What is a common by-product of the combustion of alcohols?

When alcohols undergo combustion, they react with oxygen to produce energy, water, and carbon dioxide as by-products. This process involves burning the alcohol in the presence of oxygen, leading to the formation of carbon dioxide due to the carbon atoms in the alcohol. This is a common characteristic of hydrocarbon-based fuels, including alcohols, where carbon combustion typically results in carbon dioxide being released into the atmosphere.

In this context, carbon dioxide is considered a significant by-product of burning alcohols, making it the correct answer. Other options do not fit within the standard products of alcohol combustion. Methane, for instance, is not produced during the combustion process of alcohols; rather, it is a different type of alcohol or a distinct hydrocarbon altogether. Oxygen, while initially present in the combustion reaction, is consumed rather than produced, and hydrogen gas is not a typical by-product in this scenario. Thus, the production of carbon dioxide is a key aspect of alcohol combustion, aligning with the expectations for this chemical reaction.