Chapter 1: Water Chemistry

Water

Image from: Prentice Hall

Water is an incredibly unique molecule. There is really nothing like it in the universe and it is one of the compounds that makes life on Earth possible. 71% of the Earth is covered by water and 97% of this water is in the oceans.

Water is made up of two atoms of hydrogen and one atom of oxygen.  Oxygen is an extremely electronegative atom and it has a strong attraction to Hydrogen’s electrons. Because of this, the oxygen atom in water has a slightly negative charge and the hydrogen atoms have a slightly positive charge. This gives a water molecule a bent shape, a bit like the head of mickey-mouse. It also causes water molecules to act like magnets.

Special Properties of water

  • Water has unusual changes in state. Water is the only natural substance that can be found in the same place and same time as a gas (water vapor), a liquid and a solid (ice).  Click to see a great animation of this.
  • Water has a high heat of vaporization. It takes a lot of heat to move water from one state to another (i.e. from liquid to a gas). Water also has a high heat capacity. This means that water can absorb and store a lot of heat.
  • Water tends to stick to itself (cohesion) and stick to other surfaces (adhesion.)
  • Water is a universal solvent. Water can break apart and dissolve most compounds, particularly if the bonds that form them are ionic or polar- covalent.
  • Water molecules form hydrogen bonds with each other.  Click on the link to see a flash demo of H Bonding.
  • Liquid water is highly viscous.  It is 800 times more difficult to move through than air.

Solid water (ice) floats in the liquid form of water. Image from: Wikipedia Commons

Water pH

Water is considered to be a universal solvent.  This means that it dissolves most things readily. When water dissolves things that produce an excess of hydrogen or hydronium ions it will change its pH.  pH is a measure of the acidity or alkalinity of a solution. Aqueous solutions with a pH less than seven are considered acidic, while those with a pH greater than seven are considered basic (alkaline).   Currently, the oceans are in a state of increasing acidification. The ocean’s pH is lowering. This is the result of increases in greenhouse gas emissions, particularly CO2, which is largely absorbed by the oceans.  Sea water easily dissolves carbon dioxide gas, forming the weak acid – Carbonic Acid.  Increasingly, this is seen by scientists as a threat to marine life.

Ocean Water is Salty

By mass, seawater is on average 3.5% salt.  This is often described as 35 parts per 1,000.  The percentage of salt in sea water can vary a great deal.  Where there is large freshwater input (from melting snow and ice or from significant rain fall or fresh water rivers,) the amount of salt is much lower.  In areas of high temperatures and lots of sun (think Baja, Mexico in the summer,) salinity can be much higher.  The higher the salinity of seawater, the denser the water gets.  Water density is also affected by temperature (cold water is denser than warm water.)  The differences in seawater density can create a layer cake affect in parts of the ocean (with warmer or fresher water on top of colder or saltier water.)  To get a nice visual on the how differences in salinity can affect density, check out a clip from Planet Earth, Caves on YouTube.

Why Is Ocean Water Salty?

Screen shot 2014-07-14 at 6.25.37 AMAs noted above, water is a great solvent.  It can dissolve almost anything and ultimately, the ocean is salty because water dissolves minerals out of rocks. This happens whenever water flows over or through rocks. The nature of the water cycle means that all dissolved minerals eventually end up in the ocean.  As ocean water evaporates, salts are left behind.  There is no other place for the salt to go, and as a result the ocean is much saltier than other water on Earth.The table to the right shows the most common minerals in ocean water . The main components are sodium and chloride. Together they form the salt known as sodium chloride, also known as table salt.  Click to learn more about what stuff can be found in seawater and how it affects marine life.

Questions to Research:

  1. Take the United States Geological Survey (USGS) true or false quiz. Record two things that you learned from the quiz or two mistakes that you made
  2. Read the second paragraph from the text above.  Then use the link in the second paragraph (to the Wikipedia page on the properties of water) to draw a picture of a water molecule.  Label the oxygen and the hydrogen.
  3. Also from Wikipedia, examine the density vs. temperature graph for water.  In your own words, explain why water gets so much less dense at 0 degrees Celcius.
  4. What should we learn about the density of liquid and solid water from the picture of the iceberg above.  Explain how this property of water makes life in the arctic possible for marine organisms, like polar bears or ring seals.
  5. Water has a high heat capacity.  Explain how this property of water causes the oceans moderate the temperatures of adjacent land masses.  If your not sure, check out the average monthly temperature in Ketchikan, AK (surrounded by water,) and Edmonton, Alberta (on nearly the same latitude but no water.)  
  6. Identify the average salinity of sea water______. The salinity of the ocean is far from uniform. How does an increase in salinity affect the density of sea water?
  7. Why do changes in salinity affect living organisms?
  8. Write down the following sentence, adding the words adhesion and cohesion in the correct places.  “________________ is the property of water that allows plankton to float near the water’s surface.   ________________ is the property of water that allows insects to walk across the surface of a sill pond.”
  9. Explain the correlation between atmospheric CO2 and ocean pH and how human-caused increases in green house gases are affecting ocean pH.
  10. Describe how increased absorption of CO2 can impede the formation of calcification in marine organisms, and how this will impact shelled organisms.