Preparing salts, whether soluble or insoluble, involves specific chemical reactions tailored to the solubility of the desired product. Soluble salts are typically prepared by reacting an acid with a suitable reactant, followed by crystallization. Insoluble salts are prepared through a process called precipitation.
Methods for Preparing Salts
The method used to prepare a salt depends directly on whether the salt is soluble or insoluble in water. Understanding solubility rules is crucial before attempting salt preparation.
Preparing Soluble Salts
Soluble salts can be made by reacting acids with soluble or insoluble reactants. The specific reactant chosen determines the technique used. Common reactions include:
- Acid + Metal: (if the metal is reactive enough) e.g., Zinc + Sulfuric acid -> Zinc sulfate + Hydrogen
- Acid + Metal Oxide: e.g., Copper(II) oxide + Sulfuric acid -> Copper(II) sulfate + Water
- Acid + Metal Hydroxide (Alkali): e.g., Sodium hydroxide + Hydrochloric acid -> Sodium chloride + Water
- Acid + Metal Carbonate: e.g., Calcium carbonate + Hydrochloric acid -> Calcium chloride + Water + Carbon dioxide
Procedure Steps (General for insoluble reactants like oxides/carbonates/metals):
- Reaction: React the acid with an excess of the solid reactant (e.g., metal oxide, carbonate) until no more solid dissolves or no more gas is produced (if using a carbonate). Using excess solid ensures all the acid reacts.
- Filtering: Filter the mixture to remove the excess insoluble solid. The filtrate contains the soluble salt solution.
- Evaporation: Heat the filtrate gently to evaporate most of the water. Stop heating when crystals just begin to form around the edge of the solution (saturation point).
- Cooling: Allow the concentrated solution to cool slowly. Crystals of the soluble salt will form as the solution cools.
- Drying: Filter the crystals from the remaining solution (mother liquor) and dry them, often between filter paper or in a warm oven.
Special Case: Using Soluble Reactants (Acid + Alkali)
If both reactants are soluble (like an acid and a soluble metal hydroxide/alkali), you cannot use excess solid and filter. Instead, the preparation requires a technique called titration. Titration must be used if the reactants are soluble.
- Titration: Carefully add one reactant (e.g., alkali) from a burette to a measured volume of the other reactant (acid) in a flask, using an indicator to determine the exact volume needed for complete neutralization.
- Preparation: Repeat the reaction without the indicator, using the precise volumes determined by the titration.
- Crystallization: Evaporate, cool, and dry the resulting soluble salt solution as described in steps 3-5 above.
Preparing Insoluble Salts
Insoluble salts are made by precipitation reactions. This method involves mixing solutions of two soluble salts. When the solutions are mixed, the ions rearrange, and if one of the possible new combinations forms an insoluble compound, it will appear as a solid precipitate.
Procedure Steps:
- Solution Preparation: Prepare separate solutions of two soluble salts, one containing the cation needed for the insoluble salt and the other containing the anion needed. For example, to make insoluble barium sulfate (BaSO₄), you could mix solutions of soluble barium chloride (BaCl₂) and soluble sodium sulfate (Na₂SO₄).
- Mixing: Mix the two solutions together. The insoluble salt will immediately form as a precipitate (a cloudy solid).
- Filtering: Filter the mixture to separate the solid insoluble salt from the soluble spectator ions remaining in the solution.
- Washing: Wash the precipitate on the filter paper with distilled water to remove any traces of the soluble salts or spectator ions.
- Drying: Dry the insoluble salt, often in a warm oven or by leaving it in a warm place, to obtain a pure sample.
Example Precipitation Reaction:
Mixing Lead(II) nitrate solution with Potassium iodide solution produces insoluble Lead(II) iodide precipitate:
Pb(NO₃)₂(aq) + 2KI(aq) → PbI₂(s) + 2KNO₃(aq)
Here, Lead(II) iodide (PbI₂) is the insoluble salt that precipitates out. Potassium nitrate (KNO₃) is soluble and remains in solution.
Summary Table
| Salt Type | Preparation Method(s) | Reactants | Key Technique for Soluble Reactants |
|---|---|---|---|
| Soluble | Reaction of Acid + Base/Metal/Carbonate/Metal Oxide | Acid + soluble/insoluble base, reactive metal, metal oxide/carbonate | Titration (if reactants are soluble) |
| Insoluble | Precipitation Reaction | Mixing solutions of two soluble salts | N/A |
Understanding the solubility of the desired salt is the first step in choosing the appropriate preparation method.
