163 votes
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Why is absolute zero unattainable?

There was a story in my days about a physical chemist who was asked to explain some effect, illustrated by a poster on the wall. He did that, after which someone noticed that the poster was hanging ...
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73 votes

Why is absolute zero unattainable?

Absolute zero is a tricky concept, particularly once you start getting precise about it. Thermodynamics and quantum mechanics is a tricky business! I'll try to avoid the precise parts, and see if I ...
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38 votes
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Why is −78 °C a magic temperature for organic reactions?

Dry ice (solid carbon dioxide) sublimes at −78 °C. Dry ice and acetone are a common cold bath for chemical reactions. The melting point of acetone is -95 °C so the bath never gets cold enough to ...
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38 votes

Why do we still need to know about the Rankine temperature scale?

Rankine is commonly used in the aerospace industry in the United States. Rankine is to Fahrenheit what Kelvin is for Celsius. So when people in the United States were creating programs and using ...
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33 votes
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A glass of water with ice-cubes in it. Where's the water the coldest; at the top or bottom?

Interesting question! A few things first: As the ice melts, it cools the water around it. Technically, the ice cube melts because the water cools down. This may sound ridiculous at first, but you ...
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30 votes
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There's an absolute zero, is there an "absolute hot"?

In the actual theories of physics the highest temperature which has a physical meaning is the Planck's temperature. $$T_\mathrm{P} = \frac{m_\mathrm{P} c^2}{k} = \sqrt{\frac{\hslash c^5}{G k^2}} \...
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20 votes

Is there an uncertainty associated with the value 0 K for absolute zero?

As another answer explains, absolute zero is defined in the Kelvin temperature scale as precisely $\pu{0 K}$. But this is not the entire story, as all measurements have an associated uncertainty (all ...
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18 votes

Why do we still need to know about the Rankine temperature scale?

Most of us in the world use the Celsius scale to measure temperature for day-to-day purposes. The Kelvin scale has been designed in such a way, it is not only an absolute temperature scale, but also 1°...
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18 votes

What is the difference between "vapour" and "gas"?

Vapor is a much older word alluding to dampness and it was not coined by scientists. It is in use since the 1300s. The actual meaning of meaning of vapor is "Matter in the form of a steamy or ...
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17 votes
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Can paper burn without oxygen or air?

No, the paper will not burn without oxygen being present. Paper is made primarily of cellulose which is a polymer of glucose. If you heat paper in a vacuum the cellulose simply decomposes to $\ce{...
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17 votes
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What is the significance of 273.16 K?

The Celcius scale was originally based on the freezing and boiling points of water, so 0 °C was chosen as the freezing point until 1954. But now, the size of one degree on both Celcius and Kelvin ...
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17 votes
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When can a molecule be considered freely rotating at room temperature?

I'm not a transition-state physical chemist, but I think a good approach to this problem is transition-state theory, specifically the Eyring equation: $$k = \kappa \frac{k_b T}{h} e^{\frac{-\Delta G^{...
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17 votes
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Why is the Haber process carried out at such high temperatures?

As others have pointed out, it is purely kinetics, but you may still wonder, why. For a reaction to actually occur (in both directions) and thus for an equilibrium to be reached, you need to overcome ...
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17 votes

Why is absolute zero unattainable?

Leaving quantum mechanics aside (it gives me a headache) the second law of thermodynamics prevents absolute zero from being reached in practice. To cool something down, its heat must be transferred ...
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17 votes
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Why do helium balloons expand in volume as they go higher?

I didn't know that balloons expanded during the fly because of thermodynamics, and I didn't know how high they can fly, but a rapid search tells that a partially unfilled regular balloon can fly until ...
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16 votes
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Do all gases occupy same volume at equal temperature and pressure conditions?

Why do all gases occupy 22.4 L [per mol] at STP? The question is based on a false premise. Only ideal gases are guaranteed to occupy 22.4 L/mol at STP. There are many gases that are not ideal. ...
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14 votes

There's an absolute zero, is there an "absolute hot"?

It depends on what you mean by ceiling. Are we talking about a practical or theoretical limit? At a high enough energy, the stress-energy tensor will be large enough that you're going to make a black ...
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14 votes

Does motor oil get more viscous when heated & what is the meaning of the SAE motor oil code?

The numbers in the code have nothing to do with viscosity index. However, your understanding of 'W' for winter is correct. The real code for your car is actually SAE 5W-30 where SAE stands for the ...
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14 votes

What is the difference between "vapour" and "gas"?

The normal use distinguishes "vapour" from permanent gas At normal lab conditions there is a (fairly obvious) distinction between things that could exist as liquids and things where no ...
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13 votes

There's an absolute zero, is there an "absolute hot"?

Depends on what you mean by "temperature". In statistical mechanics, a system of interacting parts is in thermal equilibrium if the probability of finding a given part in a state with energy $E$ is ...
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12 votes
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Temperature of an atom

Thermodynamic functions are strictly defined only for macroscopic systems (systems that have an essentially infinite number of atoms). You can't apply them to individual atoms because that would be ...
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12 votes
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How is Negative Temperature Hotter than Infinite Temperature?

The other answer given is correct, but for completeness, I will give the thermodynamic, rather than statistical mechanics, explanation. Thermodynamics gives us that $$\frac{\partial E}{\partial S}=T$$...
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12 votes

Why is the Haber process carried out at such high temperatures?

The answer mainly has to do with kinetic considerations, as aml points out. I want to point out another thing. In a typical industrial setting, you don't just mix the $\ce{N2}$ with the $\ce{H2}$ at ...
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12 votes

A glass of water with ice-cubes in it. Where's the water the coldest; at the top or bottom?

The convection to produce uniformity depends on a number of nebulous factors: How much ice? How tall is the glass? Diameter of the glass? Is the "glass" really a glass or paper cup, styrofoam cup, ...
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  • 21.4k
12 votes

Why do we still need to know about the Rankine temperature scale?

I am currently studying mechanical engineering in the US, and I have used Rankine. It is used similarly to Kelvin. For example, in my thermodynamics class we used it to analyze various heat engines. ...
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12 votes

What exactly is temperature?

Heat is the transfer of energy to or from the body in forms other than matter flow or work (organized energy transfer, such as pushing). Temperature is only a well-defined property for a collective ...
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12 votes

What exactly is temperature?

Temperature vs kinetic energy [OP:] I've read at many places that temperature is the average kinetic energy of particles present in an object. Temperature has to do with the average kinetic energy ...
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11 votes
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At what temperature (in kelvin) are most of the elements on the periodic table liquids?

Now that's a great question indeed! Evidently, at 0K all elements except helium are solids, at 10000K they are all gases, so someplace in between the number of liquids must reach a maximum; what and ...
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