Law of Conservation of Mass
The law of conservation of mass states that during a chemical change, the total mass of reactants is equal to the total mass of products.
Practice This ConceptMain explanation
Teacher explanation
In a chemical reaction, atoms are rearranged to form new substances, but atoms are neither created nor destroyed. Therefore, if the reaction happens in a closed system, the total mass before and after the reaction remains the same.
Example
If 12 g of carbon combines completely with 32 g of oxygen, 44 g of carbon dioxide is formed. Total reactant mass is 44 g and product mass is also 44 g.
Simple analogy
Same atoms, same total mass.
Common confusion
Students often think mass decreases because a gas escapes. The law is checked correctly only when all products, including gases, are included.
Exam tip
When a question gives masses before and after a reaction, add all reactant masses and all product masses before comparing.
Answer writing and exam use
1-mark use
Write the exact meaning of law of conservation of mass in one clean line.
2-mark use
Define law of conservation of mass and add one example or condition.
3-mark use
Explain law of conservation of mass, show the method or example, and mention the common mistake.
Practice this concept with focused MCQs
Open the concept quiz intro first, review the test details, and then start a focused MCQ set from this concept only. Instant score and answer review are live now.
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