Curriculum
Science — Second Preparatory
Unit 1: Matter and Energy
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Unit 2: Matter and Chemical Reactions
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Unit 3: Energy Flow in Photosynthesis and Respiration
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Unit 4: Geological Processes
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Text lesson
Chemical Reactions
Chemical Reactions
Conservation of mass in a reaction
Conservation of mass in a reaction
- According to the law of conservation of mass, matter is neither created nor destroyed in a chemical reaction.
- The total mass of the reactants equals the total mass of the products.
- This can be shown with the reaction between hydrogen and oxygen to form water:
- 2H₂ + O₂ → 2H₂O
| Side | Total mass |
|---|---|
| Reactants: 2H₂ + O₂ | 4 + 32 = 36 g |
| Products: 2H₂O | 2 × 18 = 36 g |
Key points
Worked exampleIf 2 g of hydrogen react exactly with 16 g of oxygen, what mass of water is formed?
SolutionFrom the reaction, the mass ratio of hydrogen to oxygen is 4 : 32, which is 1 : 8. So 2 g of hydrogen needs 16 g of oxygen.
Mass of water formed = 2 + 16 = 18 g
Simple experiment→Weigh a sealed flask containing a reacting mixture before and after the reaction. You will find that the total mass has not changed, which confirms the law of conservation of mass.
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