Final answer:
The substance NOT in the standard state at 25 °C is D. 1 M of CO₂ (aq), as the standard state for carbon dioxide is as a gas at 1 atm pressure, not in an aqueous solution.
Step-by-step explanation:
The student asked to identify which substance is NOT in the standard state at 25 °C. The standard states for elements and compounds are defined at 1 atm pressure and 25 °C, except for substances in solutions, which are typically defined at 1 M concentration. A standard state is a reference point used to define properties of substances such as enthalpy, entropy, and Gibbs free energy.
Considering the options given:
- N₂(g) at 1 atm and O₂(g) at 1 atm are both in their standard states, being pure substances (gases) at 1 atm pressure.
- 1 M KNO₂ (aq) is a solute in aqueous solution at 1 M concentration, which qualifies as the standard state for a dissolved substance.
- CO₂ (aq) is somewhat tricky, as the standard state for carbon dioxide is as a gas at 1 atm pressure.
Considering these points, the substance NOT in the standard state at 25 °C must be 1 M of CO₂ (aq) because the standard state for carbon dioxide should be as a gas, not in an aqueous solution. Therefore, the correct answer is D. 1 M of CO₂ (aq).