# Tag Info

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Thermodynamics. The second law of thermodynamics states that entropy always increases in an isolated system. This is taken as a fundamental postulate---we simply accept this statement as a fact regarding how the world works, and our justification is that no experiment has ever shown the second law incorrect. In the framework of macroscopic thermodynamics, ...

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It appears you're looking for an ELI5-style answer, not an elaborate definition. Entropy just happens – as long as the universe isn't frozen solid, things will always be moving around, and that movement tends to introduce randomness more than it tends to introduce order. Consider a deck of cards. Shuffle it. Is it perfectly sorted? No. Why? There are 10^67 ...

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Do not think of entropy as 'disorder' as this is misleading, better is that it is a 'measure of disorder' but this is equally vague. It is better to think of entropy as the number of ways that 'particles' or quanta (say vibrational or rotational quanta in a molecule) can be placed among the various energy levels available. Thus at zero energy all the ...

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There is a general belief that plastics are impermeable, probably because many liquids are stored in plastic containers. This is, however, not correct. Plastics are permeable to water and also to gases. The permeability is low but is not zero and therefore, when very long periods of time are involved, a perceptible change occurs. If absolute impermeability ...

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Diffusion also occurs in solids. For example, (intentional or undesirable) movement of impurity atoms or other crystal defects can be very important for the fabrication of semiconductors. The classical experiment to demonstrate diffusion (and to measure the diffusion coefficient) in solids is the diffusion of gold in lead. A thin layer of gold that is ...

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Osmotic pressure for non-electrolytic solutes is given by $$\pi = CRT$$ where $C$ is the effective concentration of all the solutes. In our case, with multiple solutes, we simply add all their concentrations to obtain the effective concentration. This gives us \begin{align} \pi_\mathrm{cell} &= 0.05RT\\ \pi_\mathrm{environment} &= 0.03RT \end{... 4 Are the balls exactly the same size as when purchased? Perhaps the skin of the ball has expanded by absorption of some of the contents. If this has happened, an indentation could occur even without escape of contents. On the other hand, I have stored aqueous solutions in PETE bottles (from Diet Coke), and over a couple of years, water has evaporated out and ... 4 Whole milk with food coloring solved my problem. Whole milk has a specific gravity of 1.01, which allowed it to settle to the bottom around the main drains. The diffusion rate was extremely slow over regular pool dye, which allowed the dyed milk to linger around the drains for an extended period of time (5+ minutes) to allow for the detection of the slow ... 3 You start from a false premise, "Diffusion occurs from low to high concentration". Within a phase the expected behavior is exactly opposite. However between phases things can get more complicated. The direction of diffusion at the interface (on the resolution scale of your diagram) is based on relative solubility. At higher resolution, the higher ... 3 (1) determine at which side substance A will diffuse out of the membrane should I answer at the right side or that the substance stays within the membrane? Neither, because the concentration is higher in the membrane than either side, the substance will flow out of both sides initially at least. After a while the material will flow from left to ... 3 I am sorry, this is not an answer but a comment, too long to be edited in the comments section. I don't agree with "straightforward" in the sentence : " It is relatively straightforward to understand that the more the diffusion layer grows, the shallower the concentration gradient gets and therefore that the current decreases. " This would be an ... 3 Spray perfume in the air. Ask the people around you to raise their hands when they can smell it. The people closest to you will raise their hands first. People farther away will raise then hands after that. A perfect example of diffusion. 3 As you mention most colored gases (Cl2, Br2, NO2, I2) are toxic and being a gas difficult to handle. Therefore not suitable for such demonstration. Maybe, you could try two small colored smoke bombs in a transparent container although the colour is not really a gas (more like fine particulate) in this case. I would use liquids with the most common diffusion ... 3 There is at least a fair degree of consensus (there will always be naysayers) that the results that were performed back in the 80s and 90s by Berner and Landis were false positives that could not be confirmed by further investigations. A good historical account of this can be found for instance in Nick Lane's book "Oxygen" (first published in 2002). Quoting ... 3 Its not diffusion, it's effusion. The difference is that with diffusion the particles/molecules are migrating through a permeable medium where as for effusion the particles/molecules are migrating through a pore/hole. For example, a scent travelling across a room is diffuision, a scent from the room traveling outside is effusion. Also fun fact: the helium ... 3 There is no set answer to your problem, it depends on the geometry of the diffusional process. I give two examples. (a) Suppose there is a volatile solute in a solvent placed in a beaker and the solute evaporates from the surface, then there is a concentration gradient of solute away from the surface as the solute is lost into the air. Similarly, if there ... 3 Could anyone please explain how to determine the direction of the mass transfer [...] In order for concentrations to change (given constant volumes and no chemical reactions), there has to be mass transfer. If the concentration in a solvent increases over time, mass transfer is to that solvent, if it decreases over time, it is away from that solvent. So ... 3 More a set of queries and comments than an answer. I don't understand why you make A and B vary with time irrespective of your reaction scheme. You should really analyse using Fick's diffusion equations with the reaction scheme added and do so for each species. This will have to be a numerical calculation in space and time. (There are well established ... 2 Diffusion isn't fast enough, so you're trying to make sure that you're making the pH of the solution as uniform as possible. You can't want a pocket of acidity or basicity to throw you off from the equivalence point. People also use stir bars to do this if they don't want to swirl all day long. 2 I would recommend you to do a research to find scientific articles on using DS cells. In this case if you have any specific questions you can try to contact authors directly, hopefully they will reply and be able to help you. You can start from this article (Mike J. Danielson, Corrosion Science 2002 44 (4), pp. 829-840) http://www.sciencedirect.com/science/... 2 This should be a comment but I don't have enough reputation to comment yet. Diffusion coefficient, as defined by Fick's equation D_\mathrm {AB} =\frac{\text{flux of mass diffusing } (\pu{kg m^-2 s^-1})}{\text{concentration gradient }(\pu{kg m^-4})} = -\frac{J_\mathrm A}{\mathrm dC_\mathrm A/\mathrm dx}  increases with temperature as pointed out by @...

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Temperature is directly proportional to diffusion rate. Diffusion, being the dispersion of molecules throughout a space, is affected by the speed of molecules and the density of the space through which the molecules are being dispersed. An increase in temperature increases the speed at which molecules move at. This is because the average kinetic energy ...

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My reference is Concepts in Thermal Physics, Blundell and Blundell, Ch. 7, 2nd ed. Everything here is from there. Consider a gas trapped in a rigid box. We begin by evaluating the molecular flux incident on one part of one wall of the box. Let there be $n$ particles per unit volume, so $n$ is the particle density of the gas in the box. Let $A$ be the area ...

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If we assume that the solvent is the same on both sides of the membrane (or that the solvent itself can penetrate the membrane) then the final equilibrium will have the same concentration across the membrane. That only translates to "50% of solute b to occur on side 1 and 50% on side 2" if the volumes are equal. The rate at which solute A moves from left ...

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You can double-check the area after you're done: Poke a small hole in the membrane and drain it completely. Stretch gently by pulling the clips (don't let them slip) until the area between them is rectangular. Multiply that area (L x W) by two, because the membrane has two sides around the now-flattened middle. There will always be some small error, ...

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This greatly depends on macro structure. If the phases were say arranged in a sheet-like structure (like a diffusion front) then the low diffusivity phase would be the dominating determinate of the diffusion rate. For this macrostructure, if we assume steady state with no change in phase or length from diffusion or time, then mass transport of the bulk can ...

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There are two possibilities. The first is that it is simple dilution, but for more likely is that osmosis is occurring. In this case the concentrated solution and the water, or a dilute solution, are separated by a semi-permeable membrane, i.e. one that allows passage of small water molecule but not large solute ones. To lower the overall energy (as ...

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Given the diffusion equation $D(\frac{d^2y}{dr^2}+(\frac{2}{r})\frac{dy}{dr})=\frac{dy}{dt}$ You can put in $y=z/r$ And then $\frac{dy}{dr}=(\frac{1}{r})(\frac{dz}{dr}-\frac{z}{r})$ $\frac{d^2y}{dr^2}=(\frac{1}{r})(\frac{d^2z}{dr^2}-(\frac{2}{r})\frac{dz}{dr}+\frac{2z}{r^2})$ $\frac{dy}{dt}=(\frac{1}{r})\frac{dz}{dt}$ Plug these into your diffusion ...

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Yes. Let's improve the wording here: that is not just an expression, it's the definition of a dimensionless variable that eases the understanding of the solution of the transient diffusion is a semi-infinite medium. Because there is no clear characteristic length as in a finite diffusion path problem, Buckingham π theorem will tell us the two dimensionless ...

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