Free sample activity · Grades 6–8
Chemical Reactions Practice
Review basic reaction ideas after learning atoms and chemical formulas. Questions concern ordinary chemical reactions, not nuclear reactions. This is a reading and calculation activity; no substances need to be mixed or heated.
6 questions · No account · No timer · Immediate feedback
Before you begin
How to use this activity
Allow five to ten minutes. Count each type of atom separately. A coefficient multiplies an entire formula, while a subscript belongs to that substance's formula.
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Put it into practice
Choose an answer, read the explanation, then move to the next question. Your answers stay in this page and are not saved.
What distinguishes a chemical reaction from simply changing a substance's shape?
A chemical reaction forms substances with different chemical identities. Changing shape alone does not establish that new substances have formed.
In a reaction written with a single arrow pointing right, what are the starting substances on the left called?
Reactants are the starting substances on the left. Products are shown to the right of the reaction arrow.
In 2H₂ + O₂ → 2H₂O, which substance is the product?
Water, H₂O, is on the product side of the arrow. Hydrogen and oxygen are the reactants in this equation.
Which coefficient balances 2H₂ + O₂ → __ H₂O?
Two H₂ molecules supply four H atoms, and O₂ supplies two O atoms. Two H₂O molecules contain those same counts.
A sealed system contains 35 g of matter before an ordinary chemical reaction. No matter enters or leaves. What total mass is expected afterward?
Mass is conserved in an ordinary chemical reaction when all matter stays in the system. Forming a gas does not remove mass from a sealed system.
Which change is a change of state rather than a chemical reaction?
Boiling changes water's state while it remains H₂O. Rusting and burning form new substances.
Practice complete
Use counters or a drawing to model 2H₂ + O₂ → 2H₂O. Show that atoms are conserved without changing any formula's subscripts.
Review every answer and explanation below, or try the activity again.
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All questions, answers & explanations
Use this review before or after practice. You can read and print it without JavaScript.
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Chemical change
What distinguishes a chemical reaction from simply changing a substance's shape?
Answer: New substances form.
A chemical reaction forms substances with different chemical identities. Changing shape alone does not establish that new substances have formed.
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Reactants
In a reaction written with a single arrow pointing right, what are the starting substances on the left called?
Answer: Reactants
Reactants are the starting substances on the left. Products are shown to the right of the reaction arrow.
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Products
In 2H₂ + O₂ → 2H₂O, which substance is the product?
Answer: H₂O
Water, H₂O, is on the product side of the arrow. Hydrogen and oxygen are the reactants in this equation.
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Balancing counts
Which coefficient balances 2H₂ + O₂ → __ H₂O?
Answer: 2
Two H₂ molecules supply four H atoms, and O₂ supplies two O atoms. Two H₂O molecules contain those same counts.
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A closed system
A sealed system contains 35 g of matter before an ordinary chemical reaction. No matter enters or leaves. What total mass is expected afterward?
Answer: 35 g
Mass is conserved in an ordinary chemical reaction when all matter stays in the system. Forming a gas does not remove mass from a sealed system.
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A physical change
Which change is a change of state rather than a chemical reaction?
Answer: Liquid water boiling into water vapor
Boiling changes water's state while it remains H₂O. Rusting and burning form new substances.
Take it one step further
Use counters or a drawing to model 2H₂ + O₂ → 2H₂O. Show that atoms are conserved without changing any formula's subscripts.
Background reading
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