Prepare an Insoluble Salt
2 guided lessons · 18 checks · 2. Inorganic Chemistry → Acids, Bases & Salt Preparations
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The precipitation reaction pattern
An insoluble salt is prepared by precipitation, in which two clear solutions are mixed and a solid forms and sinks. The solid that appears is the precipitate, and it is the salt itself. The reaction pairs two soluble starting solutions and swaps their partners, so one product turns solid while the other stays dissolved.
What you know already
From Solubility Rules you already know which salts dissolve and which do not. This note uses that table as a design tool, choosing two soluble reactants whose swapped partners give the insoluble salt you want to collect.
Key points of understanding
- Insoluble salts are made using a precipitation reaction.
- The solid salt formed must be insoluble in water for the method to work.
- Two soluble salts swap partners: soluble salt 1 plus soluble salt 2 gives the insoluble salt plus a third soluble salt.
- In symbol form the pattern is AB plus CD giving AD plus CB.
- The partner-swapping pattern is why chemists also call it double decomposition.
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1.The precipitation reaction pattern
An insoluble salt is prepared by precipitation, in which two clear solutions are mixed and a solid forms and sinks. The solid that appears is the precipitate, and it is the salt itself. The reaction pairs two soluble starting solutions and swaps their partners, so one product turns solid while the other stays dissolved.
Key points
- Insoluble salts are made using a precipitation reaction.
- The solid salt formed must be insoluble in water for the method to work.
- Two soluble salts swap partners: soluble salt 1 plus soluble salt 2 gives the insoluble salt plus a third soluble salt.
- In symbol form the pattern is AB plus CD giving AD plus CB.
- The partner-swapping pattern is why chemists also call it double decomposition.
First pass
Mix two clear solutions whose salts can swap partners, and the wanted insoluble salt falls out as a solid. Everything else stays dissolved, so the solid can be filtered off as the product.
Analogy — Two dancing couples swapping partners is the picture: each dancer leaves with a new partner, and one of the new pairs, the insoluble salt, immediately sits down out of the dance.
Exam pointer — Use the AB plus CD pattern to predict products quickly in precipitation questions, then check the solubility table to see which product falls out.
Going deeper
The method only works when the wanted product is insoluble, because the solid that separates is the product and is collected as such. If both possible products were soluble, nothing would separate and there would be no solid to collect, which is why the solubility table decides whether this route is available at all.
Analogy — It is like mixing two teams where only one new pairing is incompatible: the incompatible pair walks off the pitch together, which is exactly the solid you wanted to remove from the game.
Exam pointer — Prediction questions usually hinge on the solubility rules, so name the insoluble product explicitly rather than saying a precipitate forms.
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2.Recovering and drying the precipitate
A fixed volume of the first solution is measured out, and the second solution is added until it sits in a slight excess, which squeezes the maximum amount of solid out of the mixture. The precipitate is collected on a filter, washed with distilled water to strip away traces of the other chemicals, and then left to dry. Silver and lead(II) salts suit this method, and their nitrates make convenient starting materials because every nitrate dissolves.
Key points
- One salt solution is measured out as a fixed volume.
- The second solution is added until it is in slight excess.
- The slight excess ensures the maximum amount of precipitate.
- Filtering collects the solid precipitate from the mixture.
- Washing with distilled water removes reactants contaminating the residue, which then dries.
First pass
Measure one solution, stir in the second until there is a little too much of it, then filter the solid out. Wash the solid with distilled water and leave it to dry, giving the pure, dry insoluble salt.
Analogy — The slight excess is like inviting one extra guest so nobody leaves alone: it guarantees every particle of the first salt finds a partner and joins the solid.
Exam pointer — Method marks live in the order: measure, mix to slight excess, filter, wash, dry. Quote them in that order.
Going deeper
A slight excess of the second solution drives the shared-ion product all the way out of solution, maximising the yield of solid. Washing matters because the wet residue is soaked in the soluble by-product and leftover reactants, and only distilled water can carry those away without adding fresh impurities of its own.
Analogy — Washing the precipitate is like rinsing a jumper after dyeing it: the garment keeps its shape and colour, but the loose dye and soap of the process leave in the rinse water.
Exam pointer — If asked why distilled water rather than tap water, say distilled water carries no dissolved minerals that could add new impurities to the product.
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