Mass Changes when a Reactant or Product is a Gas Simplified Revision Notes for GCSE AQA Chemistry

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3.1.3 Mass Changes when a Reactant or Product is a Gas

Why Does Mass Seem to Change in Some Reactions?

Even though mass is conserved, it might seem like the mass changes if one of the reactants or products is a gas. This is because gases can escape or enter a reaction container, making it look like mass is lost or gained.

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Example 1: Apparent Loss of Mass:

When heating calcium carbonate $(CaCO₃)$ , it decomposes into calcium oxide (CaO) and carbon dioxide $(CO₂)$ . If the reaction is done in an open container, the carbon dioxide gas can escape, making it look like the mass has decreased.

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Example 2: Apparent Gain of Mass:

If you leave a piece of iron $(Fe)$ exposed to air, it reacts with oxygen $(O₂)$ to form iron oxide (rust). The iron appears to gain mass because oxygen from the air has combined with it.

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