This question is about metal carbonates. When heated, some metal carbonates decompose to form a metal oxide and carbon dioxide gas. A student is given a solid metal carbonate with the formula $$\(\mathrm{XCO}_{3}\)$$ $$\(\mathrm{X}\)$$ represents the symbol of a Group 2 metal. A student uses this apparatus to heat a sample of $$\(\mathrm{XCO}_{3}\)$$ until it all decomposes. The equation for the decomposition of $$\(\mathrm{XCO}_{3}\)$$ is The student records the mass of $$\(\mathrm{XCO}_{3}\)$$ and the mass of carbon dioxide that escapes through the cotton wool plug. These are the student's results. mass of $$\(\mathrm{XCO}_{3}=7.40 \mathrm{~g}\)$$ mass of $$\(\mathrm{CO}_{2}=2.20 \mathrm{~g}\)$$ Give a reason why the student uses a cotton wool plug. (1)

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