Solubility, precipitation and insoluble salts

The following rules apply to common substances in water. Check both parts of a compound’s name: sodium carbonate, for example, is soluble because it is a sodium salt, despite most carbonates being insoluble.

The hydroxide rule is a general classification: calcium hydroxide is slightly soluble, so a small amount can dissolve in water.

These rules also help us choose starting solutions. If we need silver ions, silver nitrate is suitable because all nitrates are soluble. If we need chloride ions, sodium chloride is suitable because all common sodium salts are soluble.

Predicting and naming a precipitate

To predict what happens when two salt solutions are mixed, identify the positive and negative ions supplied by each solution. Consider the new combinations of positive and negative ions, then check each possible product against the solubility rules. An insoluble product forms a precipitate; soluble substances remain dissolved.

For example, mixing silver nitrate solution with sodium chloride solution gives silver chloride and sodium nitrate:

$$\text{silver nitrate} + \text{sodium chloride} \rightarrow \text{silver chloride} + \text{sodium nitrate}$$

Silver chloride is one of the insoluble chloride exceptions, so silver chloride is the precipitate. Sodium nitrate stays dissolved because all nitrates are soluble.

The balanced symbol equation makes this distinction clear:

$$\mathrm{AgNO_3(aq) + NaCl(aq) \rightarrow AgCl(s) + NaNO_3(aq)}$$

Here, $(aq)$ means dissolved in water and $(s)$ means solid. Silver ions and chloride ions form the solid, while sodium ions and nitrate ions remain in solution.

Mixing two solutions does not always produce a precipitate. Consider sodium chloride solution and potassium nitrate solution. The alternative pairings are sodium nitrate and potassium chloride. Both are soluble, so the ions remain dissolved and no precipitate forms.

Preparing a pure, dry insoluble salt

An insoluble salt can be made by precipitation, then separated from the liquid. To prepare silver chloride, use silver nitrate solution to supply silver ions and sodium chloride solution to supply chloride ions.

The apparatus needed includes a beaker, a stirring rod, a filter funnel and filter paper, a vessel to collect the filtrate, and distilled water for washing. Wear eye protection when handling the solutions.

1. Mix the two solutions in a beaker and stir. Silver chloride forms as a precipitate. The mixture also contains water and dissolved substances, including sodium nitrate.
2. Filter the mixture through filter paper in a funnel. Collect the liquid beneath the funnel. The insoluble silver chloride stays on the paper as the residue; water and dissolved substances pass through as the filtrate.
3. Wash the residue with distilled water. Pour small amounts of distilled water over the solid and allow them to drain through. This removes soluble impurities, including sodium nitrate and any remaining soluble reactants. The insoluble silver chloride remains on the paper. Distilled water avoids introducing additional dissolved salts.
4. Leave the washed solid to dry on filter paper. Water evaporates, leaving a dry sample of the insoluble salt.

Filtration, washing and drying do different jobs. Filtration separates the solid from the solution; washing makes it purer by removing soluble contamination; drying removes the remaining water. There is no need to evaporate the whole reaction mixture to obtain the salt: the desired product is already a solid.

Identify the ions → consider the new pairings → check solubility → name any insoluble product.

An insoluble product forms a precipitate. If all possible products are soluble, no precipitate forms.

Preparing an insoluble salt

Mix → filter → wash with distilled water → dry.

• Mix suitable soluble reactants to form the precipitate.
• Filter: keep the residue, not the filtrate.
• Wash: remove soluble impurities.
• Dry: allow the remaining water to evaporate.