Balancing Chemical Equations — Drills · Grade 8 Science

Chapter: Chemistry

What these drills covers

By the end of Balancing Chemical Equations you should be able to:

8 questions, numeric, multiple choice and written answer, each with a worked explanation.

Vocabulary used: chemical equation, coefficient, subscript, balanced equation, model, evidence, system boundary, assumption.

Problem 1

In the equation 2H₂ + O₂ → 2H₂O, how many hydrogen atoms are on the left?
Answer and explanation

4

2 molecules × 2 atoms each = 4 hydrogen atoms.

Problem 2

In 2H₂ + O₂ → 2H₂O, how many oxygen atoms are on the right?
Answer and explanation

2

2 water molecules × 1 oxygen each = 2, matching the one O₂ molecule on the left.

Problem 3

When balancing an equation you may change
  1. the subscripts in a formula
  2. the coefficients in front of formulas
  3. both freely
  4. neither
Answer and explanation

the coefficients in front of formulas

Changing a subscript changes the substance itself; only coefficients may be adjusted.

Problem 4

Balance: __CH₄ + 2O₂ → CO₂ + 2H₂O. What coefficient goes before CH₄?
Answer and explanation

1

One methane supplies one carbon and four hydrogens, matching CO₂ and 2H₂O.

Problem 5

Balance: 2Mg + O₂ → __MgO. What coefficient goes before MgO?
Answer and explanation

2

Two magnesium atoms and two oxygen atoms give 2MgO.

Problem 6

In N₂ + 3H₂ → 2NH₃, how many nitrogen atoms appear on each side?
Answer and explanation

2

N₂ supplies 2; 2NH₃ contains 2. The equation is balanced for nitrogen.

Problem 7

Explain why changing H₂O to H₂O₂ is not a legitimate way to balance an equation.
Answer and explanation

Changing the subscript changes the substance: H₂O is water and H₂O₂ is hydrogen peroxide, a different compound with different properties. Balancing must keep the identity of every substance and adjust only how many of each are involved, which is what the coefficients in front do.

Response criteria

  • State a scientifically accurate conclusion.
  • Show the data, mechanism, or calculation that supports it.
  • Include units, conditions, or limits where relevant.

Problem 8

Describe a reliable order for balancing an equation containing carbon, hydrogen and oxygen.
Answer and explanation

Balance carbon first, then hydrogen, and leave oxygen until last, because oxygen usually appears in more than one product and adjusting it early forces repeated corrections. After setting the coefficients, count every kind of atom on both sides to confirm they match, and check that the coefficients are the smallest whole numbers that work.

Response criteria

  • State a scientifically accurate conclusion.
  • Show the data, mechanism, or calculation that supports it.
  • Include units, conditions, or limits where relevant.

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