When Do Double Displacement Reactions Occur

Often when solutions of two ionic compounds are mixed a precipitate (solid) is formed. The mixture may turn cloudy, or may even separate, with the solid settling at the bottom of the container.

Occasionally, the mixture may bubble as a gas is produced.

Other times however, nothing happens and the two solutions will mix and remain clear.

Predicting whether a double displacement reaction will occur is based on the following rule:

Rule: Double displacement reactions occur if ONE of the products is water, a gas or insoluble (a solid).

The first part of the rule is easy. The following reaction occurs because one of the products is water:

H₂SO4 + 2 NaOH → Na₂SO₄ + 2 H₂O

(If you were just switching partners you might have ended up with HOH. That is absolutely okay and is exactly the same thing as H₂O. In fact, it's easier to balance if you write the product as HOH.)

To use the second part of the rule, you need to know the four common gases that are produced in double displacement reactions.

They are:

• H₂S,
• H₂CO₃,
• H₂SO₃ and
• NH₄OH.

So the following reaction occurs because one of its products is a gas:

HCl + Na₂S → NaCl + H₂S

A complication that your instructor may or may not care about...

The third part of the rule for double displacement reactions is also simple, if you know what things are soluble and what things are insoluble. 

There are two ways to know whether or not something is soluble. Your instructor may give you a series of rules about solubility to learn. (One such set of rules can be found here.) Or your instructor may have you use a solubility table, like the one found here. On the table, S is for soluble and I is for insoluble. So the last part of the rule could be that double displacement reactions occur if one of the products is an I on the solubility table.

For example, the reaction between Na₂SO₄ and Ba(NO₃)₂ occurs because one of the products (BaSO₄) is insoluble.

Na₂SO₄ + Ba(NO₃)₂ → BaSO₄ + 2 NaNO₃