# Questions tagged [temperature]

A measure of the average kinetic energy of the particles in a sample of matter; may be expressed either in kelvin (symbol K) or in degree Celsius (symbol °C).

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### Why is absolute zero unattainable?

We were dealing with the Third Law of Thermodynamics in class, and my teacher mentioned something that we found quite fascinating: It is physically impossible to attain a temperature of zero ...
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### A glass of water with ice-cubes in it. Where's the water the coldest; at the top or bottom?

Suppose that I fill a glass with ice water. As the ice melts, it cools the water around it. Given that cold water is denser than hot water, I would presume that the cold water would sink to the ...
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### Why is −78 °C a magic temperature for organic reactions?

In many organic reactions that I have seen, running the reaction at $\mathrm{-78\ ^\circ C}$ seems to be quite a popular choice, but I've never seen any explanation for this. What makes this ...
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### There's an absolute zero, is there an “absolute hot”?

This might be more of a physics question, but is there a ceiling on how hot things can get? What happens at this temperature?
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### Why do we still need to know about the Rankine temperature scale?

I am learning process principles in chemical engineering and I was taught various temperature units like Fahrenheit, Celsius, Kelvin, and Rankine. I see the first three being used quite commonly, but ...
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### When can a molecule be considered freely rotating at room temperature?

This question sparked from a long discussion in chat about the nature of $\ce{H2O2}$ and whether that molecule can be considered to rotate around the $\ce{O-O}$ axis (and hence display axial chirality)...
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### Why does water evaporate at room temperature?

When water temperature reaches $100\ ^\circ \mathrm{C}$, the molecules get so excited that the hydrogen atoms lose the bonds to the oxygen atom and therefore the water starts to become gas. I get that,...
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### Does the Maxwell-Boltzmann distribution apply to gases only?

The Maxwell-Boltzmann distribution can be used to determine the fraction of particles with sufficient energy to react. I know that the curve applies to gaseous reactants and would like to know whether ...
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### What exactly is temperature?

I've read at many places that temperature is the average kinetic energy of particles present in an object. I just don't intuitively get how kinetic energy is connected with temperature. And how is ...
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### Is there an uncertainty associated with the value 0 K for absolute zero?

When I say absolute zero, I’m not talking about the hypothetical temperature 0 K; I’m talking about the temperature at which a thermodynamic system has the lowest energy. Everywhere I look, sites ...
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### Why is the Haber process carried out at such high temperatures?

On a large scale, ammonia is prepared via the Haber process: $$\ce{N2(g) + 3H2(g)->2NH3(g)} \qquad \Delta _\mathrm{f}H^\circ = -46.1~\mathrm{kJ \cdot mol^{-1}}$$ The optimum conditions for the ...
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### Why no thermal expansion for camping tents?

I know that solid materials expand when heated, which is called thermal expansion. But what is happening to tent square or tent canvas of (plastic?) camping tents? I have noticed, that during the day,...
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### Why do helium balloons expand in volume as they go higher?

I realize as balloons go higher, the atmospheric pressure decreases, doing less to counteract the force of the gas particles pushing against the inner walls of the balloon. But at the same time, doesn'...
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### Are complex ions thermochromic?

Does raising the temperature of a complex ion (formed from a transition metal ion and ligand(s)) affect its color? For example, if the temperature of $\ce{[Cr(H2O)6]^3+}$ is raised, will its color ...
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### Do all gases occupy same volume at equal temperature and pressure conditions?

Why does one mole of all gases occupy 22.4 L at standard temperature and pressure? According to my thinking, it may be due to equal diffusion of all particles when at a particular temperature and ...
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### Negative Kelvin Temperature

I remember my Physical Chemistry Professor saying that very tiny negative Kelvin temperatures have been achieved on the quantum level. Is this true?
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### What is the significance of 273.16 K?

Today, at my chemistry class, we were learning how to convert Celsius to Kelvin with exact values. I said we add 273.15 K to given Celsius but my colleague said we add 273.16 K. Yes, I did check on ...
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### What is the reason for anomalous expansion of water?

What is the reason for anomalous expansion of water? Why doesn’t it simply expand on heating or contract on cooling? Why it shows anomalous behavior at 4 degrees Celsius? Why not on 4.6 or 10 (or ...
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### Is there a structured data source containing information on the World Health Organisation's Stability Testing Policies?

Hoping to automate some product testing guidance software, I've been attempting to find a (hopefully live, up-to-date) Database, Web-service, or other structured data source containing the ...
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### Can paper burn without oxygen or air?

If you put a paper inside an oven vacuum packed and go beyond $250~\mathrm{^\circ C}$, will it burn? There's no oxidizer. If not, what will happen?
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### Why does the distance between molecules increase when the temperature is raised?

I have learnt that when we heat ice-like substances it changes to water and when I asked my teacher she said that the distance between molecules increases. When I thought about it a bit more a ...
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### How to determine omitted units in a publication

I have found a 1954 paper in J. Am. Chem. Soc. which gives various temperatures in degrees but does not specify units. A snippet of the introduction is: Titanium reacts appreciably with fluorine ...
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### Thermochromism in PbI2

I noticed while heating some PbI2 powder that it reversibly changes from a yellow to a dark red. However, I was unable to find anything on this thermochromism. There are related cases in TlI and AgI ...
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### How do I control the negative temperature in an experiment?

If one were to adjust the positive temperature in an experiment, they would use a heating plate, or a stovetop. How would one adjust negative temperatures in an experiment? (Not keeping the ...
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### How is Negative Temperature Hotter than Infinite Temperature?

Recently I have read articles stating that negative Kelvin has been achieved. I was a bit speculative at first as I though how can you get cold than 0 Kelvin, but after doing some research it makes ...
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### Temperature of an atom

I read somewhere that the temperature of an atom is not defined. The definition of temperature is only for larger systems. Why is this so?
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### Is there any scenario in which the size of a molecule increases due to an increase in temperature?

Take an ice cube for example. Heat is applied in a closed container until it is vaporized completely. Will the molecule's size be larger (on average)? Is there a substance that you know of that has ...
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### What is the difference between temperature and thermodynamic temperature?

My course book says that kelvin is the unit of thermodynamic temperature, then what is the difference between temperature and thermodynamic temperature.
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### How are hot and cold defined?

My teacher asked me this question a few days ago. "How to define hot and cold?" And I said, "The higher the temperature, the hotter you are. The lower the temperature the colder you are." But my ...
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### What causes the colour when conducting flame tests on solid salts?

The normal technique for flame tests is to dip a clean nichrome or platinum wire into a solution of the relevant salt, and observe the resulting flame colour when inserting the wire into a non-...
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### What determines humidity limit / dew point of the air? - Why can air only hold a certain amount of water?

From Wikipedia: The dew point is the temperature at which the water vapor in a sample of air at constant barometric pressure condenses into liquid water at the same rate at which it evaporates. At ...
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### What are the degrees of freedom that define the temperature of an ionic solid (such as sodium azide)?

Degrees of freedom describe the different ways atoms move in a sample. For a pure ideal gas made of non-linear molecules, there are 3N degrees of freedom (N is the number of atoms in the molecule), 3N-...
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