Grade 8 · Science

8th grade Chemical reactions: signs of a new substance, and counting atoms worksheet

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

Grade 8 Science Worksheet - Building Confidence

NameDate

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

Bo writes 4 H₂O for four water molecules. Atoms = coefficient × subscript. How many hydrogen atoms is that?
Esme writes 5 CO₂ for five carbon dioxide molecules. Atoms = coefficient × subscript. How many oxygen atoms is that?
3.
Jun mixes baking soda and vinegar in a cup, and it fizzes. Which shows a new substance formed?
- The vinegar boils
- The cup gets a little heavier
- A new gas bubbles out
4.
Kira leaves damp steel wool in a dish for a week. Which change shows a new substance?
- It gets shinier
- It melts
- It turns flaky and reddish brown
5.
Luz makes breakfast. Which change is a chemical reaction?
- Ice melting in juice
- Bread browning into toast
- Butter melting on the toast
6.
Milo stirs sugar into water. Is that a physical or a chemical change?
- Physical
- Chemical, since the water tastes sweet
- Chemical, since the sugar disappears
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.
In a reaction, what happens to the atoms of the starting substances, Ren?
- New atoms are made to replace the old ones
- Some are destroyed
- They rearrange into new substances
9.
Baking soda and vinegar fizz. Where are the atoms of the baking soda now, Tala?
- Gone; they were used up
- In the new substances, including the gas
- Turned into energy
10.
A steel nail rusts. Where did the iron atoms go, Zee?
- They vanished
- They turned into air
- Into the rust, joined with oxygen

Chemical reactions: signs of a new substance, and counting atoms. Every version has 10 questions, and the answer key prints on the second page.

I can name signs of a new substance (gas, color, temperature change, a new solid). I know a sign is evidence, not proof: boiling water bubbles too.
I can tell a physical change (melting, dissolving, mixing) from a chemical one (rusting, browning, burning).
I can count atoms with coefficient × subscript, and check whether an equation is balanced with an atom-count table.
I can explain mass in an open and a sealed container: atoms rearrange, none are made or destroyed, and gases have mass.

Standard: MS-PS1-2, MS-PS1-5

chemical reaction: Atoms rearrange to make a new substance

physical change: Same substance, new shape or state

reactant / product: What goes in / what comes out of a reaction

coefficient: The big number in front: how many molecules

subscript: The small number after a symbol: atoms in one molecule

balanced: The same count of each kind of atom on both sides

conserved: Kept the same; not made or lost

Your child added the big number and the small number. The big number counts molecules, the small one counts atoms in each, so multiply.

Your child added every mass, including the one left after the reaction.

Your child thought a gas has no mass. Weigh a sealed bottle of soda, open it to let the fizz out, and weigh again: it gets lighter.

Ask: what was there before, and what is still there after?

What is the most common mistake with chemical reactions: signs of a new substance, and counting atoms?

Your child added the big number and the small number. The big number counts molecules, the small one counts atoms in each, so multiply.

Are these 8th grade science worksheets free to print?

Yes. All 6 printable versions at each level are free to print, with the answer key, and no account is needed.

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