A solid white substance \(A\) is heated strongly in the absence of air. It decomposes to form a new white substance \(\mathrm{B}\) and a gas C. The gas has exactly the same properties as the product obtained when carbon is burned in an excess of oxygen. Based on these observations, can we determine whether solids A and \(\mathrm{B}\) and gas \(\mathrm{C}\) are elements or compounds?

A solid white substance (A) decomposes to form a new white solid substance (B) and a gas (C). The gas has the same properties as the product obtained when carbon is burned in an excess of oxygen.

Since the gas produced has the same properties as the product obtained when carbon is burned in an excess of oxygen, we can identify the gas as carbon dioxide (CO₂). Carbon dioxide is a compound, as it is formed by the combination of carbon (an element) and oxygen (another element).

We have limited information on A and B, but we do know that when solid A is heated strongly, it decomposes into solid B and the gas C (carbon dioxide/CO₂). The decomposition of solid A into two different substances suggests that it is most likely a compound, as elements tend to form compounds when heated, rather than breaking down further. As to solid B, we know a new white substance is formed from the decomposition process. Since we have no information beyond this, we cannot definitively conclude if it is an element or a compound based solely on the given observation. In conclusion, we can determine that gas C (carbon dioxide) is a compound, and solid A is most likely a compound. However, we do not have enough information to make a definitive conclusion about the nature of solid B as an element or a compound.