Preparing an Insoluble Salt by Precipitation

Introduction

Some salts do not dissolve in water. Barium sulfate, silver chloride and lead(II) iodide are examples. We cannot make these by crystallising a solution, because they will not stay in solution. Instead, we use their insolubility to our advantage: mix two solutions that each contain one of the ions needed, and the insoluble salt forms immediately as a solid called a precipitate . The solid can then be separated, washed and dried.

Core explanation

The idea. An insoluble salt is made of a positive ion and a negative ion that cling together too strongly to be pulled apart by water. If we bring those two ions together in solution, they combine and fall out as a solid. So we need:

- one soluble compound that supplies the metal ion , and - one soluble compound that supplies the non-metal ion .

Choosing reactants. Solubility rules guide the choice. All sodium, potassium and ammonium salts are soluble, and all nitrates are soluble, so these make ideal sources of ions. For example, to make insoluble barium sulfate:

BaCl₂(aq) + Na₂SO₄(aq) → BaSO₄(s) + 2NaCl(aq)

Barium chloride supplies Ba²⁺; sodium sulfate supplies SO₄²⁻. The other product, sodium chloride, is soluble and stays dissolved. Dilute sulfuric acid could replace sodium sulfate as the sulfate source.

To make silver chloride:

AgNO₃(aq) + NaCl(aq) → AgCl(s) + NaNO₃(aq)

Stage one: mix. The two solutions are mixed in a beaker and stirred. A cloudy solid appears at once — white for barium sulfate and silver chloride, bright yellow for lead(II) iodide.

Stage two: filter. The mixture is poured through a filter paper in a funnel. This time the product we want is the residue on the paper. The filtrate contains the soluble by-product and any leftover reactant ions.

Stage three: wash. The solid is still wet with solution containing soluble impurities such as sodium chloride. Rinsing it on the filter paper with small amounts of distilled water carries these impurities away. Several small rinses remove more than one large one.

Stage four: dry. The washed solid is left in a warm oven at a low temperature, or on fresh filter paper in a warm place, until dry.

Safety in outline. Many insoluble salts contain metals that are toxic, such as lead or barium compounds, and some reactant solutions are harmful or stain skin (silver nitrate). In school settings, eye protection and gloves are worn, small amounts are used and residues are disposed of as instructed, never down the sink.

Step-by-step reasoning

To plan a precipitation:

1. Write the formula of the insoluble salt and identify its two ions. 2. For the metal ion, choose a soluble nitrate of that metal. 3. For the non-metal ion, choose a soluble sodium or potassium salt containing it. 4. Mix, filter, wash with distilled water and dry the residue.

Visual explanation

Picture two clear, colourless liquids poured together. Instantly the mixture turns milky white, like skimmed milk. Through the filter, the white solid collects as a paste on the paper while clear liquid drips through. Distilled water trickles over the paste, then the paper is opened on a warm tray.

Real-world analogy

Precipitation is like a dance in a busy hall. Two strangers who happen to be perfect partners meet, lock arms and leave the dance floor together, while everyone else keeps moving around. The partners are the ions of the insoluble salt; the others are spectator ions.

Real-world example

Barium sulfate is made by precipitation for medical use. It is swallowed as a "barium meal" before X-ray imaging of the digestive system. Although barium ions are toxic, barium sulfate is so insoluble that almost none is absorbed, while the dense barium blocks X-rays and outlines the gut.

Why?

Why must the precipitate be washed? When it is filtered, the solid is still coated with filtrate containing dissolved ions. If it were dried straight away, those soluble salts would crystallise on the product and make it impure.

Common misconception

"A precipitate forms because one of the solutions was already cloudy." Both starting solutions are clear. The solid appears only when the two particular ions meet and combine to form an insoluble compound.

Worked example

Question: Choose reactants and write the equation to prepare lead(II) iodide, PbI₂.

Reasoning: Pb²⁺ comes from soluble lead(II) nitrate; I⁻ from soluble potassium iodide. Two iodide ions are needed per lead ion, so two KI.

Answer: Pb(NO₃)₂(aq) + 2KI(aq) → PbI₂(s) + 2KNO₃(aq). The yellow precipitate is filtered, washed with distilled water and dried.

Quick check

1. In a precipitation preparation, is the product in the residue or the filtrate? Answer: In the residue, because the insoluble salt is trapped on the filter paper.

Exam focus

State that both reactants must be soluble and the product insoluble. Examiners reward the four stages — mix, filter, wash with distilled water, dry — and balanced equations with correct state symbols, especially (s) for the precipitate.

Advanced insight

Particle size depends on how fast the solid forms. Mixing dilute solutions slowly, often while warm, gives larger particles that filter more easily, whereas rapid mixing of concentrated solutions gives very fine particles that can pass through or clog filter paper. Analytical chemists use this in gravimetric analysis, where a precipitate is weighed to find how much of an ion was present.

Summary

Insoluble salts are made by mixing two solutions: one containing the metal ion and one containing the non-metal ion. The insoluble salt forms as a precipitate, which is filtered off, washed with distilled water to remove soluble impurities, and dried. Nitrates and sodium or potassium salts are convenient soluble sources of ions.

Practice questions

1. Suggest two solutions that could be mixed to make calcium carbonate. Answer: Calcium nitrate (or calcium chloride) solution and sodium carbonate solution. 2. Write the equation for making silver bromide from silver nitrate and potassium bromide. Answer: AgNO₃(aq) + KBr(aq) → AgBr(s) + KNO₃(aq) 3. Why is distilled water, rather than tap water, used to wash the precipitate? Answer: Tap water contains dissolved ions that could leave impurities or form further precipitates on the product. 4. Why is barium carbonate not a good choice as a source of barium ions for this method? Answer: Barium carbonate is insoluble, so it cannot supply free barium ions in solution.