What is the primary reason carbon dioxide is considered a hot gas?

Carbon dioxide is classified as a hot gas primarily due to its ability to retain heat, a characteristic linked to its relatively low thermal conductivity. Low thermal conductivity means that carbon dioxide does not transfer heat away as efficiently as gases with high thermal conductivity, thus allowing it to effectively trap heat in the atmosphere.

This property becomes particularly significant in understanding the behavior of greenhouse gases, of which carbon dioxide is a major component. It plays a crucial role in the greenhouse effect by absorbing and emitting infrared radiation, leading to an increase in the temperature of the atmosphere. As it retains heat, it contributes to the warming of the environment, which is why it is often referred to as a "hot gas" in discussions about climate change and atmospheric science.

In contrast, the other options focus on aspects that do not primarily relate to the heat-retaining properties of carbon dioxide. For instance, while it does result from combustion and is expelled during exhalation, these facts alone do not define its role as a hot gas. Thus, it is the ability of carbon dioxide to retain heat that fundamentally categorizes it as a hot gas.