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

Answer Key - Grade 8 Science Worksheet

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

1. 1.

   In 4 Fe + 3 O₂ → 2 Fe₂O₃, how many oxygen atoms are in 2 Fe₂O₃, Ari?

   6 atoms
2. 2.

   In 6 CO₂ + 6 H₂O → C₆H₁₂O₆ + 6 O₂, how many oxygen atoms are in 6 O₂, Bo?

   12 atoms
3. 3.

   In 6 CO₂ + 6 H₂O → C₆H₁₂O₆ + 6 O₂, how many hydrogen atoms are in 6 H₂O, Esme?

   12 atoms

4. Read this first

   Equation

   Fe + O₂ → Fe₂O₃. An atom-count table sits under it with one row per element and columns for left and right, left blank.

5. 4.

   Kira writes the rusting equation without numbers in front. Is it balanced?

   • No: Fe and O counts differ
   • Yes: each element is on both sides
   • Yes: atoms can be added in rusting
   • Yes: 2 molecules can make 1

6. Equation

   2 H₂O₂ → 2 H₂O + O₂. An atom-count table sits under it with one row per element and columns for left and right, left blank.

7. 5.

   A teacher adds yeast to hydrogen peroxide, and it foams. Luz checks the equation. Is it balanced?

   • Yes: 4 H and 4 O on each side
   • No: the oxygen gas came from nowhere
   • No: 2 molecules become 3
   • No: water has no oxygen
8. 6.

   A teacher boils water, and bubbles rise. Is this a chemical reaction, Milo?

   • No: the bubbles are water as a gas
   • Yes: bubbles mean a reaction
   • Yes: a gas formed
   • Yes: the water became a new substance
9. 7.

   A teacher lights a candle while the class watches. Which part is a chemical change, Pia?

   • Wax atoms vanishing as it burns
   • Wax burning into carbon dioxide and water
   • Wax melting near the wick
   • Melted wax hardening again
10. 8.

    Ren mixes baking soda and vinegar in an open cup on a scale. The reading drops by about 2 g. Why?

    • The scale got wet
    • The gas weighs less than nothing
    • Gas escaped into the air
    • Some atoms were destroyed
11. 9.

    Tala repeats the test in a sealed zip bag. What does the scale show after the fizzing?

    • Twice as much
    • Less, since atoms were used up
    • The same mass
    • Less, since gas has no mass
12. 10.

    Damp steel wool on a scale weighs a little more after it rusts for a week. Why, Zee?

    • Oxygen from the air joined the iron
    • New iron atoms were made
    • Oxygen has no mass, so it must be dust
    • Iron gets heavier when cold