What is a precipitation rule, and how is it used to predict whether a precipitate will form?

• A. A precipitation rule states all chlorides are insoluble.
• B. Solubility rules tell which salts are soluble or insoluble; if the combination yields an insoluble salt, a precipitate forms.
• C. Solubility rules predict color changes.
• D. Precipitation rules apply only to gaseous reactions.

Answer: (B)

When solutions mix, ions from the dissolved salts can pair up to form new compounds. A solid will form (a precipitate) if one of these potential products is insoluble in water. Solubility rules are handy because they tell you which salts dissolve and which don’t. By swapping the partners of the ions from the two solutions and checking the solubility of the possible products, you can predict whether a precipitate will form. If any possible product is insoluble, a precipitate will appear; if all are soluble, nothing solid forms.

For example, mixing a solution of sodium ions with chloride ions from one salt and silver ions from another often yields silver chloride, which is insoluble and forms a white precipitate, while the other product—sodium nitrate—stays dissolved.

This approach isn’t about predicting color changes or about reactions involving gases; it’s about whether the combination of ions can produce an insoluble salt.