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When we talk about the decomposition of calcium carbonate, the law of conservation of mass is central to understanding this process. The principle of conservation of mass states that mass is neither created nor destroyed in a chemical reaction. Instead, it is merely rearranged. In the context of the decomposition of calcium carbonate, the mass of the original compound (calcium carbonate) should equal the sum of the masses of the products (calcium oxide and carbon dioxide).
For the given exercise, the initial mass was 100.00 grams, and after the decomposition, we are left with 56 grams of calcium oxide and 44 grams of carbon dioxide—adding up to the initial mass. This fulfills the law of conservation of mass, confirming that no mass has been lost; it has just been transformed into different substances.

Metal carbonates, like calcium carbonate (CaCO₃), are compounds composed of a metal cation and the carbonate anion (CO₃^2−). One of the key characteristics is that they often decompose upon heating to yield a metal oxide and carbon dioxide gas. This decomposition reaction is commonly observed in carbonates of metals positioned in the second group of the periodic table.
In this scenario, when the unknown white powder A is heated, it decomposes to form gas B and another white powder C. Upon analyzing the properties of gas B and our understanding of metal carbonates, we can deduce that the decomposition of a metal carbonate is indeed taking place.

Decomposition reactions are a type of chemical reaction where a single compound breaks down into two or more products. This kind of reaction is typically endothermic, requiring heat to proceed. In our exercise, when the white powder A is heated, it breaks down into gas B and white powder C. The description of these byproducts corresponds with a typical decomposition reaction—an initial compound breaking down into a simpler substance and a gas.
Understanding decomposition allows students to predict the possible outcomes of heating a compound, especially if it is a metal carbonate, which predictably decomposes into a metal oxide and carbon dioxide.

Stoichiometry is the calculation of reactants and products in chemical reactions. It is a crucial concept in chemistry that lets us predict the amount of products formed from a given amount of reactants. In the context of this exercise, stoichiometry helps to confirm that the mass change corresponds to the expected products of the decomposition reaction of calcium carbonate.
Using stoichiometry, we can see that the molar mass of calcium carbonate is 100 g/mol which decomposes to give calcium oxide (molar mass 56 g/mol) and carbon dioxide (molar mass 44 g/mol). The calculation showed that 44 g of carbon dioxide were formed, which supports the reaction equation given. Through stoichiometry, we can prove that the substances involved in the reaction follow the law of conservation of mass and align with our understanding of the decomposition of metal carbonates and the stoichiometric ratios defined by the balanced chemical equation.