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

Answer Key - Grade 8 Science Worksheet - Challenge Me

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

1. 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?

   2.2 g
2. 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?

   4.4 g
3. 3.

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

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

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

   • Change O₂ to O instead
   • 2
   • 1
   • 3
5. 5.

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

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