Chemical Reactions Quiz

Grade 8 · Science · Quiz

10 questions on reaction types, signs of reactions, balancing concepts, and conservation of mass.

Questions

1. Which of the following is NOT a sign of a chemical reaction?

   1. Color change
   2. Gas production
   3. Change in shape
   4. Temperature change
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   Answer: Change in shape

   Changing shape (like cutting paper) is a physical change, not a chemical reaction. No new substance is formed.

2. The reaction 2Mg + O₂ → 2MgO is an example of:

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

   Two reactants (Mg and O₂) combine to form one product (MgO) — the pattern A + B → AB defines synthesis.

3. Burning wood is a chemical change because new substances are formed.

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   Combustion produces carbon dioxide, water vapor, and ash — entirely new substances with different properties than the original wood.

4. What does the law of conservation of mass state?

   1. Mass can be created during reactions
   2. Mass is destroyed when energy is released
   3. The total mass of reactants equals the total mass of products
   4. Only solids conserve mass
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   Answer: The total mass of reactants equals the total mass of products

   Atoms are rearranged but never created or destroyed, so mass is always conserved.

5. In the reaction Fe + CuSO₄ → FeSO₄ + Cu, what type of reaction is this?

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

   Iron (Fe) replaces copper (Cu) in the compound — one element displaces another, fitting the pattern A + BC → AC + B.

6. In a chemical equation, the substances formed are called ___.

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   Products appear on the right side of the arrow in a chemical equation.

7. A catalyst affects a reaction by:

   1. Increasing the amount of product
   2. Being consumed in the reaction
   3. Lowering activation energy and speeding up the reaction
   4. Reducing the amount of reactant needed
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   Answer: Lowering activation energy and speeding up the reaction

   Catalysts provide an alternative pathway with lower activation energy, speeding reactions without being consumed.

8. Dissolving sugar in water is a chemical reaction.

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   Dissolving is a physical change — the sugar molecules are separated but not chemically altered. Evaporating the water recovers the sugar.

9. Combustion reactions always produce:

   1. Only water
   2. Carbon dioxide and water (when burning hydrocarbons)
   3. Only carbon dioxide
   4. Nitrogen and oxygen
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   Answer: Carbon dioxide and water (when burning hydrocarbons)

   When hydrocarbons (fuels containing carbon and hydrogen) combust completely, they produce CO₂ and H₂O.

10. If 10 g of reactants are used in a chemical reaction, how much product is formed (assuming nothing escapes)?

    1. Less than 10 g
    2. Exactly 10 g
    3. More than 10 g
    4. It depends on the reaction type
    Show answer

    Answer: Exactly 10 g

    By the law of conservation of mass, mass is neither created nor destroyed — 10 g of reactants produce 10 g of products.

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