What Is a Precipitate? A Solid Appearing Out of a Clear Solution
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Mixing two clear liquids and watching a solid appear is one of the more satisfying things that happens in a chemistry laboratory. The solid was not hidden in either liquid, since it forms at the moment of mixing from ions that were separately dissolved.
Why a solid appears
An ionic compound dissolved in water exists as separate charged particles surrounded by water molecules rather than as intact units, so a solution of one salt and a solution of another contain four kinds of ion between them. Mixing brings every combination into contact, and if any pairing forms a compound that water cannot keep dissolved, those ions leave the solution and build a solid instead. Solubility is the deciding property, and it varies enormously between compounds for reasons involving the strength of attraction between the ions and the strength of their interaction with water. The solid forms when the concentrations exceed what the solution can hold, which means the same combination may precipitate at high concentration and stay dissolved at low, and that dependence is exploited deliberately in separating and purifying materials.
What it is used for
The phenomenon does real work across chemistry and beyond:
- •Identifying ions, since a characteristic colour and appearance of solid indicates what was present, which is the basis of classical qualitative analysis
- •Measuring quantities, by collecting, drying and weighing the solid formed, which is one of the oldest accurate methods of analysis
- •Purification, since a target compound can be precipitated away from impurities that stay dissolved
- •Water treatment, where dissolved contaminants including phosphate and heavy metals are removed by adding something that precipitates them
- •Making pigments, many of which are prepared by deliberately precipitating an insoluble coloured compound
- •Photographic chemistry, which depended entirely on precipitating light-sensitive silver compounds
Controlling what forms
The same reaction can produce a fine suspension that will not settle or large crystals that filter easily, and the difference is controlled rather than accidental. Rapid mixing at high concentration produces enormous numbers of tiny particles, since many nuclei form at once and none has material to grow on, and the resulting solid is difficult to filter and traps impurities. Slow addition at lower concentration allows fewer nuclei to form and grow larger, producing a solid that filters cleanly and is purer. Heating generally helps by increasing solubility slightly and allowing crystals to redissolve and reform, which is called digestion and improves the product substantially. Stirring, temperature, order of addition and the presence of other ions all influence the outcome, and the procedures in analytical methods specify these steps in detail for exactly that reason rather than out of tradition.
Predicting whether one forms
Chemists use a compact set of rules for whether a given combination will produce a solid, learned early and applied constantly. Compounds of the alkali metals and of ammonium are soluble almost without exception, as are nitrates, which is why these are the standard choices when a reaction needs an ion in solution without anything else precipitating. Chlorides are generally soluble apart from a short list of exceptions including silver and lead. Sulphates are generally soluble apart from barium, lead and calcium. Carbonates, phosphates, sulphides and hydroxides are generally insoluble apart from the alkali metals and ammonium, which is the reverse pattern. Committing those rules to memory allows a prediction for any pairing, and the exceptions are few enough to learn as a list. Beyond the rules, a quantity called the solubility product gives the exact concentration at which a given compound begins to come out of solution.
Where it happens outside a laboratory
The same chemistry operates at scales from the kitchen to the planet. Limescale in kettles and pipes is calcium carbonate precipitating as heating drives off dissolved carbon dioxide and reduces solubility, which is why the problem is worse in hard water areas and worse where water is heated. Kidney stones form when compounds in urine exceed their solubility, and the treatments aim at keeping concentrations below that point. Caves and their formations are built by dissolved limestone precipitating as water evaporates or loses carbon dioxide, over intervals long enough to produce structures metres in length. Marine organisms precipitate calcium carbonate to build shells and reefs, and ocean acidification threatens that by shifting the chemistry against it. And enormous mineral deposits, including several of economic importance, formed when hot mineral-rich water cooled or changed chemistry and dropped its load.
The takeaway
Dissolved salts exist as separate ions, so mixing two solutions brings new pairings into contact and any pairing water cannot hold leaves as a solid. Rapid mixing gives fine particles that filter badly and trap impurities, while slow addition gives large clean crystals. The same chemistry produces limescale, kidney stones, cave formations, shells and several major mineral deposits.