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?

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

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: atoms can be added in rusting
   • No: Fe and O counts differ
   • 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?

   • No: the oxygen gas came from nowhere
   • Yes: 4 H and 4 O on each side
   • No: water has no oxygen
   • 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: bubbles mean a reaction
   • Yes: a gas formed
   • Yes: the water became a new substance
10. 7.

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

    • Yes: new dye atoms formed
    • No: the dye only mixed in
    • 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 burning into carbon dioxide and water
    • Melted wax hardening again
    • Wax atoms vanishing as it burns
    • Wax melting near the wick
12. 9.

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

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

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

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