Chemical Reactions and Stoichiometry Quiz

Grade 9 · Science · Quiz

Test your knowledge of chemical reactions, equation balancing, reaction types, and stoichiometry with 10 questions.

Questions

1. Which is a sign that a chemical reaction has occurred?

   1. A change in shape
   2. A change in state only
   3. Gas production
   4. Mixing two liquids
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   Answer: Gas production

   Signs of a chemical reaction include: gas bubbles, color change, precipitate formation, temperature change, or light emission. A change in shape alone is a physical change.

2. The total number of atoms is conserved in a chemical reaction.

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   Answer: True

   The Law of Conservation of Mass means atoms are neither created nor destroyed — the same atoms are present before and after, just rearranged into different compounds.

3. 1 mole of any substance contains 6.022 × 10²³ particles, known as ___ number.

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   Answer: Avogadro's

   Avogadro's number (6.022 × 10²³) defines the mole. Just as a dozen = 12, a mole = 6.022 × 10²³. It allows chemists to count atoms by weighing them.

4. What type of reaction occurs when iron reacts with oxygen to form iron oxide (rust)?

   1. Decomposition
   2. Synthesis
   3. Single replacement
   4. Combustion
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   Answer: Synthesis

   Synthesis (or combination) reactions join simpler substances into one more complex product: A + B → AB. 4Fe + 3O₂ → 2Fe₂O₃ fits this pattern perfectly.

5. Balance the equation: _H₂ + _O₂ → _H₂O. What are the correct coefficients?

   1. 1, 1, 1
   2. 2, 1, 2
   3. 1, 2, 2
   4. 2, 2, 2
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   Answer: 2, 1, 2

   2H₂ + O₂ → 2H₂O: 4 H on each side (2×2 = 4), 2 O on each side (1×2 = 2). Only coefficients are adjusted — subscripts are never changed.

6. In a combustion reaction, a fuel reacts with oxygen to produce carbon dioxide and water.

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   Answer: True

   Combustion of hydrocarbons always follows the pattern: fuel + O₂ → CO₂ + H₂O. Complete combustion produces these products; incomplete combustion can also produce CO.

7. Which of the following is a decomposition reaction?

   1. 2H₂ + O₂ → 2H₂O
   2. C + O₂ → CO₂
   3. 2H₂O₂ → 2H₂O + O₂
   4. NaCl + AgNO₃ → AgCl + NaNO₃
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   Answer: 2H₂O₂ → 2H₂O + O₂

   This is hydrogen peroxide decomposing into water and oxygen — one compound breaking into two products. The other options are synthesis, synthesis, and double replacement.

8. In a chemical equation, numbers placed in front of chemical formulas to balance the equation are called ___.

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   Answer: coefficients

   Coefficients are the large numbers before chemical formulas in a balanced equation. They represent the molar ratio of reactants to products and are the ONLY numbers changed when balancing.

9. The molar mass of an element in grams per mole equals:

   1. Its atomic number
   2. Its number of electrons
   3. Its atomic mass from the periodic table
   4. Its number of neutrons
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   Answer: Its atomic mass from the periodic table

   Molar mass (g/mol) = the element's atomic mass in amu. Carbon has atomic mass 12 amu, so its molar mass is 12 g/mol. This links atomic scale to laboratory scale measurements.

10. Zn + 2HCl → ZnCl₂ + H₂ is an example of which reaction type?

    1. Synthesis
    2. Decomposition
    3. Single replacement
    4. Double replacement
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    Answer: Single replacement

    Zinc (A) displaces hydrogen (B) from HCl (BC) to form ZnCl₂ (AC) + H₂ (B). This A + BC → AC + B pattern defines single replacement reactions.

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