MORE INFORMATION- WATER

Properties of Water

The table below summarises the basic properties of water.  Following the table is an explanation of the terms used within it.

Table 1:  Summary of the Chemistry of Water

Molecular Formula
Melting/Freezing Point
Boiling Point
Surface Tension
Cohesion 
Adhesion 
Solvent 

H2O
0oC (273.15K) (320F)
100oC (373.15K) (2120F)
High
Yes, high
Yes, high
Yes, strong

 

Molecular Formula – This refers to the correct scientific way to write ‘water’.  This is put together from the periodic table of elements and tells you how many hydrogen atoms join with how many oxygen atoms in order to have the correct structure.

Molar Mass – This is the mass of one mole of a chemical element.  One mole of any element is defined as 6.023x1023 (which is known as Avogadro's number).  To get the molar mass for water you need to follow these steps:

  • Hydrogen’s atomic weight (from periodic table) = 1.0079

  • Oxygen’s atomic weight (from periodic table) = 15.9994

  • 1.0079 x 2 = 2.0158  (it is times by two because there are two hydrogen atoms in water)

  • 15.9994 x 1 = 15.9994 (it is times by one because there is one oxygen in water)

  • 15.9994 + 2.0158 = 18.0153 (which is the number you see above in the table)

Melting/Freezing Point – This is the temperature (Celsius and Fahrenheit) that water changes from either a liquid to a solid or a solid to a liquid.


Boiling Point – This is the temperature (Celsius and Fahrenheit) that water boils and gives off water vapour.

Alternative Name – The alternative name for water is dihydrogen monoxide.  This name comes from the elements (Hydrogen and Oxygen) in water.  Dihydrogen means 2 hydrogen atoms and monoxide means 1 oxygen atom.

Surface Tension – Is the tendency of the surface of a liquid that allows it to resist an external force. Water has a high surface tension (72.8 millinewtons per meter at 20oC) due to the high attraction of water molecules for each other. Surface tension on top of a layer of water for example is a result of the greater attraction of water molecules to each other than to air. The surface layer of the water then acts as an elastic sheet.  This means that items such as leaves and small insects can land and move across the surface of the water and not sink.

The photo below shows how an insect is able to land on the surface of the water and stand there without becoming wet or dropping down below the water's surface.  If you look closely you can see that the legs of this insect are not under the water at all but are standing on top and have caused small dents in the ‘skin’ of the water.  Surface tension of water can be witnessed when the object standing (in the case of insects) or landing (in the case of leaves and twigs) on it is light enough not to break the surface. 



Figure 1: Surface Tension
(Source: http://fizyka.phys.put.poznan.pl/~pieransk/Physics%20Around%20Us/Surface%20tension%2002.jpg)
   

Cohesion – This is an intermolecular attraction between like molecules.  This means that the water molecules are very ‘sticky’ and would rather ‘stick’ together than come apart.  Mercury is another example of a liquid with high cohesion (see picture below).  This should not be confused with adhesion where H2O molecules ‘stick’ to surfaces they come into contact with.

Like water, mercury has strong cohesion as you can see in the photograph below.  The strong attraction between the individual mercury molecules make it possible for this substance to form the domed bubble-like shape seen below and not spread across a surface, without outside forces interfering.


  
Figure 2:  The Cohesion of Mercury
(Source: http://en.wikipedia.org/wiki/Image:Hg_Mercury.jpg)

Adhesion – This is a molecular attraction between bodies in contact.  This means that water holds onto surfaces it comes into contact with.  This is very obvious when it rains and water ‘sticks’ to windows and other surfaces.  This should not be confused with cohesion which is when each H2O molecule wants to ‘stick’ together.

The picture below shows how adhesion works.  The water is ‘sticking’ to the leaves and not falling away because it is adhering to the surface of the leaves.  In chemistry terms the water molecules have formed an adhesive bond with the leaf surface.  Think of glue which is also known as an adhesive except with less ‘grip’.  This bond is considered to be strong which means that the water would rather ‘stick’ to the surface of the leaves than fall away even though the leaf may be on a downwards slant.


 
Figure 3:  Adhesion of Water onto Leaves
(Source: http://content.answers.com/main/content/wp/en-commons/thumb/f/f5/300px-Drops_I.jpg)
 

Solvent – This is a liquid that dissolves a solid, liquid or gaseous solute (a solute is a substance dissolved in another substance) which results in a solution being formed.  Water is the most comment solvent on earth.  Solvents do not react with whatever they are dissolving, they are chemically inactive.  Paint thinner is an example of another solvent because it thins the paint but does not change the colour or any other property of the paint.

Electrical Conductivity – This is a measure of a material or substances ability to conduct an electrical current.  Conductivity is the inverse of resistivity.  Metal is considered to have high conductivity (silver being the highest with a reading of 63.01 × 106sm-1) and glass is considered to have fairly low conductivity.  Pure water has very low conductivity. However, water is able to conduct electricity more effectively due to dissolved ions such as minerals, salts and carbon dioxide which may be in the water as pure water does not usually exist in nature.

The diagram below shows the bonds that occur between Hydrogen and Oxygen when they join together and form water (H2O). The bonds between Hydrogen and Oxygen are single bonds as Hydrogen only has one electron to use for bonding. As Oxygen has four electrons to use for bonding it still has two spare spaces. Those are indicated by the two white rods with no ball attached to the end. These spaces are used up when water molecules ‘join’ together and help make its adhesion and cohesion (see definitions above) so strong.


 
Figure 4: The Water Molecule Showing the Bonds between Hydrogen and Oxygen,
(Source: http://library.thinkquest.org/18054/data/pics/wpwaml14.gif)