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C. Chemical Reactions · Chemical Reactions

Double Displacement: Precipitation Reactions

C2 C3 A1.12About 70 minutes
Learning goals
  • I can predict the products of double displacement reactions by swapping ions.
  • I can use a solubility table to decide whether a precipitate forms or there is no reaction.
  • I can write balanced equations with correct state symbols for precipitation reactions.
  • I can describe uses of precipitation reactions, such as medical imaging and water treatment.

Words to know

double displacement reaction
Two ionic compounds exchange ions: AB + CD → AD + CB.
precipitate
An insoluble solid that forms when two solutions are mixed.
soluble / low solubility
Dissolves well in water / only a tiny amount dissolves, so the compound appears as a solid. Chemists often say "insoluble" for low solubility.
solubility table
A table of general rules that tells which ionic compounds are soluble in water and which have low solubility.
aqueous
Dissolved in water, written (aq).

Two clear liquids make a solid

Pour clear, colourless lead(II) nitrate solution into clear, colourless potassium iodide solution and a brilliant yellow solid suddenly appears, as if from nowhere. This is a precipitation reaction, one of the three kinds of double displacement reaction (the other two, which form a gas or water, are in the next lesson).

In Grade 10 you used a simplified solubility guide to predict precipitates. In Grade 11 you will use a fuller solubility table, write formulas with multivalent metals and polyatomic ions, and explain what the ions are doing.

What really happens when the solutions mix

An ionic compound dissolved in water is not made of "molecules". It has separated into ions that move freely among the water molecules. Silver nitrate solution contains Ag⁺ and NO₃⁻ ions; sodium chloride solution contains Na⁺ and Cl⁻ ions.

When you mix the two solutions, all four kinds of ions are together. If any positive ion and negative ion form a compound with low solubility, they pull together into a solid crystal and fall out of the solution as a precipitate. Here Ag⁺ and Cl⁻ form silver chloride, which has very low solubility. The Na⁺ and NO₃⁻ ions stay dissolved; they are spectator ions.

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

silver nitrate(aq)sodium chloride(aq)after mixingAg⁺Ag⁺NO₃⁻NO₃⁻Na⁺Na⁺Cl⁻Cl⁻mixNa⁺Na⁺NO₃⁻NO₃⁻AgCl(s)Ag⁺ and Cl⁻ pair up as a solid;Na⁺ and NO₃⁻ stay dissolvedEach solution is clear, with ions spread out in water.
Mixing silver nitrate and sodium chloride solutions. Silver ions and chloride ions form solid silver chloride; sodium and nitrate ions are spectators and stay dissolved.
IonSolubility of its compoundsExceptions
Group 1 ions (Li⁺, Na⁺, K⁺ ...), ammonium NH₄⁺solublenone (for this course)
nitrate NO₃⁻, chlorate ClO₃⁻, acetate CH₃COO⁻solublenone for nitrate and chlorate; silver acetate is only slightly soluble
chloride Cl⁻, bromide Br⁻, iodide I⁻solublelow solubility with Ag⁺, Pb²⁺, Hg₂²⁺, Cu⁺
sulfate SO₄²⁻solublelow solubility with Ca²⁺, Sr²⁺, Ba²⁺, Pb²⁺, Ag⁺ (some only slightly soluble)
hydroxide OH⁻low solubilitysoluble with group 1, NH₄⁺, Sr²⁺, Ba²⁺; Ca(OH)₂ slightly soluble
sulfide S²⁻low solubilitysoluble with group 1, NH₄⁺ and group 2 ions
carbonate CO₃²⁻, phosphate PO₄³⁻, sulfite SO₃²⁻low solubilitysoluble with group 1 and NH₄⁺
A solubility table for common ionic compounds in water at room temperature. You will explore what "low solubility" means in numbers in the Solutions and Solubility unit.

Predicting a double displacement

  1. Write the ions in each reactant.
  2. Swap partners: the cation of the first compound pairs with the anion of the second, and the other way around. Write correct formulas using the charges. (A common mistake is to copy the old subscripts; always rebuild the formula from the charges.)
  3. Check the solubility table for each new compound. Low solubility → (s), the precipitate. Soluble → (aq).
  4. If neither product is a solid (and no gas or water forms), there is no reaction (NR): the ions just stay mixed.
  5. Balance.

Colour clues you may meet in the lab: lead(II) iodide is bright yellow; silver chloride is white; copper(II) hydroxide is light blue; iron(III) hydroxide is rusty orange-brown; barium sulfate and calcium carbonate are white.

1. Ionslist the ions ineach reactant2. Swapcation 1 + anion 2,cation 2 + anion 13. Solubilitycheck each newcompound in the table4. Decideany (s)? write it.none? write NR.
Four steps for predicting a double displacement reaction. The solubility table decides between a precipitate and no reaction.
Worked example

A classic precipitate

Predict the products and balance: aqueous lead(II) nitrate + aqueous potassium iodide.

  1. Ions: Pb²⁺ and NO₃⁻; K⁺ and I⁻.
  2. Swap: Pb²⁺ with I⁻ → PbI₂; K⁺ with NO₃⁻ → KNO₃.
  3. Table: iodides are soluble except with Pb²⁺, so PbI₂ is the precipitate. Potassium compounds are soluble: KNO₃(aq).
  4. Balance: 2 iodide and 2 nitrate are needed, so 2KI and 2KNO₃.

Pb(NO₃)₂(aq) + 2KI(aq) → PbI₂(s) + 2KNO₃(aq). A bright yellow precipitate forms.

Worked example

A hydroxide precipitate with a multivalent metal

Predict and balance: aqueous iron(III) chloride + aqueous sodium hydroxide.

  1. Ions: Fe³⁺, Cl⁻; Na⁺, OH⁻.
  2. Swap: Fe³⁺ with OH⁻ → Fe(OH)₃; Na⁺ with Cl⁻ → NaCl.
  3. Table: hydroxides have low solubility except with group 1 etc., so Fe(OH)₃(s). NaCl is soluble.
  4. Balance: 3 OH needed, so 3NaOH and 3NaCl.

FeCl₃(aq) + 3NaOH(aq) → Fe(OH)₃(s) + 3NaCl(aq). A rusty orange-brown precipitate forms.

Worked example

No reaction

Predict: aqueous sodium chloride + aqueous potassium nitrate.

  1. Swap: Na⁺ with NO₃⁻ → NaNO₃; K⁺ with Cl⁻ → KCl.
  2. Table: sodium and potassium compounds are always soluble, and so are nitrates.
  3. No precipitate, gas or water forms. The four ions just stay mixed in solution.

No reaction (NR).

Worked example

Choosing reactants on purpose

A technician wants to make a sample of barium sulfate. Choose two soluble reactants and write the equation.

  1. Barium sulfate contains Ba²⁺ and SO₄²⁻.
  2. Choose a soluble barium compound (barium chloride or barium nitrate) and a soluble sulfate (sodium or potassium sulfate).
  3. BaCl₂(aq) + Na₂SO₄(aq) → BaSO₄(s) + 2NaCl(aq). The precipitate is filtered off, washed and dried.

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

Precipitation at work

  • Medical imaging: barium sulfate is swallowed before some X-ray exams because it blocks X-rays and outlines the digestive tract. It is safe to swallow only because it has very low solubility; soluble barium compounds are poisonous. That is why the purity of medical barium sulfate is carefully tested.
  • Water treatment: aluminium sulfate added to water reacts to form a fluffy precipitate of aluminium hydroxide, which traps fine particles so they can settle out.
  • Hard water: washing soda (sodium carbonate) softens water by precipitating calcium and magnesium ions as carbonates, so soap works better.
Hands-on activity

Make (and filter) a precipitate

You need: Epsom salt (magnesium sulfate, from a pharmacy), washing soda (sodium carbonate, from the laundry aisle, or the batch you made from baking soda), table salt, three clear glasses, a spoon, a coffee filter and funnel (or a filter set over a jar), rubber gloves, and a grown-up.

Safety: washing soda irritates skin and eyes, so wear gloves and keep your hands away from your face. Do not taste any of the mixtures. Pour liquids down the sink with lots of water and put the solid in the garbage.

  1. Glass A: dissolve 1 teaspoon of Epsom salt in 100 mL of warm water. Glass B: dissolve 1 teaspoon of washing soda in 100 mL of warm water. Both should be clear.
  2. Pour half of B into A. Observe.
  3. Control: Glass C: dissolve 1 teaspoon of table salt in 100 mL of water, then add the rest of B. Observe.
  4. Filter the mixture from glass A through the coffee filter. Let the solid dry for a day and look at it.

What to expect: a white precipitate forms in A, mostly magnesium carbonate: MgSO₄(aq) + Na₂CO₃(aq) → MgCO₃(s) + Na₂SO₄(aq). Glass C stays clear (NR), because NaCl and Na₂CO₃ have the same cation, so no new compound can form.

Think: use the solubility table to explain both results.

Just for fun

Why was the precipitate so calm during the experiment?

It had learned how to settle down. (The spectator ions, meanwhile, just kept floating around, watching.)

Your turn

1

Is it soluble? (a) K₂CO₃ (b) CaCO₃ (c) AgBr (d) Cu(NO₃)₂ (e) BaSO₄

2

Predict and balance: CuSO₄(aq) + NaOH(aq) →

3

Predict and balance: CaCl₂(aq) + Na₂CO₃(aq) →

4

Predict or NR: KBr(aq) + NaNO₃(aq) →

5

Predict and balance: aqueous silver nitrate + aqueous magnesium chloride.

6

Identify the spectator ions in Pb(NO₃)₂(aq) + 2KI(aq) → PbI₂(s) + 2KNO₃(aq).

More practice: warm-up

1

What is a precipitate?

2

Are all nitrates soluble?

3

Are most carbonates soluble?

4

What state symbol do you give a precipitate?

5

What colour is lead(II) iodide?

More practice: on your own

1

Predict and balance: aqueous sodium phosphate + aqueous calcium chloride.

2

Predict and balance: aqueous aluminium nitrate + aqueous potassium hydroxide.

3

Predict and balance: aqueous ammonium sulfide + aqueous lead(II) nitrate.

4

Predict or NR: aqueous barium nitrate + aqueous potassium sulfate. If a reaction occurs, balance it.

5

A student writes AgNO₃ + NaCl → AgCl₂ + NaNO₃. Find and fix the error.

Challenge

1

You are given an unlabelled bottle that contains either sodium chloride or sodium nitrate solution. Design a test using one reagent, and give the expected result for each possibility.

2

Phosphate from detergents and farm runoff can cause algae to overgrow in lakes. Some treatment plants add iron(III) chloride to wastewater. Predict the precipitate when it meets sodium phosphate, write the balanced equation, and explain how this helps.

3

Explain, using the solubility table, why barium sulfate is safe to swallow for an X-ray but barium chloride is poisonous.

Check your understanding

Quiz

1.

Which compound has low solubility in water?

2.

What is the precipitate in BaCl₂(aq) + Na₂SO₄(aq)?

3.

Which mixture gives no reaction?

4.

What is the balanced equation for aqueous copper(II) chloride + aqueous sodium hydroxide?

5.

In a precipitation reaction, spectator ions are ions that:

6.

Which reagent would precipitate Ca²⁺ ions from hard water?

For parents and teachers

Your teen needs a solubility table (one is in this lesson; textbooks print similar ones) and a list of ion charges.

Common difficulties:

  • Copying subscripts from the reactants instead of rebuilding each new formula from the ion charges.
  • Forgetting that if nothing precipitates (and no gas or water forms), the answer is "no reaction".
  • Thinking "insoluble" means not a single ion dissolves. It really means very low solubility, which is why barium sulfate safety depends on its purity.

Activity safety: washing soda is a skin and eye irritant. Gloves on, no tasting, rinse everything with plenty of water.