Final answer:
At -20 ºC and 1500 atm, carbon dioxide is in a liquid state, according to its phase diagram.
Step-by-step explanation:
The state of carbon dioxide (CO₂) at -20 ºC and 1500 atm can be determined using a phase diagram. According to the phase diagram, CO₂ can only exist as a solid, liquid, or gas. The phase diagram illustrates that at a pressure of 1500 atm and temperatures below the critical point, CO₂ is likely to be a liquid.
It's important to note that CO₂ sublimates at 1 atm, meaning it transitions from solid to gas without becoming liquid. However, since the triple point of CO₂ is -56.6°C and 5.11 atm, CO₂ can exist as a liquid at -20 ºC if the pressure is sufficiently high, such as 1500 atm.