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What is Critical Temperature (Thermodynamics)?
Grade Level:
Class 12
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Definition
What is it?
Critical Temperature is the highest temperature at which a gas can be converted into a liquid, no matter how much pressure is applied. Above this temperature, a substance will always remain in its gaseous state and cannot be liquefied by pressure alone.
Simple Example
Quick Example
Imagine you're trying to make a soft drink by adding fizz (carbon dioxide gas) to water. If the water is too warm, the fizz just escapes. But if the water is cold enough, the gas dissolves and stays in. Critical temperature is like that 'cold enough' point for a gas to become a liquid, even under super high pressure.
Worked Example
Step-by-Step
Let's say we have a gas, Carbon Dioxide (CO2), and its critical temperature is 31 degrees Celsius.
Step 1: We have CO2 gas at 25 degrees Celsius. This temperature is below its critical temperature.
Step 2: We apply high pressure to the CO2 gas at 25 degrees Celsius. Because it's below its critical temperature, the CO2 gas will turn into a liquid.
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Step 3: Now, we have CO2 gas at 35 degrees Celsius. This temperature is above its critical temperature (31 degrees Celsius).
Step 4: We apply very high pressure to the CO2 gas at 35 degrees Celsius. Even with extreme pressure, the CO2 will NOT turn into a liquid. It will remain a gas.
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Answer: CO2 can be liquefied at 25 degrees Celsius but not at 35 degrees Celsius because 31 degrees Celsius is its critical temperature.
Why It Matters
Understanding critical temperature is vital for engineers designing refrigeration systems, storing industrial gases, and even in medicine for cryopreservation. It helps in fields like Climate Science to study atmospheric gases and in Space Technology for handling rocket fuels, opening doors to careers in research and development.
Common Mistakes
MISTAKE: Thinking that any gas can be liquefied by applying enough pressure at any temperature. | CORRECTION: A gas can only be liquefied by pressure if its temperature is at or below its critical temperature.
MISTAKE: Confusing critical temperature with boiling point. | CORRECTION: Boiling point is the temperature at which a liquid turns into a gas at a specific pressure (usually atmospheric). Critical temperature is the maximum temperature at which a gas can be liquefied, regardless of pressure.
MISTAKE: Believing that above critical temperature, a substance becomes a different chemical. | CORRECTION: Above critical temperature, the substance is still the same chemical, but it exists as a supercritical fluid, which has properties of both a gas and a liquid.
Practice Questions
Try It Yourself
QUESTION: The critical temperature of Ammonia (NH3) is 132.4 degrees Celsius. Can Ammonia gas be liquefied at 130 degrees Celsius by applying pressure? | ANSWER: Yes, because 130 degrees Celsius is below its critical temperature.
QUESTION: If a gas has a critical temperature of -82 degrees Celsius, and you have it at -70 degrees Celsius, will it turn into a liquid if you apply immense pressure? Explain why. | ANSWER: No, it will not. Because -70 degrees Celsius is above its critical temperature (-82 degrees Celsius), it cannot be liquefied by pressure alone.
QUESTION: Gas X has a critical temperature of 50 degrees Celsius. Gas Y has a critical temperature of 200 degrees Celsius. Which gas would be easier to liquefy at room temperature (around 25 degrees Celsius) and why? | ANSWER: Gas Y would be easier to liquefy. Its critical temperature (200 degrees Celsius) is much higher than room temperature, meaning it can be liquefied by applying pressure at 25 degrees Celsius. Gas X, with a critical temperature of 50 degrees Celsius, can also be liquefied at 25 degrees Celsius, but Gas Y offers a wider range of temperatures for liquefaction.
MCQ
Quick Quiz
What happens to a gas if its temperature is above its critical temperature?
It can be liquefied by applying very high pressure.
It turns into a solid.
It cannot be liquefied by applying any amount of pressure.
Its chemical composition changes.
The Correct Answer Is:
C
Above the critical temperature, a substance exists as a supercritical fluid and cannot be converted into a liquid by pressure alone, no matter how high the pressure. Options A, B, and D are incorrect based on the definition.
Real World Connection
In the Real World
Critical temperature is crucial for storing gases like LPG (Liquefied Petroleum Gas) in cylinders for cooking at home. LPG is stored as a liquid under pressure because its critical temperature is high enough (around 100-120 degrees Celsius) that it can be liquefied at normal room temperatures. This makes it convenient and safe for transport and use in Indian kitchens.
Key Vocabulary
Key Terms
CRITICAL TEMPERATURE: The maximum temperature above which a gas cannot be liquefied by pressure alone | LIQUEFACTION: The process of turning a gas into a liquid | SUPERCRITICAL FLUID: A substance above its critical temperature and critical pressure, having properties of both a gas and a liquid | PRESSURE: Force applied per unit area | GAS: A state of matter with no fixed shape or volume
What's Next
What to Learn Next
Now that you understand critical temperature, you can explore 'Critical Pressure' and 'Critical Volume'. These concepts are closely related and will help you fully grasp the behavior of gases and supercritical fluids, which is important for advanced chemistry and physics.
