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8th grade Chemical reactions: signs of a new substance, and counting atoms worksheet

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Chemical reactions: signs of a new substance, and counting atoms

Grade 8 Science Worksheet - Challenge Me

NameDate

Circle the best answer, or write your answer on the line.

  1. A teacher mixes 4.2 g of baking soda with 100 g of vinegar in a bottle under a balloon. The liquid then weighs 102 g. What mass of gas is in the balloon, Ari?

  2. A teacher mixes 8.4 g of baking soda with 150 g of vinegar in a bottle under a balloon. The liquid then weighs 154 g. What mass of gas is in the balloon, Bo?

  3. ___ Fe + 3 O₂ → 2 Fe₂O₃. Which number belongs in the blank, Jun?

    • 4
    • Change Fe₂O₃ to Fe₃O₃ instead
    • 3
    • 2

  4. ___ H₂O₂ → 2 H₂O + O₂. Which number belongs in the blank, Kira?

    • Change O₂ to O instead
    • 2
    • 1
    • 3

  5. 6 CO₂ + ___ H₂O → C₆H₁₂O₆ + 6 O₂. Which number belongs in the blank, Luz?

    • 3
    • Change H₂O to H₁₂O instead
    • 12
    • 6

  6. Milo says the fizzing cup lost mass because atoms were destroyed. Agree or disagree?

    • Agree: fizzing is just melting
    • Agree: the gas atoms vanished
    • Disagree: the gas carried atoms into the air
    • Agree: balancing changes the formulas

  7. Pia says the quickest way to balance an equation is to change the small numbers. Agree or disagree?

    • Agree: atoms can be added to fix it
    • Agree: subscripts are fine to change
    • Agree: melting counts as a reaction too
    • Disagree: that would make a new substance

  8. Which baking soda and vinegar equation is balanced, Ren?

    • NaHCO₃ + CH₃COOH → NaCH₃COO + H₂O
    • NaHCO₃ + CH₃COOH → NaCH₃COO + 2 H₂O + CO₂
    • 2 NaHCO₃ + CH₃COOH → NaCH₃COO + H₂O + CO₂
    • NaHCO₃ + CH₃COOH → NaCH₃COO + H₂O + CO₂

  9. Which rusting equation is balanced, Tala?

    • 4 Fe + 3 O₂ → 2 Fe₂O₃
    • 4 Fe + 2 O₂ → 2 Fe₂O₃
    • 2 Fe + 3 O₂ → 2 Fe₂O₃
    • 4 Fe + 3 O₂ → Fe₂O₃

  10. Which hydrogen peroxide equation is balanced, Zee?

    • 2 H₂O₂ → 2 H₂O + 2 O₂
    • H₂O₂ → H₂O + O₂
    • 2 H₂O₂ → 2 H₂O + O₂
    • 2 H₂O₂ → H₂O + O₂
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8th grade Chemical reactions: signs of a new substance, and counting atoms worksheet – printable
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About this worksheet

Chemical reactions: signs of a new substance, and counting atoms. Every version has 10 questions, and the answer key prints on the second page.

What your child will learn

  • I can name signs of a new substance (gas, color, temperature change, a new solid). I know a sign is evidence, not proof: boiling water bubbles too.
  • I can tell a physical change (melting, dissolving, mixing) from a chemical one (rusting, browning, burning).
  • I can count atoms with coefficient × subscript, and check whether an equation is balanced with an atom-count table.
  • I can explain mass in an open and a sealed container: atoms rearrange, none are made or destroyed, and gases have mass.

Standard: MS-PS1-2, MS-PS1-5

Words to know

chemical reaction: Atoms rearrange to make a new substance

physical change: Same substance, new shape or state

reactant / product: What goes in / what comes out of a reaction

coefficient: The big number in front: how many molecules

subscript: The small number after a symbol: atoms in one molecule

balanced: The same count of each kind of atom on both sides

conserved: Kept the same; not made or lost

Common mistakes

Your child added the big number and the small number. The big number counts molecules, the small one counts atoms in each, so multiply.

Your child added every mass, including the one left after the reaction.

Your child thought a gas has no mass. Weigh a sealed bottle of soda, open it to let the fizz out, and weigh again: it gets lighter.

What to say

Ask: what was there before, and what is still there after?

Questions parents ask

What is the most common mistake with chemical reactions: signs of a new substance, and counting atoms?

Your child added the big number and the small number. The big number counts molecules, the small one counts atoms in each, so multiply.

Are these 8th grade science worksheets free to print?

Yes. All 6 printable versions at each level are free to print, with the answer key, and no account is needed.

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