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Quizzes > Science

Balancing chemical equations online practice: tackle hard equations

Quick quiz with instant feedback. Sharpen skills with advanced equation balancing.

Editorial: Review CompletedCreated By: Reece WilcoxUpdated Aug 25, 2025
Difficulty: Moderate
2-5mins
Learning OutcomesCheat Sheet
Paper art illustration for a chemistry quiz on balancing equations on a sky blue background.

Use this quiz to practice balancing chemical equations, tackle harder cases, and build speed setting coefficients. For a focused warm-up, try the chemistry balancing equations quiz, or add variety with the equation balancing practice quiz. Work through each reaction, check your steps, and see instant feedback on every attempt.

Balance the following chemical equation: H2 + O2 ? H2O.
2 H2 + O2 ? 2 H2O
H2 + 2 O2 ? 2 H2O
H2 + O2 ? H2O2
2 H2 + O2 ? H2O
Balancing H2 and O2 to form H2O requires equal numbers of each atom on both sides. You need 2 molecules of H2 and 1 molecule of O2 to produce 2 molecules of H2O, conserving both hydrogen and oxygen atoms. This simple diatomic reaction follows the law of conservation of mass. For more details, see .
Balance the combustion reaction of methane: CH4 + O2 ? CO2 + H2O.
CH4 + 2 O2 ? CO2 + 2 H2O
2 CH4 + 3 O2 ? 2 CO2 + 4 H2O
CH4 + O2 ? CO2 + H2O
2 CH4 + O2 ? CO2 + 2 H2O
Complete combustion of methane produces carbon dioxide and water. You must balance four hydrogen atoms and two oxygen atoms on both sides. One CH4 reacts with two O2 molecules to give one CO2 and two H2O molecules. More on this reaction at .
Balance the synthesis of ammonia: N2 + H2 ? NH3.
N2 + 2 H2 ? 2 NH3
2 N2 + 3 H2 ? 2 NH3
N2 + 3 H2 ? 2 NH3
N2 + H2 ? NH3
To synthesize ammonia, two nitrogen atoms combine with hydrogen. Each N2 molecule requires three H2 molecules to form two NH3 molecules, preserving atom count. This is the basis of the Haber process. Learn more at .
Balance the combustion of propane: C3H8 + O2 ? CO2 + H2O.
C3H8 + 4 O2 ? 3 CO2 + 4 H2O
2 C3H8 + 5 O2 ? 3 CO2 + 4 H2O
C3H8 + 5 O2 ? 3 CO2 + 4 H2O
C3H8 + 3 O2 ? 3 CO2 + 4 H2O
Propane combustion yields carbon dioxide and water. To balance 3 carbons and 8 hydrogens, you need 5 O2 molecules: 3 for CO2 and 4 for H2O. The balanced equation is C3H8 + 5 O2 ? 3 CO2 + 4 H2O. See .
Balance the acid-base reaction: Ca(OH)2 + H3PO4 ? Ca3(PO4)2 + H2O.
Ca(OH)2 + H3PO4 ? Ca3(PO4)2 + 6 H2O
3 Ca(OH)2 + 2 H3PO4 ? Ca3(PO4)2 + 3 H2O
3 Ca(OH)2 + 2 H3PO4 ? Ca3(PO4)2 + 6 H2O
2 Ca(OH)2 + 3 H3PO4 ? Ca3(PO4)2 + 6 H2O
Balancing polyatomic ions as a unit helps simplify the process. Three Ca(OH)2 molecules react with two H3PO4 to form one Ca3(PO4)2 and six water molecules. This conserves Ca, P, O, and H atoms. More on polyatomic balancing at .
Balance the decomposition: KClO3 ? KCl + O2.
2 KClO3 ? KCl + 3 O2
2 KClO3 ? 2 KCl + O2
KClO3 ? KCl + 3 O2
2 KClO3 ? 2 KCl + 3 O2
When potassium chlorate decomposes, it produces potassium chloride and oxygen gas. Two KClO3 molecules decompose to give two KCl and three O2, balancing K, Cl, and O atoms. Decomposition reactions often follow this pattern of oxygen release. See .
Balance the single-displacement reaction: Fe + HCl ? FeCl2 + H2.
Fe + 2 HCl ? FeCl2 + H2
2 Fe + 2 HCl ? 2 FeCl2 + H2
Fe + HCl ? FeCl2 + H2
Fe + 3 HCl ? FeCl3 + H2
Iron displaces hydrogen in hydrochloric acid to form iron(II) chloride and hydrogen gas. One Fe atom requires two HCl molecules to balance Cl atoms, yielding H2. This type of reaction is common in metal-acid chemistry. More information at .
Balance the thermite reaction: Al + Fe2O3 ? Al2O3 + Fe.
2 Al + Fe2O3 ? Al2O3 + 2 Fe
4 Al + Fe2O3 ? 2 Al2O3 + Fe
Al + Fe2O3 ? Al2O3 + Fe
2 Al + Fe2O3 ? 2 Al2O3 + 2 Fe
The thermite reaction involves aluminum reducing iron(III) oxide. Two Al atoms are needed to reduce one Fe2O3 to aluminum oxide and two Fe atoms. This exothermic reaction is used in welding. Read more at .
Balance the cellular respiration reaction: C6H12O6 + O2 ? CO2 + H2O.
C6H12O6 + 6 O2 ? 12 CO2 + 6 H2O
C6H12O6 + 6 O2 ? 6 CO2 + 6 H2O
C6H12O6 + 5 O2 ? 6 CO2 + 6 H2O
2 C6H12O6 + 6 O2 ? 12 CO2 + 12 H2O
In aerobic respiration, one glucose molecule reacts with six O2 molecules to yield six CO2 and six H2O, conserving C, H, and O atoms. This stoichiometry underpins cellular energy production. For biochemical context, see .
Balance the precipitation reaction: Fe2(SO4)3 + Ba(OH)2 ? BaSO4 + Fe(OH)3.
Fe2(SO4)3 + 3 Ba(OH)2 ? 3 BaSO4 + 2 Fe(OH)3
2 Fe2(SO4)3 + 3 Ba(OH)2 ? 3 BaSO4 + 2 Fe(OH)3
Fe2(SO4)3 + 3 Ba(OH)2 ? BaSO4 + 2 Fe(OH)3
Fe2(SO4)3 + Ba(OH)2 ? 3 BaSO4 + 2 Fe(OH)3
Balancing ionic equations often uses polyatomic ions as units. One Fe2(SO4)3 requires three Ba(OH)2 to precipitate three BaSO4 and produce two Fe(OH)3, conserving all atoms. See .
Balance the redox reaction in acidic solution: KMnO4 + HCl ? KCl + MnCl2 + H2O + Cl2.
KMnO4 + 16 HCl ? 2 KCl + 2 MnCl2 + 8 H2O + 5 Cl2
2 KMnO4 + 8 HCl ? 2 KCl + 2 MnCl2 + 8 H2O + 5 Cl2
2 KMnO4 + 16 HCl ? 2 KCl + 2 MnCl2 + 8 H2O + 5 Cl2
2 KMnO4 + 16 HCl ? KCl + MnCl2 + 8 H2O + 5 Cl2
Balancing redox reactions in acid requires half-reaction methods. Two permanganate ions and sixteen protons from HCl yield two chloride and two manganese(II) ions, plus water and chlorine gas. The coefficients conserve mass and charge. Detailed steps at .
Balance the combustion of ethanol: C2H5OH + O2 ? CO2 + H2O.
2 C2H5OH + 3 O2 ? 2 CO2 + 3 H2O
C2H5OH + 3 O2 ? 2 CO2 + 3 H2O
C2H5OH + 2 O2 ? 2 CO2 + 3 H2O
C2H5OH + 3 O2 ? CO2 + H2O
Burning ethanol requires balancing 2 carbons and 6 hydrogens: one C2H5OH reacts with three O2 to give two CO2 and three H2O, satisfying conservation of mass. This reaction is key in biofuel studies. More at .
In acidic solution, balance the redox reaction: KMnO4 + H2SO4 + FeSO4 ? K2SO4 + MnSO4 + Fe2(SO4)3 + H2O.
2 KMnO4 + 8 H2SO4 + 10 FeSO4 ? K2SO4 + 2 MnSO4 + 5 Fe2(SO4)3 + 8 H2O
2 KMnO4 + 10 H2SO4 + 10 FeSO4 ? K2SO4 + 2 MnSO4 + 5 Fe2(SO4)3 + 8 H2O
2 KMnO4 + 8 H2SO4 + 10 FeSO4 ? 2 K2SO4 + 2 MnSO4 + 5 Fe2(SO4)3 + 8 H2O
KMnO4 + 8 H2SO4 + FeSO4 ? K2SO4 + MnSO4 + 5 Fe2(SO4)3 + 8 H2O
This complex redox reaction is balanced using the half-reaction method in acidic solution. Two permanganate ions oxidize ten Fe2+ ions, producing five Fe3+ and two Mn2+; sulfate and potassium balance to form the observed salts, plus water. Coefficients conserve both mass and charge. See detailed balancing at .
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Study Outcomes

  1. Apply Stoichiometric Principles -

    Use core stoichiometry rules to balance chemical equations accurately in our balancing chemical equations quiz.

  2. Identify Correct Coefficients -

    Recognize and assign the proper coefficients to reactants and products when you practise balancing chemical equations.

  3. Balance Complex Reactions -

    Approach multi-step and redox reactions with confidence by mastering systematic balancing strategies.

  4. Analyze Quiz Feedback -

    Interpret detailed chemistry about balancing equations answers and learn from mistakes to improve your skills.

  5. Strengthen Chemistry Fundamentals -

    Reinforce your understanding of reaction dynamics and conservation of mass through our free quiz on balancing chemical equations.

Cheat Sheet

  1. Conservation of Mass Principle -

    All atoms present in the reactants must also appear in the products, following Lavoisier's law. For example, balancing H₂ + O₂ → H₂O requires two H atoms and two O atoms on each side. Mastering this concept is the foundation for any balancing chemical equations quiz and ensures total mass stays constant.

  2. Systematic Inspection Method -

    Begin by writing the unbalanced equation and then adjust coefficients one element at a time, starting with the most complex molecule. Balance metals first, then non-metals, and leave hydrogen and oxygen for last, according to guidelines from MIT OpenCourseWare. Regularly practise balancing chemical equations using this stepwise approach to build speed and confidence.

  3. Polyatomic Ions as Single Units -

    When the same polyatomic ion appears on both sides of the equation, treat it as one block to simplify coefficient assignment. For instance, in Ba(OH)₂ + H₃PO₄ → Ba₃(PO₄)₂ + H₂O, balance PO₄ and OH units rather than individual P, O, and H atoms. This trick, recommended by the Royal Society of Chemistry, streamlines your quiz on balancing chemical equations.

  4. Fractional Coefficient Shortcut -

    For combustion reactions with odd oxygen counts, place a fractional coefficient on O₂ to quickly balance oxygen, then multiply all coefficients by the denominator. For example, C₂H₆ + 7/2 O₂ → 2 CO₂ + 3 H₂O becomes 2C₂H₆ + 7O₂ → 4CO₂ + 6H₂O after clearing the fraction. Using this method in a balancing equations quiz saves time and reduces trial-and-error.

  5. Verification via Atom-Count Tables -

    Create a simple table listing each element's count on the reactant and product sides to confirm balance. Columns for Element, Reactant Count, and Product Count help catch mistakes before finalizing answers. This tabulation strategy, endorsed by university chemistry departments, guarantees accuracy in any balancing chemical equations quiz.

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