I forgot my username or password.

In support of all the students who are displaced from school due to the Corona virus. Access to physics zone and chemistry zone lessons are now available free of charge. This will be maintained at least through August 1st 2020. Learn and be well.

Chemistry Lessons Review Links

Dissolve & Dissociate

When we place a solid in a liquid and it seems to disappear or become part of the liquid, we say that it "Dissolved".  Dissolve is a very generic or broad term to describe what is happening.  It could mean many things:

  1. It could mean that the bonds between molecules (intermolecular bonds) were broken by the liquid which would leave the individual molecules to mix within the fluid.
  2. It could mean that bonds within the molecules (intramolecular bonds) were broken by the liquid resulting in smaller molecules being formed and mixing within the fluid.
  3. Or it could mean that bonds between the atoms were broken allowing the atoms or ions (if they are charged) to mix within the fluid.

Dissociate is a very similar term which is more specific and is covered by only the last two meanings.  It can be said that everything that dissociates dissolves, but not everything that dissolves dissociates.

In the animation below, table salt (NaCl) is poured into water (H2O) where it rapidly dissolves as the Sodium ions (Na+) dissociate from the Chlorine ions (Cl-).  Notice the part that the water molecules play in the dissociation.

Since water is a polar molecule (one end has a different charge than the other end) one end will be attracted to positive ions and the other end will be attracted to negative ions.  A single molecule of water is not enough to overcome the attraction of the Na+ for the Cl- but several water molecules together can easily overcome the attraction and separate the two ions.  Table salt is said to completely dissociate in water because as long as there are enough water molecules all the sodium ions will be dissociated from the chlorine ions.