Notes: Chemistry of Water

Image of Earth landing in a drop of water  

According to data from NASA, water covers 75% of the Earth's surface. It is the single most abundant compound in living organisms. Water has several important physical properties:

Image of water molecule

The Water Molecule

Overall, water is a neutral molecule.  In other words, it has the same number of protons as electrons.  However, the electrons are unequally distributed.  The oxygen atom, with its eight positively charged protons, has a strong pull on the negative electrons which makes the probability of finding those electrons near the oxygen greater than finding them near the hydrogen atoms.  This gives the oxygen end of the water molecule a slightly negative charge and the hydrogen ends of the water molecule slightly positive charges.
 
For a better understanding of water, click here to see how water molecules are connected by hydrogen bonds.

 

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The unequal distribution of charges on a molecule results in polarity (having a positive pole on one end and negative on the other). Water molecules' positive and negative charges cause opposite ends of water molecules to attract each other like tiny magnets.

 

Another representation of a water molecule

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If a water bug can walk across the water, why can't a frog? It's an interesting question. In order to answer this question, you need to know a lot more about the properties of water.

Get some answers by viewing the following animation, A Closer Look At Water from Northland College and reading about surface tension from Boise State.

 

 

Hydrogen bonds (intermolecular or between the molecules) exist between water molecules.  Hydrogen bonding is weaker than most other types of chemical bonds, allowing them to be broken by other charged substances.  The bonding of water molecules due to their polarity allows for several unique chemical properties: 

The forces of cohesion and adhesion work together to produce capillary action, the force of water rising in a thin tube.  This force makes it possible for water to travel up the thin tubes inside plants from roots to leaves.  Adhesive forces cause the water molecules to be attracted to the insides of the tubes.  The cohesive forces cause the water molecules to cling to each other.

 

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Hydrogen molecules bonding between water molecules To learn more about the chemical behavior of water, visit and complete the tutorial, the Chemistry of Water.

Note that water often is found in nature as a mixture.  A mixture is a material that is composed of two or more substances that are physically combined. Examples of combining two substances physically include mixing iron filings with sulfur or dissolving sugar into hot tea. There are two types of mixtures that can be made with water.

 

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Solutions and Suspensions: 

Bloodbag

 

What is blood? Blood is an example of a biological fluid that is both a solution and suspension.  There are some molecules that are evenly distributed throughout the plasma and others that are not evenly distributed.

 

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Acids, Bases and pH

Water molecules can dissociate, or come apart, into two ions:

H2O → H+ + OH-

water → hydrogen ion + hydroxide ion

 

Scientists have devised a scale to measure the concentration of hydrogen ions (H+) and hydroxide ions (OH-) that exist in solutions.  The scale is called the pH scale.  The pH scale ranges from 0 to 14.

 

When a solution such as water has equal amounts of H+ and OH- ions, the solution has a neutral pH.  The pH scale is based on 7 as the reading for neutral.

When a solution has a greater amount of H+, then it is assigned an acidic pH which is a number below 7 on the pH scale.

When a solution has a greater amount of OH, then it is assigned a basic pH which is a number greater than 7 on the pH scale.

 

The image below from the Environmental Protection Agency webpage about Acid Rain shows the pH of some common substances:

pH scale and link to EPA information about pH related facts  

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All living organisms have a set range of pH levels that are optimal for their growth and development.  Sudden changes in pH levels can harm organisms.

The pH inside human cells must be kept close to neutral in the range of 6.5 to 7.5.  If a chemical with a lower or higher pH enters the cell, it could disrupt chemical reactions and disrupt the cells' homeostasis.  Therefore, cells contain weak acids and weak bases called buffers which work to neutralize substances inside cells that are too acidic or too basic.