Dipoles Of Water Molecules

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Summary

  • Water is the main constituent in all organisms
  • A water molecule is said to be dipolar because it has a positive and a negative pole as a result of the uneven distribution of electrons within it
  • The dipole nature within a water molecule creates attractive forces known as hydrogen bonding, allowing them to stick together
  • One of the many types of properties water has is a high specific heat capacity, meaning it takes a lot of energy to heat up a given mass of water
  • A high number of hydrogen bonding in water molecules also means it requires a lot of an energy to evaporate one gram of water. The energy is called the latent heat of vaporisation
  • Water is also used to break many complex molecules down by hydrolysis for example; disaccharide carbohydrates broken down into monosaccharides

Introduction to water

Water is a major chemical component of all cells in building up new cells and is also used in breaking down molecules. It has many important roles within organisms and is a key reactant used in photosynthesis. Essentially, water is known as the medium of life where around 70% of the Earth’s surface is covered with water. It’s useful and unique properties occur due to its structure. It is a chemical compound and is known as a polar molecule where under standard temperatures and pressure water is a liquid, aqueous form and the solid state of water is known as ice.

Chemical structure of water

  • Water is much smaller in structure compared to other molecules
  • It is an universal solvent
  • Water consists of 2 hydrogen atoms bonded to one oxygen atom, giving the molecular formula H2O. The hydrogen atoms are bonded to oxygen atoms through covalent bonds which are a type of chemical bond that connects the hydrogen and oxygen atoms together
  • A covalent bond is one in which two atoms share the same pair of electrons
  • Covalent bonds are formed when electrons are shared between two atoms in order to fill each of the outer shell. For the hydrogen atom it needs one more electron and the oxygen atom needs to gain one electron for the outer shell to be complete

The polar nature of water molecules

An important feature of the water molecule is its polar nature. Polar molecules once placed into water dissolve and dissociate into water ions. Ionic molecules when they are placed into water dissociate into ions. Water is what provides different chemical reactions to take place:

  • The electrons in the covalent bonds of water are not equally shared and they lie closer to the oxygen nucleus than the hydrogen nuclei. This is because the negative electrons in the electron pair are more attracted to the oxygen nucleus as there are more positively charged protons. The oxygen nucleus is much larger in nature as there 8 protons whereas, the hydrogen nucleus is 1 proton (Figure 1). Protons are positive in nature and electrons are negatively charged. The covalent bonded electrons are going to be attracted to positive things and because there is more protons in the oxygen nucleus than the hydrogen they are going to be more pulled towards the oxygen atom.
  • The unequal sharing of the electrons result in the oxygen atom being polar and slightly negative. There is a slightly a negative oxygen atom and the hydrogen atoms being slightly positive.

The uneven distribution of charge across the water molecules makes it a polar molecule.
Polar molecules are molecules that have an uneven distribution of charge. The polarity of water causes there to be an attraction between water molecules. This force of attraction is known as hydrogen bond.

Hydrogen bonding in water

A bond between molecules where the slightly positive hydrogen atoms in one water molecule are attracted to the slightly negative oxygen atom in another water molecule. The type of attraction is a weak interaction that occurs between a slightly negatively charged atoms and a slightly positively charged atom. It is weaker than a covalent bond but is the strongest intermolecular forces. Each of the individual hydrogen bonds is weak, where water forms many of these hydrogen bonds.

A hydrogen bond is a weak interaction that occurs between slightly negatively charged atoms and a slightly positively charged atom.

Water forms hydrogen bonds within itself. The many hydrogen bonds within water gives it a high stability causing for a large amount of energy to be required to raise the temperature of water.

Figure 1: A water molecule

Physical Properties of Water

While water is the most abundant liquid on Earth the structure of a water molecule causes for it to have several unusual properties.
Specifically many of the properties stated below are due to its dipolar nature and subsequent hydrogen bonds it forms and allows.

Solvent

Water is a good solvent since it is a charged molecule allowing them to easily be transported alongside this its water molecule’s dipole nature allows other polar molecules to be readily dissolved into water. Such examples of charged polar molecules such as salts, amino acids and sugars are able to readily dissolved in water. They are termed as hydrophilic; “water-loving” molecules. Non-polar molecules such as lipids are termed as hydrophobic, “water-hating”.

High heat capacity

The tendency of water molecules to stick together is known as cohesion. Hence it would take a lot of heat energy to separate molecules that are stuck together rather than if they were not bonded together. Relating this to water, water does not change temperature very easily as it has a specific heat capacity of 4.2 J g-1 °C-1 which in simple terms means it takes 4.2 joules of energy to heat 1 g of water by 1 °C. This is remarkedly high and is what keeps aquatic and cellular environments stable.

High Latent heat of Vaporisation

Hydrogen bonding between the water molecules allows it to have a high latent heat of vaporisation. Therefore, a large amount of energy is needed to change water from a liquid state into a gaseous state, where evaporation has a cooling effect on organisms such as sweating in animals and transpiration in plants

Density

Water has a unique property that in a solid state (ice) it is less dense that the liquid state and is able to float on water. This property is different for the normal situation as most substances are in a gas form when they are less dense and more dense are in a solid state. This shows how water is different. The property of water makes it crucial for aquatic organisms to be able to survive in freezing sub-zero temperatures such as in ponds, lakes etc.

Cohesion and surface tension in water

Water has properties of being cohesive, where they have a tendency to stick to other molecules together. This is due to the hydrogen bonding within water molecules that causes it to have a large cohesive forces which allows for the water to be pulled through a tube for example in plants; xylem vessels which are long tubes that help in the transportation of water to provide the plan with mechanical support. Another force called surface tension is also an unique property of water. Surface tension is the attractive force that can be exerted upon the surface of other molecules of a liquid  which gives the tendency of the fluid surface to shrink into minimum surface area rather than the body of water escaping. Surface tenion of water allows for skaters to walk on water or allow insects that are usually denser than water, float on a water surface.

pH

Water itself is partially ionized and has a source of H+ ions that cause for several biochemical reactions to be sensitive to pH changes. Pure water is not buffered at a neutral pH unlike the cytoplasm and tissue fluids within living organisms that are buffered at neutral pH 7.5.

Ionization

Ionization is the process for forming or splitting of molecules to their cations and anions. An example is when sodium chloride (NaCl) dissolve into water they ionize and separate into positive and negative ions (Na++, Cl).

References:

[1]. https://commons.wikimedia.org/wiki/File:H2O_Polarization_V.1.svg
[2]. https://openstax.org/books/biology-2e/pages/2-2-water