Chemical Reactions
Grade 8 · Science · Study Guide
Discover how substances transform into new materials and learn to identify the signs and types of chemical reactions.
A chemical reaction occurs when substances (reactants) are transformed into new substances (products) with different properties. The law of conservation of mass states that atoms are rearranged but never created or destroyed — the total mass of reactants always equals the total mass of products.
Signs that a chemical reaction has occurred include a color change, gas production (bubbling), temperature change, formation of a precipitate, or production of light. These clues help distinguish chemical changes from physical changes.
Common types of chemical reactions include synthesis (A + B → AB), decomposition (AB → A + B), single replacement (A + BC → AC + B), and combustion (fuel + oxygen → carbon dioxide + water + energy).
| Type | General Form | Example |
|---|---|---|
| Synthesis | A + B → AB | 2H₂ + O₂ → 2H₂O |
| Decomposition | AB → A + B | 2H₂O → 2H₂ + O₂ |
| Single Replacement | A + BC → AC + B | Zn + CuSO₄ → ZnSO₄ + Cu |
| Combustion | Fuel + O₂ → CO₂ + H₂O | CH₄ + 2O₂ → CO₂ + 2H₂O |
| Physical Change | Chemical Change |
|---|---|
| No new substance formed | New substance(s) formed |
| Usually reversible | Usually irreversible |
| Examples: melting, cutting | Examples: burning, rusting |
Key terms
- Reactant
- A substance that enters into and is changed in a chemical reaction (left side of equation).
- Product
- A substance formed as a result of a chemical reaction (right side of equation).
- Chemical Equation
- A symbolic representation of a chemical reaction showing reactants and products.
- Catalyst
- A substance that speeds up a chemical reaction without being consumed in the process.
Practice questions
In a chemical equation, reactants are written on the ___ side of the arrow.
- Right
- Left
- Both sides
- Above the arrow
Show answer
By convention, reactants appear on the left side of the arrow, and products appear on the right.
Which observation is the best evidence that a chemical reaction has occurred?
- A substance changes shape
- Ice melts into water
- A gas is produced and a color change is observed
- Salt dissolves in water
Show answer
Gas production and color change indicate new substances have formed — a sign of a chemical reaction.
In a chemical reaction, mass is created to form the new products.
Show answer
The law of conservation of mass states that mass is neither created nor destroyed — it is rearranged into new substances.
A substance that speeds up a reaction without being consumed is called a ___.
Show answer
Catalysts lower the activation energy needed for a reaction to proceed, speeding it up without being used up.
Identify the type of reaction and explain: 2H₂O → 2H₂ + O₂
Show answer
Decomposition follows the pattern AB → A + B.
Fun facts
- The human body performs thousands of chemical reactions every second, from digesting food to producing energy in cells.
- Fireworks get their colors from specific metal salts: copper produces blue, strontium produces red, and barium produces green.
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