Solubility and precipitation — Chemistry, 14–17
Some substances dissolve only up to a limit. When dissolved particles can no longer stay in solution, a solid may form as a precipitate.
Dissolving has a limit
When a solid dissolves, its particles spread among the particles of the solvent. At a particular temperature, there is a maximum amount that can stay spread out; extra solid remains, or a new solid appears when two solutions react. That new solid is called a precipitate. Tu can often see it as cloudiness or grains.
Why chemists use precipitation
Chemists needed a way to separate invisible dissolved substances and identify unknown ions. A precipitation reaction solves both problems: a chosen reagent turns one dissolved ion into a solid, while other ions may remain in solution. The solid can then be filtered, washed and tested, rather than guessed from the clear liquid.
Making silver chloride
Mix 50.0 mL of 0.100 mol/L silver nitrate with 50.0 mL of 0.100 mol/L sodium chloride. Step 1: each solution contains 0.0500 L × 0.100 = 0.00500 mol of its useful ion. Step 2: Ag⁺ + Cl⁻ → AgCl(s), so they react 1:1. Step 3: 0.00500 mol of white AgCl precipitates; Na⁺ and NO₃⁻ stay dissolved.
A clear solution is not empty
A clear liquid looks as if it contains nothing, so it is reasonable to assume that no solid-forming particles are present. In fact, dissolved ions are still there, spread too finely to see. A precipitate appears only when their new combination is too insoluble to remain dispersed. Tu should write the ionic equation, not judge by appearance alone.
Water testing and purification
Precipitation is used to test water for ions such as chloride, sulfate or metal ions. It also helps remove unwanted substances: a treatment chemical makes a solid that traps impurities, and filters then remove it. Tu can find the same logic in laboratory analysis, wastewater treatment and some household water filters.
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