Final answer:
The label 'at room temperature' is important for clarity about the physical state of elements, considering that states can change with temperature.
Step-by-step explanation:
It is important to include the label "at room temperature" in the second column heading because the state of a substance can change with temperature. Substances like nitrogen, oxygen, and chlorine exist as gases at room temperature, whereas others like mercury and bromine exist as liquids, and most metals such as aluminum are solids. By specifying "at room temperature," we acknowledge that the physical state of these substances can vary according to the temperature.
For example, at room temperature, nitrogen is a gas, carbon is a solid, chlorine is a gas, bromine is a liquid, and aluminum is a solid. However, if we change the temperature, each of these elements could exist in a different state. Therefore, the label "at room temperature" provides clarity about the conditions under which these states are observed.