# Tag Info

40

Think of temperature as average kinetic energy of the water molecules. While the average molecule doesn't have enough energy to break the inter-molecular bonds, a non-average molecule does. Water is a liquid because the dipole attraction between polar water molecules makes them stick together. At standard atmospheric pressure (acting somewhat like a vice), ...

28

I actually went ahead and spent some hours experimenting. Used two 500ml aluminum beer cans filled with water at room temperature, 21.4°C. One can wrapped in a paper towel soaked with an additional 20ml of water, one left bare as control. Shoved both in my small, non-ventilated house freezer at -14°C and measured temperature and weight every twenty minutes ...

21

That is the Leidenfrost effect. If the surface is hot enough, a layer of vapor exists between the hot surface and the droplet, insulating the droplet from the full heat. The droplet levitates above the hot surface.

19

It's not poop. It's fly barf. A fly spends about 25% of its time re-digesting and it only can eat liquids. It mixes the eaten food with the appropriate enzyme for digestion. The fly does this by retrieving the eaten food from its digestive system (a vomit of sorts), and drop by drop it is placed on the surface on which the fly is sitting. Only then is it ...

18

Evaporation is a different process to boiling. The first is a surface effect that happens at any time, while the latter is a bulk transformation that only happens when the conditions are correct. Technically the water is not turning into a gas, but random movement of the surface molecules allows some of them enough energy to escape from the surface into the ...

14

At high enough pressure you can keep water as a liquid above 100°C. With even more pressure you can even keep ice above 100°C. Similarly you can boil water at room temperature with a low pressure. (https://en.wikipedia.org/wiki/Phase_diagram) The phase diagram of water shows what state it is in at any given temperature and pressure. edit: To answer ...

11

The relative humidity of air is pressure dependent. Your method of popping your ears involves increasing the pressure of the air in your mouth. And if you sufficiently compress a volume of air that has a high relative humidity, you can increase the air's relative humidity beyond it's saturation limit, which causes the water vapor in the air to start to ...

11

Sprinkling water on the sidewalks will cool them down in exactly the same way that sweating cools you. In both cases it is due to evaporative cooling. The same idea has been used for millenia to cool water in hot climates. The only downside is that it will increase the humidity of the air, and humid air feels hotter than dry air because humidity slows ...

10

It actually gets a bit complicated, since several effects are involved: Evaporating water does require heat, which comes primarily from the hot stones. So throwing water on the stones does cool them down. (This is where the claim one occasionally hears, that "throwing water on the stones makes the sauna colder", comes from. Technically it's true, if one ...

8

For every temperature, there is some amount of water vapor that can exist as gas mixed in with the air. This is called the saturation pressure of water at that temperature. The relative humidity is the amount of water vapor, expressed as a percentage of the saturation pressure. As you increase the temperature, the saturation pressure increases. Steam is ...

8

I mean, they are heavier than air. No. Water is $H_2O$ which has a molecular weight of 18. Nitrogen is $N_2$ which has a molecular weight of 28. Oxygen is $O_2$ which has a molecular weight of 32. Argon is $Ar$ which has an atom weight of 40. So a water molecule has a mass that is less than that of all the significant components of air. But ...

7

When cooking, keeping the lid on a pot does a few things if you think about it: it decreases air circulation significantly so the air in the pot stays hotter, this cooks the food faster. if it's snug, it should increase the air pressure.* The pot lid captures condensation, so it will return water back into what you're cooking. That is, I think, the ...

6

Temperature is a measure of the average kinetic energy of particles, characterized by a Maxwell-Boltzmann distribution. Basically, that's a fancy way to say, if something is at 25°C, a large percentage of its particles have a temperature close to 25°C, but some have a temperature farther away. When some of those particles that happen to have a temperature of ...

6

Evaporative cooling works by removing the high-velocity tail of the kinetic energy distribution. That is, only the fastest molecules escape the liquid, leaving the rest to thermalize at a lower temperature. If there is capillary action taking water to the outside of the pot and that is evaporating, then the pot cools down as it is losing heat to the leaving ...

6

yes, this is called Sublimation. Ice has a vapor pressure: Molecules will go from the solid phase to the gas phase or visa versa depending upon whether the partial pressure of water vapor in the gas phase is above or below the vapor pressure of the ice, until equillibrium is reached.

6

It may actually work, as evaporating liquids need heat to evaporate, and water will somewhat evaporate even in the fridge. I am not sure it works in practice, because the paper also causes an adverse effect, it provides insulation, Hard to tell which effect is dominant. I'm pretty sure that the balance of both effects depends in a very large part on the ...

6

Temperature is a measure for how much kinetic energy the molecules in a substance have. If the temperature is high, they are moving pretty fast, if the temperature is low, they are moving a lot slower. If molecules are moving slow, they bundle up and you get a solid. Once you heat it up a bit, the substance starts to become liquid. When you heat it up even ...

5

I will make it an answer instead of a comment. My guess is that the convergence is an optical illusion. This plane is flying at a level where the relative humidity is small.This means that the trail evaporates, it will evaporate faster from outside (the trail itself is humid) and finally what is left is merged the dissolution giving the impression of ...

4

The way your question is phrased, it looks like you are expecting the mass to change. In that case the only change will be a slight mass loss due to evaporation, but the rate of evaporation is a variable - dependant on room temperature, air pressure humidity and how still the air is above the sample.

4

While the capillary pressure in soil is many orders of magnitude lower than the atmospheric pressure, you also need to remember that in soil, the water is still in contact with the atmosphere, and thus is at atmospheric pressure plus capillary pressure. Since atmospheric pressure is orders of magnitude larger than capillary pressure, the pressure on the ...

4

Imagine spinning a roulette wheel, but instead of dropping in one ball, you drop in 100. They all rattle around at different speeds, like the molecules in water. You can cool them down by spinning the wheel slower, so they bounce about less; heat them up by spinning faster so they bounce more; you can freeze them by stopping the wheel and waiting till ...

4

The equilibrium vapor pressure of water vapor over ice is well known and easy to google for (http://www.its.caltech.edu/~atomic/snowcrystals/ice/ice.htm is one possible link). It is slightly lower than the equilibrium vapor pressure of water vapor over liquid. Ice does not evaporate - it sublimates under those conditions. The equilibrium vapor pressures ...

4

Assuming no energy input, the lifetime of a black hole is related to its mass by: $$T = \frac{5120\pi G^2}{\hbar c^4}M^3$$ There is a nice summary of the derivation of the lifetime on this web site. I make the condition assuming no energy input because for large black holes the Hawking temperature is less than the temperature of the cosmic microwave ...

3

The trick is here in the temperature of the pot. As you know, evaporation does not require boiling at 100°C: else, towels would only dry in an oven, and as far as I know, nobody dries towels in their ovens. Instead, it depends on the partial pressure of water vapor near the water surface (as Hot Licks mentioned). If the lid is on the pot, the partial ...

3

Think of a pressure cooker... The higher the pressure, the higher the boiling temperature. You need a vessel that will hold the pressure at 350C. Wikipedia gives a formula for the pressure need to get the boiling temperature of water up to a certain value. The formula is $$T_b=1730.53/(8.07131-\log_{10}P) -233.426$$ where $T_b$ is the boiling temperature ...

3

No, the temperature of the water is not that important for the performance of an evaporative cooler. This is basically because the energy needed to increase the temperature of liquid water (its specific heat capacity) is very small compared to the energy needed to evaporate the same amount of water (its enthalpy of evaporation). At room temperature the ...

3

From a molecular perspective there is nothing discontinuous about a phase change. It is just molecules provided with enough energy to escape from the bulk fluid and fly into the gas phase*. From that same perspective you also know that each molecule has a certain mass, so upon moving from the bulk fluid to the steam, the steam becomes slightly heavier (more ...

3

If there is no mass leaving or coming into the system, notwithstanding the seeming complexity of phase change, and indeed one kilogram of water must evaporate to 1 kg of steam: there is no "trick" here. There is a tiny correction to this statement if you want to look at really fine details. Quite a deal of energy goes into the water to boil it. You're ...

3

A water molecule is a water molecule, irrespective if it is densly packed with other water molecules (forming a liquid phase) or if it is widely separated from other water molecules (forming a vapor phase). So it seems obvious that boiling one kilogram of water gives you exactly one kilogram of steam. But boiling off water requires energy... ... and ...

3

It's done outside many smaller stores near where I live (Tokyo) and has zero apparent effect on the temperature. Midday summer weather is high-30s temperature and rainforest-level humidity, so the half-bucket of water tossed out the front door does little except moisten the pavement. A side effect is it tends to make the place messier - dust plus lots of ...

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