Visualizing Colligative Properties

Posted by Hunter Archibald on 7/1/15 10:00 AM

To simplify colligative properties, picture what is happening when you dissolve a salt in water. We know that increasing the concentration of ions has an effect on the boiling and freezing points of water. How do you remember which way?  

A salty hot spring, [image credit]
When we dissolve NaCl in water, the salt dissociates into Na+ and Cl- ions. Each of the ions is surrounded by a “sphere of hydration.” In other words, the water molecules form bonds to the ions.

How can they form bonds to both negatively and positively-charged ions? Well, water is a dipole, so the oxygen side, which has a partial negative charge, attracts to the positive Na+ ion, and the partially-positive hydrogen side of the water molecule associates with the negative Cl- ion. Opposites attract, so these spheres of hydration are quite stable.

Spheres of Hydration [image credit]

This stability makes it harder to remove a water molecule from the liquid (boiling,) and hard to form a crystal (freeze.) Therefore, the boiling point is increased.

Similarly, the freezing point is decreased because these spheres of hydration have to be broken in order to form new crystalline bonds (which is what happens when water freezes.)

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Tags: chemistry