49 votes
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Are diamonds really forever?

how long would it take for this super-material to convert to the stuff I scribble with? No, despite the fact that James Bond said "Diamonds are Forever", that is not exactly the case. Although Bond'...
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26 votes
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At what pressure will hydrogen start to liquefy at room temperature?

$\ce{H2}$ cannot be liquified at room temperature, whatever the pressure. Generally speaking, all gases can only be liquified when the temperature is under its critical value.
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23 votes

At what pressure will hydrogen start to liquefy at room temperature?

The critical temperature of Hydrogen is $\pu{32.938 K, resp. -240.21 ^{\circ}C}$. Above this temperature, it cannot be liquified. So to answer your question, you can get as high pressure as you can ...
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22 votes

Aside from carbon, what other substances can be made "superhard"?

Interestingly, nobody addressed the reason why diamonds are hard in the first place. The pressure (and temperature) are not the reason why they're hard, only the reason why they are formed. The ...
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  • 331
21 votes

Aside from carbon, what other substances can be made "superhard"?

Here are some compounds that have other structures, followed by their hardest structure (based on Moh's Scale). Titanium dioxide: Rutile structure or Cotunnite structure Aluminum oxide: Corundum ...
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11 votes

What is "chemical pressure"?

External and internal pressure To study the effect of pressure on properties of a solid, is equivalently to learn how changes in volume transform physical parameters. For external pressure at ...
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10 votes
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Why do we use the external pressure to calculate the work done by gas

If the piston is frictonless and massless, then, if you do a force balance on the piston, you must have that the force per unit area that the gas exerts on the inside face of the piston will always be ...
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10 votes
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Pressure exerted by a particle of ideal gas

Here's your confusion: You need to consider two different things: The momentum transfer per particle per collision. There, since we assume an instantaneous collision, it doesn't make sense to try to ...
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9 votes
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Can gaseous hydrogen and gaseous oxygen be compressed without reaction?

The study Explosion Characteristics of Hydrogen-Air and Hydrogen-Oxygen Mixtures at Elevated Pressures includes data for pressures up to 200 bar. Data were collected for Hydrogen-Oxygen mixtures at ...
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9 votes
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Can silicon dioxide melt at 20 °C?

Question: Can $\ce{SiO2}$ melt at $\pu{20 ^\circ C}$? According to experimental and calculated data values, my answer is no. See the phase diagram of pure silica based on the experimental and ...
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Is it possible to store moist air at high pressure?

There is one importing principle to consider. Air does not store vapor. Space does. Less space means less vapor capacity, no matter how much of air is there. (*) If air is being compressed, the ...
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8 votes

Room-conditions supercritical fluids?

As you can see on this data page on Wikipedia helium-4 has critical pressure of 2.24 atm, but helium-3 has 1.13 atm, only slightly above atmospheric pressure. Hyperbaric chambers work with 1,3-1,5 atm,...
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8 votes

Why do helium balloons expand in volume as they go higher?

You are exactly correct that it is a matter of atmospheric pressure decreasing at a rate great enough to overcome the contraction due to decrease in temperature. On a nice, clear, dry 25°C day at sea ...
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8 votes
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Help with two containers problem, Xenon

I have calculated pressure in container one and that is p1 = 342817.92 Pa, and in container two p2 = 326979.2275 Pa. Then I have substracted this two values which gives me 15838.6925 Pa. After that I ...
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7 votes

How can you achieve negative pressure?

The term “negative pressure” is often used in engineering to refer to a situation in which an enclosed volume has lower pressure than its surroundings. If the a region is surrounded by more ...
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7 votes

Are diamonds really forever?

Well, it seems that diamond is not forever because upon exposure to sunlight it loses atoms.
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7 votes

Are diamonds really forever?

I add an other answer : it depends what kind of people you are, if you are as crazy as them, of course not ! 2012 Christmas Lectures - Burning a Diamond But ron's answer is better sure ! ;)
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7 votes
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Van der Waals real gas equation

Let me begin my discussion by first introducing the notion, using some intuitive model building, and will try to derive, or perhaps guess the van der Waals equation of state. The perfect gas ...
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7 votes

Does exotic potassium exist?

First, a point that is more like a comment: I'm not sure that just because electrons get put in to d-orbitals (and thinking of crystal band structures in terms of atomic orbitals is of questionable ...
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7 votes

Why is my solution for the partial pressures of two different gases incorrect?

The answer by camd92 is correct, but there is no reason to calculate amounts. According to the chemical equation, one third of the particles in the gas phase will be dinitrogen oxide, and two thirds ...
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7 votes

Pressure-dependence of electrolysis voltage

Of course, we're starting by acknowledging a perpetual motion machine is impossible. The question, then, is how do we understand, through chemical thermodynamics, why your specific set of steps can't ...
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7 votes

Regarding units of water potential and chemical potential

According to German Wikipedia water potential is defined as $$\psi := \frac{\mu - \mu_0}{\bar{V}},\tag{1}$$ where $\mu_0$ is the standard chemical potential (usually pure water at atmospheric pressure ...
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7 votes

At what pressure will hydrogen start to liquefy at room temperature?

As others have said, hydrogen cannot be liquified above its critical temperature, which my source (Wolfram Alpha chemical database*) says is $\pu{32.97 K} = \pu{-240.18 ^\circ C}$ However, with ...
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6 votes

Room-conditions supercritical fluids?

A number of long carbon chain molecules with large numbers of fluoride atom, have $P_c < \pu{10 atm}$ and $T_c < \pu{1000 K}$. That is within the reach of a good bicycle pump and a burner. In ...
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6 votes

Will supercritical nitrogen stay supercritical if we raise temperature and pressure high above critical values?

If we maintain nitrogen at 220 bar and 400 °C, what will be the state of $\ce{N2}$? Will it be still supercritical? Yes, it will still be supercritical. The only reasonable definition of "...
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6 votes

Entropy and specific heat capacity

It is not true that heat exchanged at constant pressure is always reversible. But, if you want to determine the change in entropy from thermodynamic equilibrium state 1 at $(T_1,P)$ to state 2 at $(...
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6 votes

What is "chemical pressure"?

First off, "chemical pressure" (CP) is often portrayed as an empirical concept or effect for linking chemical composition and physical properties in the first place, and not a physical quantity -- ...
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