Tuesday, April 6, 2010

Dissolution Magic!

Just try this at home!

Take a beaker full of water. Note the water initial level. Now, a spoon-full of common salt is added to the beaker and stirred for some-time. After adding more and more common salt, does the water level increase?? 

No! Then, where all the salt has gone??


See, not only in this case, but the dissolution of any substance into any solvent doesn't necessarily change the volume of the solution. Why?

Let us get into the molecular level. 

In this case, water being polar exists as cross-linked 3-dimensional structure due to inter-molecular hydrogen bonding. Water molecules are scattered into regular pattern with a little inter-molecular space as shown in the following picture. The molecules are a little apart from each other due to electrostatic repulsion between the highly electronegative oxygen atoms. At the same time, these molecules are held by electrostatic attraction between highly electronegative oxygen atom and hydrogen atoms carrying a fractional positive charge.
(before dissolution of common salt) 

After adding few grams of common salt (sodium chloride or NaCl), the molecules of sodium chloride dissociate into Sodium ions and Chloride ions, by Arrhenius' theory of electrolytic dissociation. These Sodium ions and Chloride ions break the regular pattern(arrangement) of water. But these ions need energy to break the bonds between water molecules. They absorb the required energy from the surrounding and thus, water gets cool. Similarly you can feel the coldness when glucose is dissolved in water. 
NaCl(s) → Na+(aq) + Cl-(aq)

Now, these ions occupy the inter-molecular spaces between the molecules of water and so there is no change in the volume of the solution. (See the picture below)
(After the dissolution of common salt)
What difference did you make out between the two pictures?? Yeah~! After dissolution, the solution is more denser than before. So, more energy would be required to boil salt water than pure water. This is called as "elevation in boiling point".

After adding more and more NaCl, at one moment, all the inter-molecular spaces will be occupied. So there is no place for further dissolution. When you add more of NaCl, it just gets precipitated simply, as there is no place to get into! This point is called "Saturation".

Try this for different salts and solvents, but make sure that you don't get them onto your mouth!
The second picture has been taken from the web and the third picture has been designed by altering the second picture for better understanding.

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