# Tag Info

175

For organic matter, such as bread and human skin, cutting is a straightforward process because cells/tissues/proteins/etc can be broken apart with relatively little energy. This is because organic matter is much more flexible and the molecules bind through weak intermolecular interactions such as hydrogen bonding and van der Waals forces. For inorganic ...

121

Wow, this one has been over-answered already, I know... but it is such a fun question! So, here's an answer that hasn't been, um, "touched" on yet... :) You, sir, whatever your age may be (anyone with kids will know what I mean), have asked for an answer to one of the deepest questions of quantum mechanics. In the quantum physics dialect of High Nerdese, ...

78

Density relates to the mass per unit volume. If your molecules are heavier but take up more space, the net result could be more or less mass per unit volume. When you look at a typical hydrocarbon, it has a lot of carbon and hydrogen. Now atom for atom, oxygen is heavier than carbon (ignoring isotopic abundance, roughly a 16:12 ratio). So if the molecules ...

74

Because water molecules are small and pack tightly together, causing water to have a greater density than petrol.

41

I'll do that teacher thing and turn your question around back at you. Why isn't the spectrum of the lithium atom just the spectrum of the hydrogen atom plus the spectrum of the helium atom? And, for that matter, why is the helium spectrum not simply two copies, somehow, of the hydrogen spectrum? Why do atoms have unique spectra in the first place? The ...

39

The fancy word terminology for this is that the concept of heat supervenes on the concept of molecules moving. We can think about heat as molecules moving. We can also think about a lot of non-heat things as molecules moving too. The kinetic energy of molecules moving is a very low-level concept, so it can be used to describe a lot of things. The only ...

36

Research has created antihydrogen, and that is about it for the present as far as antimatter in bulk, which one would need for antiwater.. Scientists in the US produced a clutch of antihelium particles, the antimatter equivalents of the helium nucleus, after smashing gold ions together nearly 1bn times at close to the speed of light. They were gone ...

28

Anti-matter is a lot less exciting than you probably think. If we could magically change all matter to anti-matter by waving a magic wand then it would make almost no difference. The anti-Dirk could drink an anti-glass of anti-water in exactly the same you drink a glass of water. The anti-water would have the same density, boiling point, ability to dissolve ...

27

I notice that online definitions of this experimental law always say, molecules or atoms. The problem with just calling them all "molecules" and being done with it is some are uncomfortable with using that term for unbound atoms. If you have a container of He, there are no "molecules" in it. So when it says "molecules or atoms", it means "molecules or ...

25

The energy levels of a diatomic molecule are $E = 2B, 6B, 12B$ and so on, where $B$ is: $$B = \frac{\hbar^2}{2I}$$ Most of the mass of the molecule is in the nuclei, so when calculating the moment of inertia $I$ we can ignore the electrons and just use the nuclei. But the size of the nuclei is around $10^{-5}$ times smaller than the bond length. This ...

22

It depends on what's being cut. When metal is cut, what happens is that, on a small or not so small scale, it shears. That means layers slide over each other. The mechanism by which they slide over each other is that there are imperfections in the crystal structure called dislocations, and the crystal layers can move by making the dislocations move in the ...

19

To start with, "water freezes faster when it starts out hot" is not terribly precise. There are lots of different experiments you could try, over a huge range of initial conditions, that could all give different results. Wikipedia quotes an article Hot Water Can Freeze Faster Than Cold by Jeng which reviews approaches to the problem up to 2006 and proposes a ...

17

I'm not sure I would have phrased it exactly that way, but I think his statement is by-and-large defensible. The crux of the issue is that, in liquid water, there is no sharp line between intramolecular and intermolecular H-O bonding. The intramolecular H-O bond is closer than the intermolecular H-O hydrogen bond, but it's a difference of degree not kind, ...

17

The idea that there must be some reason that all terrestrial DNA has a right-handed twist (or D- vs. L-glucose, or whatever your favorite chiral biomolecule is), goes all the way back to the discovery of biomolecular chirality by Pasteur. The connection to the weak nuclear interaction is apparently known as the "Vester–Ulbricht hypothesis," after its first ...

16

Common sense of touching can be expressed in "scientific means" as an event when exchange-repulsion interaction between 2 objects (you and the geek) extends some arbitrary value, say 1meV. I leave finding an agreeable threshold which is easy to measure to later discussion. :)

16

$H_2$ contains 2 electrons in the same ground-state orbital; by Pauli exclusion, one must be spin-up and the other must be spin-down. The 21cm line is generated in a normal hydrogen atom when an electron's spin flips from being aligned with the proton to being anti-aligned with the proton. In $H_2$, an electron's spin cannot flip because it would then be ...

15

I think it's tosh. A very small fraction of liquid $\rm H_2O$ will spontaneously decompose and recombine with very small amount of $\rm H_3O^+$ (hydronium ion) and $\rm OH^-$ (hydroxyl) ions. Weak hydrogen bonds may form between neighboring water molecules, but that doesn't really count as $\rm H_4O_2$ or higher order molecules. While I have a great deal ...

14

It's both: At the smallest scale, thermal energy is just kinetic energy, the energy of motion. That is, thermal energy (heating up) and kinetic energy are the same thing. When molecules vibrate, they're bumping into each other—transferring kinetic energy to other molecules, which sometimes radiate this energy as heat (on a larger scale). Note, too, that ...

12

As a useable heuristic I would go with something along the lines of the intermolecular forces between the surface molecules of the bodies are comparable to the scale of one-to-one intermolecular forces between nearby{*} molecules due to other components of the same body You could make it a little more strict by replacing "comparable to" with "non-...

12

Your body is warmer than the surrounding air and as such when heat escapes from your body it warms up that air; if it didn't, you would have overheated years ago. However, if the air isn't moving, that air around you begins to warm up. Heat transfer is faster if the temperature difference is greater. What wind does is move that warm air away, and replace ...

11

I think it is a mistake, as often happens in popularizations of science. A water or any molecule may lose kinetic energy and acquire potential energy, but it is the kinetic energy distribution that gives the temperature of an ensemble of molecules. The shape of the distribution shows that there will always be individual molecules at very high energy, in ...

11

Without intending any disrespect, I'm quite surprised that several very knowledgeable people have given a wrong, or at least incomplete, answer to this old question. For a single molecule that is in complete isolation, it is indeed generally not true (or at least not useful) to assign it a temperature, as others have said. Such a system would be more ...

11

The red, orange, yellow, and white parts of a candle flame results from glowing soot. The color in this part of the flame is indicative of the temperature. The spectrum in this part of the flame is fairly close to that of a black body. The blue part of the candle flame at the bottom of the flame results from chemiluminescence. Chemiluminescence is not black ...

11

Just an addition to John Rennie's answer. The equipartition theorem can only be derived in classical statistical physics. In quantum statistics it is not correct. For each degree of freedom there is a characteristic temperature below which the quantum effects are significant. This temperature is very high for rotation around the axis of the molecule; I guess ...

11

Answering this question inevitably leads to stumble upon one of the most controversial topics in astronomy. First let’s give some context here by reminding why tracers of $\mathrm{H_2}$ are required in the first place: Molecular hydrogen – or $\mathrm{H_2}$ – is the most abundant molecule in the universe, hence it embodies what is called the ...

11

First of all, high pressure (i.e., compressive equitriaxial stress) is simply not going to induce cracks (or failure of any sort) in a uniform solid—there's just nowhere for the material to move to reduce the sum of strain energy and surface energy. Yes, melting a metal and then freezing it might increase the density in several ways, although the ...

10

As the other answers have said, temperature is a collective property and can only be defined when you have an assemblage of particles. However by definition in a molecule you have an assemblage of atoms, and they have relative motions described by the vibrational excitations of the molecule. So if you have a large enough molecule you can look at the ...

10

This answer I once gave for What does it mean for two objects to "touch"? discusses what touching even means. It's not a direct answer to your question, but I think it may help you view the issue in a different way. Warning: It's one of my long, talky answers that some people love and others hate. The physics in it is accurate (and for many folks, unexpected)...

9

Molecules aren't just sums over their constituent atoms. There's many different kinds of bonds which involve different patterns in the overlap of electron orbitals, and which affect the energy levels those electrons can occupy - I'm assuming the QP video you watched explained how "color" relates to electron energy levels. The (hydrogen-like-)atom case is ...

9

Your intuition is spot on. If we have two atoms approaching each other with a large kinetic energy then they will have too much energy to form a stable molecule. Their electrons will interact as they approach, but the two atoms will simply whizz past each other and head off into the distance. In many cases the reaction is more like: \mathrm{AB + CD \to ...

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