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

Answer Key - Grade 8 Science Worksheet - Building Confidence

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

1. 1.

   Ari draws 3 O₂ molecules. Atoms = coefficient × subscript. How many oxygen atoms is that?

   6 atoms
2. 2.

   Bo writes 4 H₂O for four water molecules. Atoms = coefficient × subscript. How many hydrogen atoms is that?

   8 atoms
3. 3.

   Jun mixes baking soda and vinegar in a cup, and it fizzes. Which shows a new substance formed?

   • The cup gets a little heavier
   • The vinegar boils
   • A new gas bubbles out
4. 4.

   Kira leaves damp steel wool in a dish for a week. Which change shows a new substance?

   • It turns flaky and reddish brown
   • It melts
   • It gets shinier
5. 5.

   Luz makes breakfast. Which change is a chemical reaction?

   • Bread browning into toast
   • Ice melting in juice
   • Butter melting on the toast
6. 6.

   Milo stirs sugar into water. Is that a physical or a chemical change?

   • Chemical, since the water tastes sweet
   • Chemical, since the sugar disappears
   • Physical
7. 7.

   An ice cube melts on Noah's plate. Is that a physical or a chemical change?

   • Chemical, since it got wet
   • Chemical, since the ice changed
   • Physical
8. 8.

   Pia leaves a cut apple out, and it turns brown. Is that physical or chemical?

   • Chemical
   • Physical, since it is still an apple
   • Physical, since it is a color change
9. 9.

   In a reaction, what happens to the atoms of the starting substances, Ren?

   • Some are destroyed
   • New atoms are made to replace the old ones
   • They rearrange into new substances
10. 10.

    Baking soda and vinegar fizz. Where are the atoms of the baking soda now, Tala?

    • Turned into energy
    • In the new substances, including the gas
    • Gone; they were used up