Are there any substances that exist as supercritical fluids at STP?

Question

If not, why If so, please give some examples.
I know that some gases can be made into supercritical fluids by compression alone however I am not aware of any substance that is supercritical at stp

Answer 1

Plasma: To maintain a substance in the plasma phase the energy associated with the temperature, $k_\text{B}T$, has to be near to, or greater than, the ionization energy. At room temperature, $k_\text{B}T$ is only $0.025$ eV, and the lowest ionization energy of any element is $3.89$ eV, for cesium, so that is much too high (and Cs is not a gas at STP to begin with).
For molecules, the energy to ionize them (kick out an electron) will not be orders of magnitude lower, so also in that case no plasma will exist at STP. If we let an electric current run through the gas then it could definitely become a plasma, but that might be outside the scope of the question. So the answer is no.

Supercritical fluids: This was already asked in [chemistry SE question 41345]. The answer there is that helium-$3$ requires a pressure of $1.13$ atm, which is very close to STP. So that answer is almost yes (with He-$3$ as example), but strictly speaking no.

Answer 2

Substances cannot exist as supercritical fluids at standard temperature and pressure (STP) because supercritical fluids require conditions where both the temperature and pressure are above their respective critical points.

Understanding the Supercritical State:

1. Critical Temperature and Pressure:

   • The critical temperature is the highest temperature at which a substance can exist as a liquid, regardless of the pressure applied.
   • The critical pressure is the minimum pressure required to liquefy a gas at its critical temperature.

2. Supercritical Fluid:

   • When a substance is heated above its critical temperature and simultaneously subjected to a pressure above its critical pressure, it enters a supercritical state.
   • In this state, the substance exhibits properties of both a gas and a liquid. It can diffuse through solids like a gas but dissolve materials like a liquid.

Why They Cannot Be Supercritical at STP:

• Temperature at STP: $0^\circ$C ($273.15$ K) is below the critical temperature for most substances. For example, the critical temperature for carbon dioxide (CO$_2$) is about $31.1^\circ$C ($304.2$ K), and for water (H$_2$O), it is about $374^\circ$C ($647$ K). At STP, the temperature is simply too low for these substances to reach their supercritical state.

• Pressure at STP: $1$ atm ($101.3$ kPa) is much lower than the critical pressure required for substances to enter the supercritical state. For instance, CO$_2$ requires $73.8$ atm, and water requires $218$ atm to become supercritical. The pressure at STP is insufficient to achieve this state.

However, the theoretical explanation behind why substances cannot exist as supercritical fluids at standard temperature and pressure (STP) is rooted in the concepts of phase transitions, molecular interactions, and the critical point.

1. Phase Transitions and the Critical Point:

• Phases of Matter: At a given temperature and pressure, a substance exists in one of three primary phases: solid, liquid, or gas. The phase depends on the balance between thermal energy (which tends to disperse particles) and intermolecular forces (which tend to hold them together).

• Phase Boundaries: The transitions between these phases (e.g., melting, boiling) occur along boundaries in the phase diagram, where the substance changes from one phase to another based on temperature and pressure.

• Critical Point: The critical point is a specific combination of temperature and pressure where the distinction between the liquid and gas phases disappears. Beyond this point, the substance enters the supercritical phase, where it no longer behaves distinctly as either a liquid or a gas.

2. Molecular Interactions:

• Intermolecular Forces: The behavior of molecules in a substance is governed by intermolecular forces, such as van der Waals forces, dipole-dipole interactions, and hydrogen bonding. These forces determine the substance's state by influencing how closely molecules can pack together.

• Thermal Energy: Temperature reflects the average kinetic energy of molecules. At higher temperatures, molecules have more kinetic energy, which can overcome intermolecular forces, causing the substance to behave more like a gas. Conversely, at lower temperatures, intermolecular forces dominate, leading to liquid or solid phases.

• Balance at Critical Point: At the critical temperature, the thermal energy of the molecules is just sufficient to overcome the intermolecular forces enough that the molecules can no longer be distinctly categorized as being in a liquid or gas state. At the same time, the critical pressure ensures that the molecules are still close enough to interact, preventing them from completely dispersing like in a gas.

3. Why Supercritical State Requires Specific Conditions:

• Temperature Influence: At temperatures below the critical temperature, the thermal energy is not high enough to overcome the intermolecular forces sufficiently to erase the distinction between liquid and gas. The substance will either condense into a liquid (if the pressure is high) or remain as a gas (if the pressure is low).
• Pressure Influence: At pressures below the critical pressure, even if the temperature is high enough to disrupt the liquid structure, the molecules can still spread out enough to behave as a gas, preventing the formation of the supercritical phase.

4. Why Not at STP:

• Sub-Critical Conditions: At STP ($0^\circ$C and $1$ atm), the conditions are below the critical temperature and pressure for all substances. The intermolecular forces dominate over the available thermal energy, leading to distinct phase behaviour (solid, liquid, or gas) rather than the merged properties seen in a supercritical fluid.
• Phase Distinction: Under these conditions, the substance will either be in the liquid phase (if it condenses) or the gaseous phase (if it evaporates), as the balance between intermolecular forces and thermal energy does not allow for the unique properties of a supercritical fluid.

Conclusion:

The supercritical state is a unique phase that requires both the temperature and pressure to exceed specific critical values. These conditions alter the balance between thermal energy and intermolecular forces, leading to the merging of liquid and gas properties. At STP, neither the temperature nor the pressure is sufficient to achieve this balance, so substances remain in their conventional phases, unable to exist as supercritical fluids.