge results in each water molecule having a net dipole moment. The net dipole on each water molecule produces an attraction. This attraction is called a hydrogen bond.

 

The arrangement of atoms in a molecule of water.

Hydrogen bonding

 

Hydrogen bonds occur as a result of an attraction between water molecules due to their net dipole moments and can be referred to as an electrostatic dipole-dipole interaction. The hydrogen bonds are relatively strong bonds when they occur in a large number, and they are able to pull the water molecules closer together this makes it harder to separate the water molecules and is the reason for water's high boiling point (100 Celsius) compared to the other 16 hydrides. 

 

Each water molecule is able to form hydrogen bonds with up to four other water molecules. This is because the oxygen atom in each water molecule has two lone pairs of electrons, and each electron is able to form a hydrogen bond with a hydrogen atom on another water molecule. However not all of the water molecules are held together by hydrogen bonds when water is in the liquid state. This means that individual water molecules are able to move about in relation to one another and allows the liquid to flow. In addition hydrogen bonds are constantly being made and broken, this also allows movement of the water molecules. Hydrogen bonds are responsible for the characteristic crystal lattice structure of ice. 

 

Hydrogen bonding between molecules in the 3 forms of water

Density

 

The density of the solid form of water; ice, is less than that of the density of the liquid form at the same temperature, with liquid water having a density of one gram per cubic centimetre. This is a very unusual characteristic for a compound especially if compared to other natural compounds, which usually have a more dense solid form than their liquid forms. Usually, for most natural compounds, the solid form sinks when placed in the liquid form, however in terms of water, with its unusual density characteristics, ice is able to float on top of the liquid water. 

 

Water's unusual characteristics in terms of density do not become apparent until the water is cooled below four Celsius. This is because when cooled from room temperature water follows the predicted pattern of most natural compounds, and its density increases. However when the water reaches four degrees Celsius the water reaches a maximum density. If the water is then cooled further, becoming ice, the water expands and becomes less dense. In fact, when the water turns into ice it's density is decreased by 9%, compared to the maximum density at four degrees Celsius. 

 

The decrease in density below four degrees Celsius is very unusual and is called negative thermal expansion. It has been suggested that the reason for this unusual characteristic is due to the strong, orientation dependant hydrogen bonds that occur between the water molecules. The normal intermolecular vibrations that occur in the hydrogen bonds are cooled with the decrease in temperature of the water, and allows steady hydrogen bonds to form between neighbouring water molecules which locks the molecules in place. A hexagonal, crystal lattice structure forms, with shorter hydrogen bonds between the water molecules and ice is produced. 

 

These properties are important for water's role on earth, and have important consequences for ecosystems. As water at four degree's Celsius is at the maximum density it always accumulates at the bottom of lakes and ponds, no matter what the atmospheric temperature is. In addition as both water and ice are good insulators and are therefore poor conductors of heat it means that it is unlikely that deep lakes will freeze completely (unless there are strong currents which mix up the water from the top and bottom of the lake and increase the rate of cooling.) In addition as ice can float on top of the water it can act as an additional layer of insulation to the water below, further reducing the loss of heat. As a consequence, aquatic life is able to survive in lakes in the winter, despite atmospheric temperatures dropping below freezing.

 

Cohesion and adhesion

Comparison between cohesion and adhesion involving water molecules

 

Cohesion is the attraction between water molecules. Cohesive attractive forces are responsible for the characteristic surface tension of water. This allows insects to run across the waters surface without breaking the surface. 

 

Adhesion is the attraction between molecules of water and molecules of another substance. Adhesive forces when combined with surface tension can result in a phenomenon known as capillary action. This action results in the movement of water up a narrow tube, against the force of gravity for example in the xylem vessels of plants. 

 

Capillary action occurs because water molecules adhere to the inside wall of the tube. The molecules are pulled up the tube by surface tension which tends to try to straighten the waters surface. Additional water molecules are pulled up the tube as a result of cohesion. This action is very important in plants and is aided by transpiration pull. 

 

Surface tension

Cohesive forces between water molecules are the result of the polar nature of water molecules. Most of the water molecules in the liquid form of water are covalently bonded to four other water molecules that surround them. However the molecules on the surface of water cannot form bonds to four other molecules. Instead the surface molecules form stronger bonds to the molecules at the surface, that are closest to them.

 

This strong attraction between the molecules on either side of the surface molecules results in the formation of a film. This film makes it difficult for objects to pass through the water surface. However once objects have passed the surface, they can move more easily through the liquid when fully submerged because the cohesive forces within the liquid are weaker. Surface tension decreases with temperature, and also decreases with detergents and soaps. 

Bonding between water molecules the liquid form of water producing surface tension

Specific heat capacity and heat of vaporisation

 

Heat capacity is the measurable physical quantity that characterises the amount of heat that is required to change a substances temperature by a given amount. For water this is measured as the amount of heat per unit mass needed to raise the temperature of the water by one degree Celsius. Water has a high specific heat capacity at one calorie per gram (i.e. one calorie is needed to raise the temperature of one gram of water by one degree Celsius). This heat capacity is one of the highest of all natural common substances.

 

As a result of the very high specific heat capacity of water, its role in temperature regulation is very important. For example water helps to moderate the earth's climate by buffering and preventing against large fluctuations in temperature. In addition water has a high heat of vaporisation. Both waters high specific heat capacity and its high heat of vaporisation properties are due to the extensive hydrogen bonding in water. This is because the hydrogen bonds provide a place where heat can be stored. The heat is stored as potential energy, even at relatively low temperatures. 

 

Water as a universal solvent

 

As many substances can dissolve in water it is often referred to as the universal solvent. Due to the number of substances that can dissolve in water it is very rare in nature to find pure water. As a consequence of this, some of the properties of water that have been described above, may be different in natural sources of water due to the substances that are dissolved in it.

 

The reason why water is such as good solvent is due to the dipole, polar nature of the water molecules, which allow water molecules to be attracted to each other, but also to other polar molecules. As such water molecules are able to form charge-charge attractions with other polar molecules. Molecules able to form such bonds in water are called hydrophilic (or water-loving). When water surrounds non-polar molecules it cannot bind with them and rotates to maximise bonding with itself. This results in a more ordered conformation and consequently a decrease in entropy. This state is energetically unfavourable, so these molecules are called hydrophobic (water-hating).

 

For example salt, or sodium chloride is able to dissolve in water. This is because the molecule is composed of ions which makes the molecule polar in nature, with positive and negative charges on the molecule, as a result of a positively charged sodium atom and a negatively charged chloride atom. When mixed into water the salt dissolves because the chloride and sodium atoms are held together by ionic bonds which are over come by the stronger hydrogen bonds in water. 

 

The dissolving process involves the positively charged side of the water molecule, the hydrogen atoms, becoming attracted to the negatively charged chloride ions and the negatively charged oxygen atom in water becoming attracted to the positively charged sodium ions in salt. The attractions result in the water molecules pulling the sodium and chloride ions apart as the ionic bonds between them are over come and broken. Following the break up of the salt molecule the separated sodium and chloride ions are surrounded by water molecules (a solvation shell).

 

References

 

  • Tysoe, O. (2012) Proteins: Ion transport. Fastbleep.com Biochemistry Chapter. Description of how solvation shells affect ion channel selectivity.
  • H. R. Horton, L. A. Moran, K. G. Scrimgeour, M. D. Perry, J. D. Rawn (2006) Principles of Biochemistry (4th ed.) New Jersey : Pearson Education, Inc., chap 19 ; pg 583 -604
  • Campbell, N. and Reece, J. (2005) Biology 7th Edition. Pearson Education Inc.
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