Precipitation Reactions
What is a Precipitation Reaction?
A precipitation reaction is a type of chemical reaction where two soluble ionic compounds in aqueous solution react to form an insoluble solid compound (called a precipitate) and a soluble compound.
The insoluble solid that forms separates from the solution and appears as a cloudy suspension or settles at the bottom of the container.
General Form of Precipitation Reactions
Where:
- AB and CD are soluble ionic compounds (reactants)
- AD is the insoluble precipitate (solid product)
- CB remains in solution (soluble product)
- The downward arrow (↓) indicates the formation of a precipitate
Examples of Precipitation Reactions
1. Silver Chloride Formation
When silver nitrate solution is mixed with sodium chloride solution, white silver chloride precipitates out while sodium nitrate remains in solution.
2. Barium Sulfate Formation
Barium chloride reacts with sodium sulfate to form white barium sulfate precipitate, which is insoluble in water and acids.
3. Lead Iodide Formation (Golden Rain Experiment)
This reaction produces beautiful golden yellow lead iodide crystals that precipitate out of solution.
Solubility Rules for Predicting Precipitates
To predict whether a precipitation reaction will occur, we use solubility rules:
| Compound Type | Generally Soluble? | Exceptions (Insoluble) |
|---|---|---|
| Nitrates (NO3–) | Yes | None |
| Alkali metals (Group 1) and NH4+ | Yes | None |
| Chlorides (Cl–), Bromides (Br–), Iodides (I–) | Yes | Ag+, Pb2+, Hg22+ |
| Sulfates (SO42-) | Yes | Ba2+, Pb2+, Ca2+ |
| Hydroxides (OH–) | No | Alkali metals, Ba2+, Sr2+, Ca2+ |
| Sulfides (S2-) | No | Alkali metals, NH4+, alkaline earth metals |
| Carbonates (CO32-), Phosphates (PO43-) | No | Alkali metals, NH4+ |
Applications of Precipitation Reactions
- Water treatment: Removing dissolved ions from water
- Analytical chemistry: Identifying ions in qualitative analysis
- Photography: Silver halide precipitation in photographic film
- Medicine: Barium sulfate used in X-ray imaging
- Industrial processes: Producing pigments and other chemicals