Think of it like a puzzle: if you have $10$ pieces and use $5.6$ pieces to build a house, the remaining pieces must automatically account for the rest of the structure.

L-C-M (Law of Conservation of Mass): Total Reactants = Total Products.

The chemical reaction is represented as $CaCO_{3}(s) \rightarrow CaO(s) + CO_{2}(g)$. Given the initial mass of $CaCO_{3}$ is $10\text{ g}$ and the mass of $CaO$ is $5.6\text{ g}$, the mass of $CO_{2}$ is calculated by subtracting the mass of the product $CaO$ from the reactant $CaCO_{3}$: $Mass_{\text{Reactant}} = Mass_{Product1} + Mass_{Product2}$. Substituting the values: $10\text{ g} = 5.6\text{ g} + Mass_{CO_{2}}$, resulting in $Mass_{CO_{2}} = 4.4\text{ g}$.

Always verify if the system is closed; if the container were open, mass would appear to decrease due to the escape of carbon dioxide gas.