Step 1: Mixing: Measure and combine equal volumes of the two chosen soluble salt solutions in a beaker. A visible cloudiness or solid formation indicates the reaction is occurring immediately.
Step 2: Filtration: Pour the mixture through filter paper held in a funnel. The insoluble salt (the residue) stays on the paper, while the remaining liquid (the filtrate) passes through into a flask.
Step 3: Washing: Rinse the residue on the filter paper with small amounts of distilled water. This step is vital to remove any traces of the filtrate or unreacted soluble ions that would contaminate the salt upon drying.
Step 4: Drying: Transfer the clean residue to a warm oven or leave it between sheets of filter paper. This removes all moisture, leaving behind a pure, dry sample of the insoluble salt.
| Feature | Precipitation (Insoluble Salts) | Titration/Excess Base (Soluble Salts) |
|---|---|---|
| Reactant State | Two soluble solutions | Acid + Alkali or Acid + Insoluble Base |
| Product State | Solid precipitate forms immediately | Salt remains dissolved in solution |
| Separation | Filtration (collecting the residue) | Evaporation/Crystallization (collecting the solute) |
| Purification | Washing with distilled water | Recrystallization |
Predicting Products: Always check the solubility of the potential products. If an exam question asks how to make a specific salt, first determine if it is soluble; if it is insoluble (like or ), you must use the precipitation method.
Writing Ionic Equations: Focus only on the ions that form the solid. For a general reaction , ensure the state symbols are correct, as is the defining characteristic of the product.
Verification of Purity: If asked why distilled water is used for washing, the answer is always to remove soluble impurities. Using tap water would introduce new ions (like or ) into the sample.
Starting with Insoluble Reactants: A common error is trying to react an insoluble solid with a solution to make another insoluble solid. This fails because the surface of the reactant becomes coated with the new product, stopping the reaction prematurely.
Skipping the Wash Step: Students often forget that the wet precipitate is soaked in a solution of other ions. Without washing, these ions will crystallize onto the product as it dries, resulting in an impure salt.
Confusing Residue and Filtrate: In this specific method, the desired product is the residue (the solid in the paper), not the filtrate. This is the opposite of many other chemical extractions.
