Precipitation Reaction Predictor- How to Predict Products
What Is a Precipitation Reaction?
A precipitation reaction happens when two aqueous solutions mix and form an insoluble solid called a precipitate. This solid crashes out of solution—it's the cloudy stuff you see floating around after the reaction completes.
These reactions are double displacement (metathesis) reactions. The cations and anions swap partners. If the new combination is insoluble in water, it falls out.
That's it. That's the whole concept. Now here's how you actually predict what forms.
The Solubility Rules: Your Only Real Tool
You cannot predict precipitation products without knowing solubility rules. Memorize them or keep a reference table handy. There is no trick around this.
General Solubility Patterns
- All nitrates (NO₃⁻), acetates (CH₃COO⁻), and most alkali metal salts are soluble.
- Most chlorides, bromides, and iodides are soluble—except Ag⁺, Pb²⁺, and Hg₂²⁺.
- Most sulfates (SO₄²⁻) are soluble—except Ba²⁺, Pb²⁺, Ca²⁺, and Sr²⁺.
- Most hydroxides (OH⁻) are insoluble—except alkali metals and Ba²⁺, Sr²⁺.
- Most carbonates (CO₃²⁻), phosphates (PO₄³⁻), sulfides (S²⁻), and chromates (CrO₄²⁻) are insoluble—except alkali metal salts.
Quick Reference Solubility Table
| Anion | Soluble If Combined With | Notable Exceptions |
|---|---|---|
| NO₃⁻ | Almost everything | None |
| Cl⁻, Br⁻, I⁻ | Most cations | Ag⁺, Pb²⁺, Hg₂²⁺ |
| SO₄²⁻ | Most cations | Ba²⁺, Pb²⁺, Ca²⁺, Sr²⁺ |
| OH⁻ | Li⁺, Na⁺, K⁺, Ba²⁺, Sr²⁺ | Everything else |
| CO₃²⁻ | Li⁺, Na⁺, K⁺, NH₄⁺ | Everything else |
| S²⁻ | Li⁺, Na⁺, K⁺, NH₄⁺, Ca²⁺, Ba²⁺, Sr²⁺ | Most transition metals |
How to Predict Precipitation Products
Follow this step-by-step process. Every time. Without skipping.
Step 1: Write the Reactants
Identify your two aqueous ionic compounds. Write them as dissociated ions.
Example: AgNO₃(aq) + NaCl(aq) →
Break it down:
- AgNO₃ → Ag⁺ + NO₃⁻
- NaCl → Na⁺ + Cl⁻
Step 2: Swap the Cations
The positive ions pair with the negative ions from the other compound. This is the double displacement part.
Ag⁺ pairs with Cl⁻ → AgCl
Na⁺ pairs with NO₃⁻ → NaNO₃
Step 3: Apply Solubility Rules
Check if each product is soluble or insoluble.
- AgCl: Cl⁻ is usually soluble, BUT Ag⁺ is an exception. AgCl is insoluble. This precipitates.
- NaNO₃: Nitrates are always soluble. NaNO₃ stays in solution.
Step 4: Write the Complete Equation
AgNO₃(aq) + NaCl(aq) → AgCl(s) + NaNO₃(aq)
The (s) denotes solid precipitate. The (aq) denotes aqueous/dissolved.
The Ionic Equations
You need to understand three forms:
1. Molecular Equation
Shows complete formulas, like what you just wrote above. Good for balancing.
AgNO₃(aq) + NaCl(aq) → AgCl(s) + NaNO₃(aq)
2. Complete Ionic Equation
Shows all dissociated ions. Everything that dissolves goes separate.
Ag⁺(aq) + NO₃⁻(aq) + Na⁺(aq) + Cl⁻(aq) → AgCl(s) + Na⁺(aq) + NO₃⁻(aq)
3. Net Ionic Equation
Cancel the spectator ions (those that don't change). What's left is the actual reaction.
Ag⁺(aq) + Cl⁻(aq) → AgCl(s)
This is the useful part. It tells you exactly what formed the precipitate.
Getting Started: Practice Problems
Work through these. No watching, no skipping ahead. Write it out.
Problem 1
Pb(NO₃)₂(aq) + KI(aq) → ?
Dissociate:
- Pb²⁺ + 2NO₃⁻
- K⁺ + I⁻
Swap:
- Pb²⁺ + I⁻ → PbI₂
- K⁺ + NO₃⁻ → KNO₃
Check solubility:
- PbI₂: I⁻ is usually soluble, BUT Pb²⁺ is an exception. PbI₂ precipitates (bright yellow solid).
- KNO₃: Nitrates are always soluble. Stays in solution.
Answer: Pb(NO₃)₂(aq) + 2KI(aq) → PbI₂(s) + 2KNO₃(aq)
Net ionic: Pb²⁺(aq) + 2I⁻(aq) → PbI₂(s)
Problem 2
CaCl₂(aq) + Na₂SO₄(aq) → ?
Dissociate and swap:
- CaSO₄
- NaCl
Check solubility:
- CaSO₄: SO₄²⁻ is usually soluble, BUT Ca²⁺ is an exception. CaSO₄ precipitates (slightly soluble, but yes, it forms a precipitate).
- NaCl: Always soluble.
Answer: CaCl₂(aq) + Na₂SO₄(aq) → CaSO₄(s) + 2NaCl(aq)
Common Precipitates to Know
These come up constantly. Memorize them or you'll be checking the table every single time.
| Precipitate | Color | Formula |
|---|---|---|
| Silver chloride | White | AgCl |
| Lead(II) iodide | Bright yellow | PbI₂ |
| Lead(II) chloride | White | PbCl₂ |
| Barium sulfate | White | BaSO₄ |
| Calcium carbonate | White | CaCO₃ |
| Iron(II) hydroxide | Dirty green | Fe(OH)₂ |
| Iron(III) hydroxide | Reddish brown | Fe(OH)₃ |
| Copper(II) hydroxide | Blue | Cu(OH)₂ |
The color matters in lab identification. If you're asked to identify an unknown solution, these colors are your clues.
What Determines If Something Precipitates?
Two factors:
- Solubility product constant (Ksp): Every slightly soluble salt has a Ksp. If the ion product exceeds the Ksp, precipitation occurs. If not, nothing falls out.
- Concentration: Dilute solutions might not exceed the Ksp even if the salt is technically insoluble. Concentrated solutions will precipitate more readily.
For most introductory chemistry, the solubility rules are sufficient. You only need Ksp calculations when the problem specifically asks for them.
The Bottom Line
Prediction is straightforward:
- Break reactants into ions
- Swap partners
- Apply solubility rules
- Whatever is insoluble is your precipitate
That's the entire process. No magic, no guessing. Know your solubility rules and you can predict any precipitation reaction in seconds.