The total mass during a chemical reaction always remains the same. This is the requirement of the law of conservation of mass which is always obeyed. However, some reactions may appear to proceed with a change in mass which can usually be explained because a reactant or a product is a gas in such reactions.
If a reaction is carried out in an open container where gases can escape or enter, the apparent mass change can occur if a gas is either absorbed or evolved. For example, if a gas is evolved, we shall observe a decrease in mass and if a gas is absorbed, the apparent mass will increase.
Magnesium combustion example
If a ribbon of magnesium is burnt in an open crucible, the mass of the product (MgO) formed will be greater than the reactant (Mg). This increase in mass has occurred due to the reaction of magnesium with oxygen present in air. However, if the two reactants (Mg and O2) are allowed to react in a closed container, the total mass will remain the same before and after the reaction.
Formation of a gas during a chemical reaction
If a reaction is performed between marble chips and dilute hydrochloric acid in an open container, the mass of products will apparently be less than that of reactants. This apparent decrease in mass is due to evolution of CO2 gas during the reaction which escapes into the atmosphere.
When a reaction moves ahead the concentrations of the reactants decrease until all the reactants are used up. At the same time, the concentrations of the products increase and reach the maximum value at the end of the reaction. This is shown in a graph in Figure 17.2.
The graph clearly shows how reactant concentration decreases over time while product concentration increases until the reaction reaches completion. This trend is fundamental in kinetics.
Reaction Kinetics (Chapter 17, Chemistry – X) — pages 57-58. Law of conservation of mass always holds; apparent mass changes are due to gas exchange in open containers.
📌 17.2 Quick Check!
1. Why does the mass of product increase when magnesium burns in an open crucible?
Answer: Magnesium reacts with oxygen from air, so product (MgO) has additional mass from oxygen.
2. What causes the apparent decrease in mass when marble chips react with HCl in an open container?
Answer: CO₂ gas evolved escapes into the atmosphere, reducing the total mass measured.
3. If the same reaction (marble + HCl) is done in a closed container, what happens to total mass?
Answer: It remains constant (law of conservation of mass).
4. In the concentration-time graph, what trend do reactants and products show?
Answer: Reactants decrease over time; products increase over time.
🧠 Memorization tips & tricks for 17.2
✨ “Open system, gas escapes → mass seems less. Open system, gas absorbed → mass seems more.”
✨ Closed container = mass constant — think of a sealed bottle.
✨ Magnesium + oxygen: Mg + O₂ → MgO ; mass increase because oxygen from air adds.
✨ Marble + HCl: CO₂ bubbles out → fizzing means mass loss in open.
✨ Concentration graph: Reactants (R) go Down, Products (P) go Up — like a seesaw.
📚 Guidelines for students – 17.2
• Focus on the difference between open and closed systems — that’s the key to apparent mass change.
• Remember the two classic examples: magnesium combustion (mass ↑) and marble+ HCl (mass ↓ in open).
• Practice drawing the concentration-time graph with reactants decreasing and products increasing.
• Use the MCQs to test yourself — click any option to see instant correct/red feedback.
• Adjust font size for comfortable reading, and switch day/night mode as needed.