Changes in substances are broadly divided into physical changes and chemical changes. Let us organize the types and characteristics of chemical changes and master how to identify them.
Basic Knowledge
Physical change: A change in which the substance itself does not change, but its state, shape, or size changes. Examples: water becoming ice (freezing), sugar dissolving in water (dissolution), and tearing paper into smaller pieces (fragmentation).
Chemical change (chemical reaction): A change in which the type of substance changes. A new substance with properties different from those of the original is produced. Examples: combustion, decomposition, combination, oxidation, and reduction.
Summary of types of chemical changes:
① Decomposition: one type → two or more types Example: 2H2O → 2H2 + O2
② Combination: two or more types → one type Example: 2Cu + O2 → 2CuO
③ Oxidation: a change in which a substance combines with oxygen Example: 2Mg + O2 → 2MgO
④ Reduction: a change in which a substance loses oxygen Example: CuO + H2 → Cu + H2O
⑤ Combustion: a change involving rapid oxidation while producing light and heat
How to identify a chemical change: Observe gas production, color changes, formation of a precipitate, temperature changes, light emission, and so on.
Physical change（the type of substance does not change; only its state or shape changes）
Chemical change（the type of substance changes and a new substance is produced）
Oxidation（a chemical change in which a substance combines with oxygen）
Reduction（a chemical change in which a substance loses oxygen）
Combustion（rapid oxidation that produces light and heat）
Catalyst（a substance that speeds up a chemical reaction without changing itself. Example: manganese dioxide in hydrogen peroxide→oxygen）
Further Study
Organizing the characteristics of chemical changes in a table helps deepen understanding.
・Decomposition and combination are opposites: water→hydrogen＋oxygen (decomposition) / hydrogen＋oxygen→water (combination)
・Oxidation and reduction occur simultaneously (we will study this in greater detail in a later course)
Factors affecting the rate of a chemical change: ① temperature (higher is faster) ② concentration (higher is faster) ③ catalyst (promotes the reaction) ④ increasing surface area by crushing (makes it faster)
Example of a catalyst: When manganese dioxide is added to hydrogen peroxide solution, oxygen is produced even at room temperature. 2H2O2 → 2H2O + O2 (manganese dioxide does not change)
Comparison of chemical and physical changes: heating sugar→it first melts (physical change)→with further heating, it burns and turns black (chemical change)
- Physical change = the type of substance does not change (change of state, dissolution)
- Chemical change = a new substance is produced (combustion, decomposition, combination)
- Decomposition and combination are opposites
- Oxidation = combines with oxygen; reduction = loses oxygen
- Combustion = rapid oxidation (light and heat)
- Catalyst = promotes a reaction without changing itself
- Evidence of a chemical change = gas production, color change, temperature change, light emission, and so on
Points to Note
① Dissolution is a physical change (when sugar water is evaporated, the sugar returns). Combustion is a chemical change (the substance cannot return to its original form after burning). ② Difference between oxidation and combustion: combustion is a type of rapid oxidation. Rusting (oxidation) is not combustion because it is not rapid. ③ Reduction always occurs simultaneously with oxidation (while a substance that takes oxygen is reduced, the substance losing oxygen is oxidized). ④ A catalyst changes the reaction rate but does not change itself.
Practice
- Classify the following changes as physical or chemical changes: ① water evaporates ② iron rusts ③ paper burns ④ ice melts
- State the gas produced when manganese dioxide is added to hydrogen peroxide and how to confirm it.
- Explain the difference between oxidation and combustion.