Ionic reactions and tests for ions

Add a drop of silver nitrate to tap water and it turns faintly cloudy: chloride ions are there. Pour hydrochloric acid on marble and it fizzes. Almost everything that happens in a school test tube is a reaction between ions, and the CSCA syllabus names it directly: “Ionic reactions and testing methods”. The questions ask you to write or judge an ionic equation, to decide whether a group of ions can exist together, and to identify an ion from what is seen. What happens to the electrons in reactions such as Fe + Cu²⁺ is in the lesson “Redox reactions”; the equilibrium of weak electrolytes and hydrolysis are in “Theories of electrolyte solutions”.

Electrolytes: what is really in the solution

Strong electrolytes (强电解质) ionise completely in water: strong acids (HCl, HBr, HI, HNO₃, H₂SO₄, HClO₄), strong bases (NaOH, KOH, Ba(OH)₂, Ca(OH)₂) and almost all salts. Even insoluble BaSO₄ is strong, because the little that dissolves is fully ionised. Weak electrolytes (弱电解质) ionise only partly: weak acids (CH₃COOH, H₂CO₃, H₂S, HClO, HF), the weak base NH₃·H₂O and water. How far a weak electrolyte ionises is the subject of the lesson “Theories of electrolyte solutions”; here we need only the list. Conductivity depends on the concentration and charge of the free ions, not on whether the electrolyte is strong: a very dilute HCl solution can conduct worse than a concentrated acetic acid solution.

Net ionic equations

An ionic equation (离子方程式) shows what really reacts. When BaCl₂ solution is poured into Na₂SO₄ solution, Na⁺ and Cl⁻ stay in solution unchanged — they are spectator ions — and the whole change is Ba²⁺ + SO₄²⁻ → BaSO₄↓. That is why one net equation describes a whole family of reactions: any soluble barium salt with any soluble sulfate. Chinese textbooks teach four steps, 写、拆、删、查 (write, split, delete, check).

1. 1
   Write (写)

   Write the balanced molecular equation: BaCl₂ + Na₂SO₄ → BaSO₄↓ + 2NaCl.

2. 2
   Split (拆)

   Rewrite as ions only the strong electrolytes that are dissolved: strong acids, strong bases, soluble salts. Keep as formulas: precipitates, gases, weak electrolytes (water, CH₃COOH, NH₃·H₂O), oxides and elements.

3. 3
   Delete (删)

   Cross out the ions that appear unchanged on both sides (here 2Na⁺ and 2Cl⁻).

4. 4
   Check (查)

   Atoms and total charge must be equal on both sides, the coefficients must be the smallest whole numbers, and the equation must match the real reaction.

When do ions react? Coexistence of ions

An ion-exchange reaction in solution goes to completion only if the products remove ions from the solution: a precipitate (Ba²⁺ + SO₄²⁻, Ag⁺ + Cl⁻, Ca²⁺ + CO₃²⁻, Cu²⁺ + 2OH⁻), a gas (2H⁺ + CO₃²⁻ → CO₂↑ + H₂O, 2H⁺ + SO₃²⁻ → SO₂↑ + H₂O, and on warming NH₄⁺ + OH⁻ → NH₃↑ + H₂O) or a weak electrolyte (H⁺ + OH⁻ → H₂O, H⁺ + CH₃COO⁻ → CH₃COOH, NH₄⁺ + OH⁻ → NH₃·H₂O). Mixing KNO₃ and NaCl solutions only gives a solution of four ions: no reaction.

Coexistence of ions (离子共存). Ions can be present together in large amounts only if no pair of them forms a precipitate, a gas or a weak electrolyte, and no pair reacts by redox. Questions hide extra conditions in the wording:

• colourless: no Cu²⁺ (blue), Fe³⁺ (yellow-brown), Fe²⁺ (pale green) or MnO₄⁻ (purple);
• acidic (pH < 7, litmus turns red): add H⁺ to every group; it removes OH⁻, CO₃²⁻, HCO₃⁻, SO₃²⁻, S²⁻, CH₃COO⁻ and ClO⁻;
• basic (pH > 7, phenolphthalein turns pink): add OH⁻; it removes H⁺, NH₄⁺, Mg²⁺, Cu²⁺, Fe²⁺, Fe³⁺, Al³⁺ and HCO₃⁻ (HCO₃⁻ survives in neither medium: HCO₃⁻ + OH⁻ → CO₃²⁻ + H₂O);
• redox pairs: NO₃⁻ together with H⁺ (dilute nitric acid) oxidises Fe²⁺, I⁻, SO₃²⁻ and S²⁻; MnO₄⁻ oxidises Fe²⁺ and I⁻; Fe³⁺ oxidises I⁻ and S²⁻ (see the lesson “Redox reactions”).

Tests for ions

An ion test relies on a sign that only this ion gives under the chosen conditions. So every test has two parts: a reagent that gives a clear sign (a precipitate of a certain colour, a gas, a colour) and a way to exclude the ions that could give the same sign.

Why the acid first? In the Cl⁻ test, dilute HNO₃ dissolves Ag₂CO₃ (and other silver salts of weak acids), so only AgCl remains. In the SO₄²⁻ test, HCl added first removes CO₃²⁻ and SO₃²⁻ (as gases) and Ag⁺ (as AgCl, seen at once); a white precipitate that appears only after BaCl₂ is then BaSO₄. Nitric acid is not used here: it would oxidise SO₃²⁻ to SO₄²⁻ and create sulfate that was not there. For CO₃²⁻, the first step with CaCl₂ tells it from HCO₃⁻, which gives no precipitate with dilute CaCl₂, and the odourless gas tells it from SO₃²⁻, whose SO₂ also clouds limewater but smells sharp. When several ions are present, remove the disturbing one first: to test for Cl⁻ in a sulfate solution, add excess Ba(NO₃)₂ (not BaCl₂, which brings Cl⁻ itself), filter, then use AgNO₃ and HNO₃.

NaOH solution sorts cations by colour: Mg²⁺ gives a white precipitate, Cu²⁺ blue, Fe³⁺ red-brown, Fe²⁺ white that turns grey-green and then red-brown in air (4Fe(OH)₂ + O₂ + 2H₂O → 4Fe(OH)₃), and NH₄⁺ no precipitate but NH₃ on warming. How iron’s two ions turn into each other is in the lesson “Common metals and their compounds”.

1. 1
   Clean the wire

   Dip a platinum (or clean iron) wire into dilute hydrochloric acid and heat it in the outer flame until the flame shows its original colour. HCl is used because chlorides are volatile; sulfates are not.

2. 2
   Pick up the sample

   Touch the wire to the solution or the solid and hold it in the outer flame.

3. 3
   Observe the colour

   Look at potassium through blue cobalt glass: the glass absorbs the strong yellow light of any sodium traces.

4. 4
   Clean again

   Before the next sample, clean the wire with HCl and heat it again.

How CSCA asks about this

• Electrolytes: classify substances; BaSO₄ is strong, CO₂ and NH₃ are non-electrolytes, Cu and NaCl solution are neither.
• Ionic equations: pick the correct one by checking the facts, the splitting, the atoms, the charge and the ratios; watch reactions that depend on amounts (CO₂ with NaOH, acid with Na₂CO₃).
• Which reaction a given net equation represents (H⁺ + OH⁻ → H₂O, Ba²⁺ + SO₄²⁻ → BaSO₄↓).
• Coexistence: find the group that can coexist, or the reason why one cannot (precipitate, gas, weak electrolyte, redox), with colour and pH clues.
• Tests: the right reagent and order, what is observed, why an acid goes in first, one reagent that tells several solutions apart, what a set of observations proves and what it leaves open.
• Conductivity: a bulb in Ba(OH)₂ solution goes almost dark as H₂SO₄ is added and glows again when the acid is in excess.
• Time-savers (about 75 s per item): in coexistence items, first strike out options with a coloured ion or an obvious pair (Ba²⁺/SO₄²⁻, Ag⁺/Cl⁻, H⁺/CO₃²⁻, NH₄⁺/OH⁻); in equation items, check the charge first, it takes two seconds.

The test can be taken in English or in Chinese. These are the terms of this lesson in both languages: