MadSci Network: Chemistry

Re: How can dissociation of salt when dissolving not be a chemical change?

Date: Sun Feb 17 17:24:14 2002
Posted By: Tracy Cheatham, Faculty, Chemistry, Central Carolina Community college
Area of science: Chemistry
ID: 1012279794.Ch


Sodium is a metal that reacts explosively with water, check out this
site for some properties:

Chlorine is a yellow-green poisonous gas whose MSDS can be found here:

Compounds do not exhibit the properties of the individual elements.  so, 
sodium chloride does not exhibit the properties of sodium or chlorine.  
NaCl (sodium chloride, or table salt) has a long history.  Mentioned in 
the bible as collateral for trade, it was highly prized.  It has become a 
very common substance now.  We not only like sodium chloride in our diet, 
we need it to help maintain fluid controls and aid in muscular 
contractions.  The MSDS for NaCl can be found here:

When sodium and chlorine are reacted, there is a transfer of electrons.  Na
(sodium) gives away an electron and becomes Na+ (sodium ion).  The sodium 
ion does not have the properties of its parent atom, solid sodium metal.  
The electron that is given away goes to chlorine, which needs only one 
elctron to complete its outer shell.  Chlorine accepts the electron and 
becomes Cl-, or chloride ion.  Again, the chloride ion does not act at all 
like its parent atom, chlorine.  Sodium ion and chloride form an 
electrostatic bond, they are attracted to each other because they carry 
opposite charges, one is plus and one is minus.  When sodium chloride is 
dissolved in water, a process called solvation occurs.  Water is a polar 
solvent.  The water molecule has a positive end and a negative end, very 
similar to the opposite sides of a car battery.  The positive part of the 
water molecule is attracted to the negative part of the sodium chloride, 
the chloride ion.  The negative part of the water is attracted to the 
positive part of the sodium chloride, the sodium ion.  Thus, the sodium 
chloride dissociates, or breaks apart in water.  (Dissociate is the 
opposite of associate)  If the water evaporates, the sodium ion and 
chloride are forced closer and closer, due to evaporating media.  When 
they get close enough, the attraction is great enough to associate them 

The key here is that sodium (Na) and sodium ion (Na+) do not have similar 
properties.  Same for chlorine and chloride.  Therefore when Sodium 
chloride dissociates in water, sodium and chlorine is not formed.  Sodium 
ion and chloride are formed, two very different acting things.  Hope this 


Chemistry, The Central Science: Brown, LeMay, Bursten, 8th edition, 
Prentice Hall publishers.

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