Precipitation reactions

In a precipitation reaction two solutions are mixed together resulting in the formation of an insoluble solid called the precipitate.

Each of the solutions is composed of positive and negative ions. By convention the metallic ion is always named first. eg. sodium chloride is composed of positive sodium ions, Na⁺ and negative chloride ions, Cl^-

In a precipitation reaction there is an exchange of partners creating a new set of positive and negative ions.

One group of these ions becomes more attracted to each other than they do to the water molecules. This produces an insoluble compound called the precipitate. The other set of positive and negative ions remains in solution.

Analogy of two couples on a dance floor. 

Boys = (+) metallic ions and girls = (-) negative ions.

Fred and Jill + Jack and Wilma  Fred and Wilma + Jack and Jill

On the dance floor there is a change of partners. Fred and Wilma are strongly attracted to each other. They leave their partners and unite together in a new relationship due to their strong attraction to one another. They form the solid precipitate. Jack and Jill, however, never quick make it together as an item. They are more strongly attracted to the water molecules than to each other.

Physical states:

(aq) = aqueous or dissolved in water.

(s) = solid

Colorful Precipitation reactions

1. Sodium hydroxide + silver nitrate   sodium nitrate +  silver hydroxide

NaOH(aq) + AgNO₃(aq) NaNO₃(aq) + AgOH(s)

The precipitate is brown.

Note: Ag(OH)₂ actually becomes Ag₂O in aqueous solution.

2. Sodium hydroxide + copper sulfate sodium sulfate + copper hydroxide

2NaOH(aq) + CuSO₄(aq) Na₂SO₄(aq) + Cu(OH)₂(s)

A blue gelatinous precipitate of copper hydroxide, Cu(OH)₂ is produced.

3. Potassium iodide + lead nitrate potassium nitrate + lead iodide

2KI(aq) + Pb(NO₃)₂(aq) 2KNO₃(aq) + PbI₂(s)

A bright yellow precipitate of lead iodide, PbI₂ is produced.

4. Potassium iodide + mercury nitrate mercury iodide + potassium nitrate

2KI(aq) + Hg(NO   ₃  ) ₂(aq) H gI ₂ (s ) +  KNO  ₃  (aq)

A bright orange precipitate of mercury iodide, PbI₂ is produced.

Note: This is an unusual reaction as the precipitate dissolves in excess iodide solution and disappears as a complex ion is formed.

5. Potassium iodide + silver nitrate potassium nitrate + silver iodide

KI(aq) + AgNO ₃(aq) KNO ₃ (aq) + AgI(s) 

A yellow white precipitate of silver iodide, AgI is produced.

6. Sodium chloride + silver nitrate sodium nitrate + silver chloride

 NaCl(aq) + AgNO ₃ (aq) NaNO₃(aq) + AgCl(s)

A white precipitate of silver chloride is produced.

Note: Exposure of silver chloride to UV light from the sun results in the decomposition of this compound. Black metallic silver is produced. 2AgCl(s) 2Ag(s) + Cl₂(g)

7. Mercury nitrate  + potassium fluoride mercury fluoride + potassium nitrate

 Hg(NO₃)₂(aq) + 2KF(aq) HgF₂(s) + 2KNO₃(aq)

An orange brown precipitate of mercury(II) fluoride, HgF₂ is produced.

8. Copper sulfate + potassium iodide copper iodide + potassium sulfate

CuSO₄(aq) + 2KI(aq) CuI₂(s) + K₂SO₄(aq)

An olive green precipitate of copper iodide, CuI₂ is produced.

9. Mercury nitrate  + potassium sulfide mercury sulfide + potassium nitrate

Hg(NO₃)₂(aq) + K₂S(aq) HgS(s) + 2KNO₃(aq)

A black precipitate of mercury(II) sulfide, HgS is produced.

10. Silver nitrate  + potassium chromate silver chromate + potassium nitrate

2AgNO₃(aq) + KCrO₄(aq) Ag₂CrO₄(s) + 2KNO₃(aq)

A red-brown precipitate of silver chromate, Ag₂CrO₄ is produced.

A precipitation worksheet may be downloaded. The first page includes a precipitation practical experiment. The second page has equations only. The third page has the answers to the precipitation reactions.