the idea
In water, ionic compounds stop being compounds and start being crowds of ions.
Dissolve NaCl and you don't have NaCl molecules floating around. You have Na⁺ and Cl⁻ ions, each surrounded by water. Ions in solution carry current, so the solution is an electrolyte. A reaction happens only when mixing two solutions lets some ions form something that leaves the crowd: a solid (precipitate), water, or a gas.
Plant version: your CIP system already relies on this. Caustic concentration is read by a conductivity probe because NaOH is a strong electrolyte: more dissolved ions, more current. And milkstone is a precipitation reaction: calcium and phosphate ions from milk crash out as insoluble calcium phosphate on hot surfaces, which is why it builds up worst in the heating section.
Strong electrolyte
Fully ionized: soluble ionic compounds, strong acids, strong bases. NaCl, HCl, NaOHWeak electrolyte
Only a few % ionized: weak acids and bases. acetic acid, lactic acid, NH₃Nonelectrolyte
Dissolves as whole molecules, no ions. sugar, ethanolsort it
Strong, weak, or non?
Memorize the strong acids (HCl, HBr, HI, HNO₃, HClO₃, HClO₄, H₂SO₄) and the strong bases (Group 1 hydroxides plus Ca, Sr, Ba hydroxides). Every other acid or base you meet in this course is weak until proven otherwise.
reference
Solubility rules
| Ion | Usually… | Exceptions |
|---|---|---|
| Group 1 (Na⁺, K⁺…), NH₄⁺ | soluble | none worth knowing |
| NO₃⁻, CH₃COO⁻ | soluble | none |
| Cl⁻, Br⁻, I⁻ | soluble | Ag⁺, Hg₂²⁺, Pb²⁺ |
| SO₄²⁻ | soluble | Sr²⁺, Ba²⁺, Hg₂²⁺, Pb²⁺ (CaSO₄ only slightly soluble) |
| S²⁻ | insoluble | NH₄⁺, Group 1, Ca²⁺, Sr²⁺, Ba²⁺ |
| CO₃²⁻, PO₄³⁻ | insoluble | NH₄⁺, Group 1 |
| OH⁻ | insoluble | NH₄⁺, Group 1, Ca²⁺, Sr²⁺, Ba²⁺ |
try it
Mix two solutions
Pick two. The ions swap partners. If a new pairing is insoluble, it precipitates. If H⁺ meets OH⁻, water forms. If H⁺ meets carbonate, CO₂ bubbles out. Anything left floating is a spectator.
+
practice
Predict the product, write the net ionic
engineering version
Why acid washes work on scale and milkstone
Hard-water scale is mostly CaCO₃, and milkstone is largely calcium phosphate plus protein. Both are insoluble in neutral water. Acid changes that:
- Carbonate scale: CaCO₃(s) + 2H⁺(aq) → Ca²⁺(aq) + H₂O(l) + CO₂(g). A gas-forming reaction. The fizz is the scale leaving.
- Phosphate milkstone: Ca₃(PO₄)₂(s) + 4H⁺(aq) → 3Ca²⁺(aq) + 2H₂PO₄⁻(aq). Protonating the phosphate turns it into a soluble ion, so the solid dissolves.
- That's why the order is caustic first (to remove the protein and fat glued over the mineral) and acid second (to dissolve the mineral underneath).
exam traps
Common mistakes
- Splitting solids, liquids, gases, or weak electrolytes into ions. Only strong electrolytes in solution (aq) get split.
- Splitting polyatomic ions apart. SO₄²⁻ stays SO₄²⁻.
- Forgetting to reduce the net ionic equation to the lowest whole-number coefficients.
- Calling a mix "no reaction" when water, a gas, or a weak electrolyte forms. Precipitates aren't the only driving force.
- Leaving out phases. Net ionic equations need (s), (l), (g), and (aq).