Double replacement reactions, sometimes called double displacement reactions, take the form AB + CD → AC + BD. Frequently such reactions produce products that are both soluble — that is, AC and BD both are water soluble. Fairly often, one reaction product is water-soluble, whereas the other product is insoluble. Question arises as to whether double replacement reactions can produce two insoluble substances.
The best starting conditions for a reaction producing two insoluble substances is to have them dissolved in water separately beforehand. Otherwise an insoluble coating could be formed on the surface of as-yet-undissolved solids. Thus, AB + H₂O → A⁺ + B⁻ and CD + H₂O → C⁺ + D⁻.
When Precipitates are Expected
This then gives, A⁺ + B⁻ + C⁺ + D⁻ → AC↓ + BD↓, where the down arrow means the substance is insoluble and precipitates out of solution. Rare as they are, such reactions can occur if solutions are very dilute, as is exemplified by combining barium hydroxide with ferrous sulfate, Ba(OH)₂ + FeSO₄ → BaSO₄↓ + Fe(OH)₂↓. Dilute solutions are required because barium hydroxide is only slightly soluble.
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