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. 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?

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

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

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: water has no oxygen
   • No: the oxygen gas came from nowhere
   • No: 2 molecules become 3
9. 6.

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

   • No: the bubbles are water as a gas
   • Yes: the water became a new substance
   • Yes: bubbles mean a reaction
   • Yes: a gas formed
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: a color change is a reaction
    • Yes: the water became a new liquid
    • Yes: new dye atoms formed
11. 8.

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

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

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

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

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