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 hydrogen atoms are in 6 H₂O, Esme?

   12 atoms

3. Read this first

   Equation

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

4. 3.

   Jun checks an equation for copper joining with oxygen. Is it balanced?

   • Yes: 2 Cu and 2 O on each side
   • No: copper is not on the right
   • No: 3 molecules on the left, 2 on the right
   • No: oxygen disappears

5. Equation

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

6. 4.

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

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

7. 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.

8. 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
9. 6.

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

   • No: the bubbles are water as a gas
   • Yes: a gas formed
   • Yes: the water became a new substance
   • Yes: bubbles mean a reaction
10. 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
11. 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
    • Some atoms were destroyed
    • Gas escaped into the air
12. 9.

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

    • Less, since gas has no mass
    • Twice as much
    • Less, since atoms were used up
    • The same mass
13. 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
    • Iron gets heavier when cold
    • New iron atoms were made
    • Oxygen has no mass, so it must be dust