Why do only some mixtures form a solid?
In 1.8.6 you learned that a double replacement reaction happens only when a precipitate, a gas, or water forms, and you met a few solubility patterns. Arrange those patterns in a table and you can see at a glance whether any pair of ions can stay together in solution. This lesson shows how to read that table, gives the solubility rules used in US textbooks, and then puts precipitates to work as detectives: which ion is in an unlabeled bottle?
This lesson has two parts:
- Solubility and precipitation reactions: the solubility table and rules, predicting whether mixing two solutions forms a precipitate, and the colors of common precipitates.
- Testing for ions: tests for Cl⁻, SO₄²⁻, and CO₃²⁻, and why other ions have to be ruled out.
A short practice set follows Part 1; do it before you start Part 2.
Part 1: Solubility and precipitation reactions
The solubility table
How well a substance dissolves in water is its solubility溶解性. Chinese textbooks sort compounds by how many grams dissolve in 100 g of water at 20 °C: more than 10 g is soluble (易溶), 1–10 g is moderately soluble (可溶), 0.01–1 g is slightly soluble (微溶), and less than 0.01 g is insoluble (难溶). The table itself uses just three marks: soluble, slightly soluble, and insoluble.
The Chinese textbook table, “Solubility of Some Acids, Bases, and Salts (Room Temperature),” lists 14 cations against 5 anions (OH⁻, NO₃⁻, Cl⁻, SO₄²⁻, CO₃²⁻). The interactive table below starts from it, adds the Pb²⁺ row and PO₄³⁻ column that show up in US solubility rules, and drops the Mn²⁺ row.
A dash (—) means one of two things. The ions may not be able to coexist in water: Al³⁺ and CO₃²⁻ react to form Al(OH)₃ and CO₂, so Al₂(CO₃)₃ cannot be made this way. Or the compound may be too unstable: AgOH breaks down into Ag₂O as soon as it forms.
The solubility rules
US textbooks usually turn the table into rules: first what is always soluble, then the exceptions.
- Compounds of Group 1 cations (Na⁺, K⁺, …) and of NH₄⁺ are soluble.
- Nitrates (NO₃⁻) and acetates (CH₃COO⁻) are soluble.
- Chlorides, bromides, and iodides are soluble, except those of Ag⁺, Pb²⁺, and Hg₂²⁺.
- Sulfates are soluble, except BaSO₄ and PbSO₄ (insoluble) and CaSO₄ and Ag₂SO₄ (slightly soluble).
- Carbonates and phosphates are insoluble, except those of Group 1 cations and NH₄⁺.
- Hydroxides are insoluble, except those of Group 1 cations and Ba(OH)₂; Ca(OH)₂ is slightly soluble.
These are the same patterns as in 1.8.6, extended to more ions such as Br⁻, I⁻, and PO₄³⁻, plus the common exception Pb²⁺.
Tap a cell to see the compound and the rule behind it, or switch to Quiz me to test yourself:
| OH⁻ | NO₃⁻ | Cl⁻ | SO₄²⁻ | CO₃²⁻ | PO₄³⁻† | |
|---|---|---|---|---|---|---|
| H⁺ | ||||||
| NH₄⁺ | ||||||
| K⁺ | ||||||
| Na⁺ | ||||||
| Ba²⁺ | ||||||
| Ca²⁺ | ||||||
| Mg²⁺ | ||||||
| Al³⁺ | ||||||
| Zn²⁺ | ||||||
| Fe²⁺ | ||||||
| Fe³⁺ | ||||||
| Cu²⁺ | ||||||
| Ag⁺ | ||||||
| Pb²⁺† |
S = soluble, sS = slightly soluble, I = insoluble, v = volatile, — = does not exist or breaks down in water. The row and column marked † come from US solubility rules and are not in the Chinese textbook table. Tap a cell for details.
Precipitation reactions
When solutions of two soluble ionic compounds are mixed, the ions can swap partners. If a new combination is insoluble, a solid forms. This is a precipitation reaction沉淀反应, and the solid is a precipitate沉淀. To predict one:
- List the ions in each solution.
- Swap partners and write the formulas of the two new combinations.
- Check the rules. If one is insoluble, it precipitates; if both are soluble, write NR.
- Write the balanced equation, marking the precipitate (s) and everything dissolved (aq).
Precipitate colors help identify them: AgCl, BaSO₄, CaCO₃, BaCO₃, and Mg(OH)₂ are white, Al(OH)₃ is white and gelatinous, Cu(OH)₂ is blue, and Fe(OH)₃ is reddish-brown. Fe(OH)₂ starts out white but quickly turns gray-green and then reddish-brown in air.
Part 1 practice
- CaCO₃
Carbonates are insoluble except with Group 1 cations and NH₄⁺. CaCO₃ is limestone and marble.
- Ba(NO₃)₂
All nitrates are soluble.
- AgCl
AgCl is the classic exception among chlorides.
- CaSO₄
CaSO₄ is slightly soluble. It is the main compound in gypsum.
- K₃PO₄
Compounds of K⁺ are always soluble.
- Mg(OH)₂
Most hydroxides are insoluble; Mg(OH)₂ is a white precipitate.
- (NH₄)₂CO₃
Ammonium compounds are always soluble.
- BaSO₄
BaSO₄ is an insoluble sulfate. It is the “barium meal” used for X-rays of the digestive tract.
- Ca(OH)₂
Ca(OH)₂ is slightly soluble; its saturated solution is limewater.
Part 2: Testing for ions
- Cl⁻: add AgNO₃ solution and dilute nitric acid. A white precipitate of AgCl that does not dissolve in the acid shows Cl⁻. The acid rules out CO₃²⁻.
- SO₄²⁻: add dilute hydrochloric acid first (nothing should happen), then BaCl₂ solution. A white precipitate of BaSO₄ shows SO₄²⁻. Adding the acid first rules out CO₃²⁻ and Ag⁺.
- CO₃²⁻: add dilute hydrochloric acid. A gas forms that turns limewater cloudy.
Chinese textbooks write these equations with ↓ for a precipitate and ↑ for a gas instead of state symbols.
Be a detective. Each bottle holds one of four sodium salts. Identify it in as few tests as you can:
Example 1: does a precipitate form?
Predict whether a precipitate forms when each pair of solutions is mixed. If one does, write the balanced equation.
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The method. Swap partners to get the possible products, then check the solubility rules: if any product is insoluble, a precipitate forms. Check the products, not the reactants.
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FeCl₃(aq) and NaOH(aq). Swapping gives Fe(OH)₃ and NaCl. Hydroxides are insoluble except with Group 1 cations and Ba²⁺, so Fe(OH)₃ forms a reddish-brown precipitate. Fe³⁺ needs three OH⁻:
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K₂SO₄(aq) and NaNO₃(aq). Swapping gives KNO₃ and Na₂SO₄. Group 1 compounds are all soluble, so there is no precipitate, gas, or water: NR. The four ions just mix.
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Na₃PO₄(aq) and CaCl₂(aq). Swapping gives Ca₃(PO₄)₂ and NaCl. Phosphates are insoluble except with Group 1 cations and NH₄⁺, so white Ca₃(PO₄)₂ precipitates. Write Ca₃(PO₄)₂ from the charges first (three Ca²⁺ for two PO₄³⁻), then balance:
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Check. In the last equation, Na: 2 × 3 = 6 on the left and 6 on the right; Cl: 3 × 2 = 6 on each side.
Example 2: which ions are present?
A colorless solution may contain any of Cl⁻, SO₄²⁻, and CO₃²⁻. A sample is tested in this order: (1) excess dilute nitric acid is added, and bubbles form; (2) excess Ba(NO₃)₂ solution is added, and a white precipitate forms; (3) the mixture is filtered, and AgNO₃ solution added to the filtrate gives a white precipitate. Which ions are present?
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Step 1. Bubbles with acid show CO₃²⁻:
The acid is in excess, so all the carbonate is removed and can’t form BaCO₃ or Ag₂CO₃ later.
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Step 2. The solution is now acidic, so a white precipitate with Ba²⁺ can only be BaSO₄: SO₄²⁻ is present. The excess Ba²⁺ also removes all the SO₄²⁻, so it can’t form slightly soluble Ag₂SO₄ in the next step.
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Step 3. Filtering removes the BaSO₄ first. A new white precipitate with Ag⁺ is AgCl, so Cl⁻ is present.
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Conclusion. All three ions are present.
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Why these reagents. Steps 1 and 2 used nitric acid and barium nitrate, not hydrochloric acid and barium chloride. Adding Cl⁻ earlier would make step 3 useless. The order is: remove each interfering ion, then test.
Common mistakes
Mistake 1: “Insoluble means none of it dissolves.” Insoluble means very little dissolves (less than 0.01 g per 100 g of water at 20 °C), not none. A tiny amount of AgCl does dissolve, and AP Chemistry calculates exactly how much with the solubility product, Ksp.
Mistake 2: “A white precipitate with AgNO₃ proves Cl⁻ is present.” CO₃²⁻ also precipitates with Ag⁺. Add dilute nitric acid: AgCl stays, but Ag₂CO₃ dissolves and gives off gas.
Mistake 3: “For the sulfate test, the order of BaCl₂ and HCl doesn’t matter.” If the solution contains Ag⁺, adding BaCl₂ first forms AgCl, a white precipitate that also does not dissolve in acid, and it looks just like BaSO₄. Add the hydrochloric acid first; if nothing precipitates, then add BaCl₂.
Quick check
Which pair of solutions forms a precipitate when mixed?
Ba²⁺ and SO₄²⁻ form insoluble BaSO₄. In the other three pairs, the swapped compounds are all soluble, so there is no reaction.
Adding AgNO₃ solution to a sample gives a white precipitate. When dilute nitric acid is added, the precipitate dissolves and bubbles form. Which ion was most likely in the sample?
Ag₂CO₃ dissolves in nitric acid and releases CO₂. An AgCl precipitate would not dissolve.
Summary
- A solubility table shows how soluble each cation–anion combination is; a dash means the compound does not exist or breaks down in water.
- The rules: Group 1, NH₄⁺, and nitrate compounds are soluble; chlorides are soluble except with Ag⁺ and Pb²⁺; sulfates except with Ba²⁺ and Pb²⁺; most carbonates, phosphates, and hydroxides are insoluble.
- Mixing two solutions gives a precipitate if a swapped combination is insoluble. Precipitates and gases identify Cl⁻, SO₄²⁻, and CO₃²⁻, and the order of the reagents prevents false results.
On to AP (for later)
AP Chemistry expects you to know only that all salts of Na⁺, K⁺, NH₄⁺, and NO₃⁻ are soluble; any other solubility you need will be given. AP questions do ask you to identify a precipitate from a particle diagram or lab observations and to write the net ionic equation (next lesson). Unit 7 uses Ksp to calculate how much of an “insoluble” compound actually dissolves.