Oceanography: Chemistry of Seawater

The building blocks of what gives sea water its unique properties are known as atoms.  First, we need to look at a little bit of chemistry to fully understand why water is so special.

Basic Chemical Notions

An atom is a single element in the periodic table.  Try to think of an atom.  It’s relatively straight forward.  Look at the periodic table and pick one, any one.

So, an atom can be Na (Sodium) or Cl (Chloride) or any number of the abundant elements which make up the infamous periodic table.

Na (Sodium) and Cl (Chloride) were the first elements that sprung to mind when thinking about seawater.  Why?  Well instinctively sea water is salty and NaCl or Sodium Chloride is commonly known as salt.  A good place to start!

However, we are bringing together two elements or atoms Na (Sodium) and Cl (Chloride) so these must be termed differently, right?  That’s correct, two or more atoms chemically bonded together are known as molecules.  Therefore, we can better find NaCl as a molecule of NaCl or salt.

The molecule NaCl can be separated into two individual atoms but first we need to understand what creates the bond between the two.  Here are some key facts:

Charges are Important

Think of a magnet where +ve and -ve charges attract, opposites attract.  Easy right!  The opposite is also true of course, a -ve and -ve or +ve and +ve will repel and pull away.

Here are some key facts on the electrical charges present in Na and Cl, the two key constituent atoms that make up the molecule NaCl:

  • Each atom Cl, and Na have different charges
  • -Cl and +Na therefore attract due to this dipole moment or differences in polarity
  • Basic chemistry determines an atom is made up of a central nucleus which contains neutrons and protons.  Around the nucleus several electron shells orbit in a function of probability state of motion.
  • The outermost shell furthest away from the nucleus and therefore the electrons with the least resistance is known as the valence shell.
  • An atom with an equal number of protons within the nucleus to electrons within the electrons shell configuration is considered electrically neutral
  • So, we understand that in an atom if the number of protons is = to the number of electrons then the result would be a stable state

Applying these Rules to Water

From what we have learnt already, we know that water is H2O.  So, we should be able to derive from this that the molecule water (H2O) is made up of two H (Hydrogen) atoms and one O (Oxygen) atom.

Take H (Hydrogen).  H has 1 electron in its valence shell and 1 proton in its nucleus.  The +ve charge of its single proton is neutralised by its -ve charge of its single electron.  Therefore, H is electrically neutral.

Take O (Oxygen) on the other hand.  O has 8 protons in its nucleus and is balanced electrically by 8 electrons orbiting through a probability function in its shells.  2 of these electrons orbit near to the nucleus in an inner shell and 6 orbit in the valence shell.

Key point: If electrons are added or removed from any atom then this results in an imbalance of charges.

Such as if an atom has more -ve charged electrons than +ve charged protons then it will have a net -ve charge.  Conversely the opposite is true.

Another key concept to realise is that an atom with either a -ve or +ve charge is an ion.  Any ion is simply an atom with a given charge.

A great example of two ions is +Na and -Cl, both are ions with opposite charges.  The charge of an ion is the single most important reason for its ability to bond with other elements (atoms).

How H2O Bonds

O (Oxygen) has 6 electrons in its valence shell and 2 electrons sit closer to the nucleus.  H (Hydrogen) that makes up the molecule H2O has a single electron in its valence shell which is shared with the O (Oxygen) atom to make up a stable electrical configuration and secure what is termed a covalent bond.

Important to note that for O to be completely stable two hydrogen atoms are required to covalently bond as illustrated in the figure above.

Important Properties of Water

In addition to the polar attraction of ions +H with -O to form the molecule H2O the following characteristics are also observed:

  • The water molecule H2O has an asymmetric shape
  • Rather than two H atoms being attracted with symmetry they are separated by an angle of 105o.  This is true for water in both liquid and gaseous states
  • H and O atoms share electrons to form a very strong covalent bond.  This sharing of electrons occurs due to O requiring 2 electrons to fulfil a stable electronic state

More About H20 Charges & Dipole

Looking at the figure to the left it is crucial to understand the unique relationship that water has with polarity.

Each H atom within the water molecule possesses a single +ve charge, whilst the O atom a double -ve, as such the water molecule H2O is considered electrically neutral due to the equal balance of charge.

However, and this is crucial, the structural asymmetry present in H20 is explained by the 105o angle of separation result i charge.  The imbalance of charges in the molecule creates what is known as a dipole moment with a stronger -ve pull from the -O ion.  All this despite the electrically neutral state of the H2O molecule.

The consequence of the asymmetric 105o structure of the water molecule and the dipole moment toward the more electrically active -O means that water is not merely a collection of freely moving molecules.  The residual -ve charge that results from the dipole moment acts to encourage nearby H20 molecules to form bonds as +ve H ions become attracted to the -O ions.  This electrostatic bonding is known as hydrogen bonding and results in irregular chains and cluster of H20 molecules.  The implications of all this is liquid water remains in the liquid state at surface temperatures on earth.

H20 Cluster and Irregular Chains

Important concepts to grasp are bulleted below:

  • Size of water molecule clusters decrease with size with an increase in temperature
  • H bonding is responsible for the many wonderful properties of water

But why?

The higher than expected melting and boiling points of water depend directly on the dipole structure of the H2O molecule.

The energy required to melt ice to vaporise liquid water is significant.  This is because the H bonds that link the H2 molecule to the H2O molecule must first be broken before the solid structure can melt and the liquid can vaporise.

High Heat Capacity of Sea Water TIP

* The reason for waters high heat capacity is due to the substantial amounts of energy required to break the hydrogen bonding in the H2o molecule.

Key Concept – Heat Capacity of Water H2O

  1. When heat is added to water, only a fraction of the energy is used to increase the vibrational energy of the molecules which would be detected as a rise in temperature T
  2. Much of the energy is used to break H bonds that link H2O molecules together in irregular clusters
  3. In contrast, when water is cooler, water releases more heat than is expected from the decrease in temperature T

Fundamental Water Facts

Water has a unusually high heat capacity and tremendous solvent power.  What does this mean?  Simply put, more energy is required to raise the temperature of water per gram than would be the case with a substance that had a low heat capacity.

The ocean has a higher heat capacity than the adjacent landmass. Therefore, more radiation energy is required to heat the sea then the land results in the land warming more rapidly than the sea.  During the summer, this explains why the land warms and cools much more rapidly than the cooler ocean, and partly explains why it is often cooler by the coast than inland.  The land has a lower specific heat capacity than the sea!

In other words, adding the same amount of heat will raise the temperature of a substance with a low heat capacity such as the land to a greater degree than one with a very high heat capacity such as the ocean.

The Impact All These Facts Have on The Oceans, Land and Our Climate

Take a step back to imagine yourself by a lake in the summer.  Lakes have a high heat capacity, much like the ocean, due to the presence of the water molecule and the H bonding we have discussed.  This means that lakes absorb a lot of the incident solar radiation helping to keep the air temperature above and around it cooler.  Conversely in the winter large lakes and the ocean act to reradiate a lot of this stored heat back out into the atmosphere as the water cools helping to warm the immediate shore or Lakeland.  Incidentally the energy that is absorbed sits within a different electromagnetic spectrum from the energy that is reradiated, but this will be covered in another chapter on energy balances and climate.

Important Concept – Water Is a Powerful Solvent

The capability for water to dissolve material matter is remarkable and is like nothing else on the planet.  The ability for water to dissolve material matter for efficiently means water has a fantastic solvent power.

But what is makes water have such a high solvent power?  Well it’s a good question and one that scientists have been able to answer.  The solvent power is reliant on the dipole structure of a molecule and in waters case the 105o separation of the +H ions with the -O that creates the asymmetric dipole -ve dipole moment.

If we take a look at seawater, the most comment elements or ions dissolved are Na (Sodium) and Cl (Chloride) with Na +ve charge and Cl -ve charge.

Important Concept: A -ve ion is more commonly referred to as a anion and a +ve ion a cation

As has been touched upon already in this article when two ions exhibit opposing charges +ve and -ve they become ionically attracted to each other, in other words they form an ionic bond.  In the example of Na with Cl the ionic bond formed creates Halite, rock salt or what we commonly known as table salt.

When Halite crystals are submerged in water or in other words when NaCl encounters H2O the following things happen to form a common chemical reaction:

  • The -ve end of the H2O dipole dislodges the +Na cations from the solid
  • The -ve end of the H2O also acts to tear off the -Cl anions
  • Dissolution, otherwise known as the noun term of dissolve in chemistry occurs.  Dissolution of Cl and Na ions into the H2O solution occurs until either the entire volume of crystal halite is consumed or the volume of water can no longer contain any more ions due to complete saturation

Other Terms We Need to Understand

Hydration is the process of water surrounding an ion, whereas Hydrolysis is the process of a water molecule H2O breaking the bonds that hold other molecules (particularly ionic bonds) those with opposing dipole attractions +ve and -ve.

Useful Facts

Water efficiently dissolves and is termed a solvent.  Ions that dissolve with the solvent are known as solutes.  The solutes in ocean water are +Na and -Cl for example.  Water dissolves ionic salts, gases such as diatomic O2 and Co2, as well as polar organic molecules such as ethanol.  Conversely water does not dissolve covalently bonded minerals such as quartz, clays and other non-polar organic material matter such as wax, oils and methane.  Therefore, when you get an oil spill the risk of an ecological disaster is significant because oil does not dissolute with water.

Copyright Weather Scientifc 2017

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