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

   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.

5. 4.

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

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

6. Equation

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

7. 5.

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

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

8. 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.

9. 6.

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

   • No: the oxygen gas came from nowhere
   • No: 2 molecules become 3
   • No: water has no oxygen
   • Yes: 4 H and 4 O on each side
10. 7.

    Noah adds a drop of food coloring to water, and the color spreads. Is this a reaction?

    • No: the dye only mixed in
    • Yes: new dye atoms formed
    • Yes: the water became a new liquid
    • Yes: a color change is a reaction
11. 8.

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

    • Wax atoms vanishing as it burns
    • Melted wax hardening again
    • Wax melting near the wick
    • Wax burning into carbon dioxide and water
12. 9.

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

    • Some atoms were destroyed
    • Gas escaped into the air
    • The gas weighs less than nothing
    • The scale got wet
13. 10.

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

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