Balanced, ionic and test-tube equations

Key terms

• Atom: the smallest part of an element.
• Molecule: two or more atoms covalently bonded.
• Ion: a charged particle formed when an atom or group gains or loses electrons.
• Compound: two or more elements chemically combined.
• Empirical formula: the simplest whole-number ratio of atoms; molecular formula: the actual number of atoms in a molecule.

Balancing and state symbols

Balance by changing the numbers in front of formulae, never the formulae themselves. Add state symbols: (s) solid, (l) liquid, (g) gas, (aq) dissolved in water.

Mg(s) + 2HCl(aq) -> MgCl2(aq) + H2(g)

Ionic equations

Write soluble ionic compounds as separate ions, cancel the spectator ions that appear unchanged on both sides, and keep what is left. Charges must balance as well as atoms.

• Precipitation: Ag+(aq) + Cl-(aq) -> AgCl(s), a white precipitate
• Acid and carbonate: CO3²-(aq) + 2H+(aq) -> CO2(g) + H2O(l), fizzing (for an insoluble carbonate keep the full formula, for example CaCO3(s) + 2H+(aq) -> Ca2+(aq) + CO2(g) + H2O(l))
• Displacement: Zn(s) + Cu2+(aq) -> Zn2+(aq) + Cu(s), blue colour fades and a pink-brown solid forms
• Neutralisation: H+(aq) + OH-(aq) -> H2O(l)

Linking to observations

Every product you can see should match an observation: a gas means fizzing (effervescence), an insoluble product means a precipitate, and a change in the ions present can change the colour of the solution.

Worked example

Write the ionic equation for the reaction of barium chloride solution with sodium sulfate solution.

1. Full: BaCl2(aq) + Na2SO4(aq) -> BaSO4(s) + 2NaCl(aq)
2. Split the soluble ionic compounds into ions; Na+ and Cl- appear on both sides, so cancel them.

Answer: Ba2+(aq) + SO4²-(aq) -> BaSO4(s), a white precipitate

Check yourself

1. What is the ionic equation for the precipitation of silver bromide from silver nitrate and potassium bromide solutions?
   • AgNO3(aq) + KBr(aq) -> AgBr(s) + KNO3(aq)
   • Ag+(aq) + Br-(aq) -> AgBr(s)
   • K+(aq) + NO3-(aq) -> KNO3(s)
   • Ag(s) + Br2(aq) -> AgBr(s)
   Show answer

   Ag+(aq) + Br-(aq) -> AgBr(s). Only Ag+ and Br- change; K+ and NO3- are spectator ions.

2. In the reaction of zinc with copper(II) sulfate solution, which ion is a spectator ion?
   • Cu2+
   • Zn2+
   • None, every ion changes
   • SO4²-
   Show answer

   SO4²-. Sulfate ions are in solution before and after, unchanged.

3. Balance: _C3H8 + _O2 -> _CO2 + _H2O. What number goes in front of O2 when the C3H8 coefficient is 1?
   Show answer

   5. C3H8 + 5O2 -> 3CO2 + 4H2O. Oxygen atoms: 3(2) + 4(1) = 10, so 5 O2.

4. Dilute hydrochloric acid is added to solid sodium carbonate. What would you observe?
   • A white precipitate forms
   • A brown solid forms
   • Fizzing as a colourless gas is given off
   • The solution turns blue
   Show answer

   Fizzing as a colourless gas is given off. CO3²- + 2H+ -> CO2 + H2O; the CO2 gas causes effervescence.

5. Which state symbol should be used for sodium chloride dissolved in water?
   • (aq)
   • (l)
   • (s)
   • (g)
   Show answer

   (aq). Dissolved in water is aqueous, (aq). (l) is only for a pure liquid such as water itself.

6. Which ionic equation is correctly balanced for charge?
   • Fe(s) + Fe3+(aq) -> 2Fe2+(aq)
   • Cu(s) + Ag+(aq) -> Cu2+(aq) + Ag(s)
   • Fe(s) + 2Fe3+(aq) -> 3Fe2+(aq)
   • Al(s) + Cu2+(aq) -> Al3+(aq) + Cu(s)
   Show answer

   Fe(s) + 2Fe3+(aq) -> 3Fe2+(aq). Left: 2(3+) = 6+; right: 3(2+) = 6+. The others have unequal total charge.

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Sources

• Pearson Edexcel IAL Chemistry (2018) scheme of work
• Pearson Edexcel International Advanced Level Chemistry (2018) qualification page
• Physics and Maths Tutor: IAL Unit 1 notes, Formulae, Equations and Amounts of Substance

Last updated 7 November 2026. Spotted a mistake? Tell us.