Acids, Bases and Salts – Preparation of Salts
Solubility Rules for Salts
Imagine a club where only certain members (ions) are allowed to stay in the water (the club). These rules help you predict whether a salt will dissolve or stay solid. Below is a quick reference guide, followed by some fun analogies and examples to make it easier to remember.
| Category | Generally Soluble | Exceptions (Insoluble) |
|---|
| Alkali & Ammonium | \$Na^+\$, \$K^+\$, \$NH_4^+\$ | None |
| Nitrates | All | None |
| Chlorides | All | \$PbCl_2\$, \$AgCl\$ |
| Sulfates | All | \$BaSO4\$, \$CaSO4\$, \$PbSO_4\$ |
| Carbonates | \$NaCO3\$, \$KCO3\$, \$NH4CO3\$ | All others (e.g., \$CaCO_3\$) |
| Hydroxides | \$NaOH\$, \$KOH\$, \$NH4OH\$, \$Ca(OH)2\$ (partial) | All others (e.g., \$Fe(OH)_3\$) |
Why These Rules Work
- 💧 Alkali and ammonium ions are like the “VIPs” of the water club – they always get in because they’re very comfortable in aqueous solutions.
- ⚡ Nitrates are the “universal pass” – every nitrate salt is soluble because the nitrate ion (\$NO_3^-\$) doesn’t like to hold onto other ions.
- 🧊 Chlorides, sulfates, carbonates and hydroxides have their own “bouncer” rules. Most are allowed, but some heavy metal ions (lead, silver, barium, calcium) are too heavy or form strong bonds that keep them out.
Quick Test: How to Predict Solubility
- Write down the cation (top ion) and anion (bottom ion).
- Check the cation against the “always soluble” list (Na⁺, K⁺, NH₄⁺).
- Check the anion against the “always soluble” list (NO₃⁻, Cl⁻, SO₄²⁻).
- Look for any exceptions (e.g., PbCl₂, CaCO₃).
- If it’s not an exception, the salt is soluble; otherwise, it’s insoluble.
Examples & Analogies
- 🧂 Table salt – \$NaCl\$ is soluble because both Na⁺ and Cl⁻ are “VIPs.”
- 🧪 Lead(II) chloride – \$PbCl_2\$ is insoluble; think of lead as a heavy backpack that can’t fit through the club’s doorway.
- 🧊 Calcium carbonate – \$CaCO_3\$ is insoluble; it’s like a stubborn rock that refuses to dissolve.
- 🧪 Calcium hydroxide – \$Ca(OH)_2\$ is only partially soluble; it’s a “partial‑member” that dissolves a bit but leaves a solid residue.
Remember: “If it’s not on the VIP list or an exception, it’s in!” Use this rule to quickly decide whether a salt will dissolve in water. Happy experimenting! 🎉