What is the difference between physical and chemical dissolution?

Answer 1

Physical dissolution is purely mechanical; an example would be shaking a bottle of Italian salad dressing, the different ingredients mix but don't form a new substance. Chemical dissolution produces new substances; for example, mixing NaOH with HCl forms water and table salt, two chemicals different from what you had to begin with. Also, dissolving table salt in water is a chemical change, though it may not seem like it.

When adding water:

NaCl (s) => Na+ (aq) + Cl- (aq)

This is a very soluble ionic compound and it changes from a neutral compound to ions, a chemical change.

The dissolution of non-ionic compounds, such as sugar, is not a chemical reaction because sugar doesn't separate to ions in solution. i.e., this is a physical dissolution.

When adding water:

C6H12O6 (s) => C6H12O6 (aq)

The dissolution of weak electrolytes is somewhat more complicated. Let's consider acetic acid (vinegar).

When adding water:

CH3COOH (s) => CH3COOH (aq)

Although we write the dissolution for a weak acid or weak base this way, it differs from the sugar example because we know that, actually, some of the molecules go to CH3COO- and H3O+. We don't write this, though, because although some molecules have undergone a chemical reaction, that population is a minority.