Hot answers tagged

20

One boring Monday morning in the lab a group of us did the experiment, and to our surprise we found that the hot water (in sealed containers) did freeze faster. On closer examination we discovered that the shelves in our freezer were covered in frost, like I imagine most freezers, and the hot water was melting the frost and creating a good thermal contact ...


19

As a supplement to @tpg2114's answer, it also depends on the "wetness" of an object. As most people should know the evaporation of water requires energy and this lowers the temperature. The lowest temperature a wet object can reach is what is called the "wet-bulb temperature." This can be several degrees lower than the "dry-bulb temperature," the amount ...


17

Water is an unusual substance in that it expands when it freezes. Evidently this expansion wasn't enough to burst the bottle in your case, but it left the bottle's contents under pressure. After you'd defrosted it for a while there was, presumably, some ice and some water in the bottle. Because the ice was taking up more volume than it did when it was water, ...


14

Typically freezing rain falls as water because the air is warm enough that it won't be ice, but then it freezes when it's on the surface of things because those surfaces are below freezing. This usually happens with roads and the ground when it's been a long time of very cold temperatures. Bridges also tend to get colder, faster, than other roads because ...


12

You can have a look at the phase diagram pressure-temperature of water: [Phase diagram taken from Martin Chaplin's webpage, http://www.lsbu.ac.uk/water/phase.html#b\ , under license CC-BY-NC-ND. This webpage is highly recommended, with tons of useful links and articles.] The transition between solid and liquid is the red line separating the blue (solid) ...


11

Liquid nitrogen boils when it comes in contact with skin, so small amounts of spatter are no danger at all-- the droplets just bounce off. I regularly pour a liter or so (a bit at a time) out on a lab table when I do liquid nitrogen demos, with no problems or safety gear. The biggest risk from the low temperature is getting it into fabric of some sort, ...


10

Pipes are damaged when ice forms a complete blockage, and the expansion of water trapped by it puts too much pressure on them. Now, ice is a pretty good thermal insulator, so once a little ice forms on the inside of the pipe further freezing proceeds slowly. If the water is flowing there will not be enough time for it to freeze between leaving the (...


9

As mentioned in the comments, this is an instance of supercooling. When you cool a liquid below its freezing point, the molecules are still moving around quite a lot and any two that stick together are likely to be broken up by a subsequent impact. Liquids freeze better when the molecules have something to latch onto -- either a block of the same ice they ...


8

First of all, when you say that trying to crack a pipe is hard work, what you probably mean (in physics terms) is that it takes a large force. But that doesn't necessarily mean that it requires a lot of energy. The energy used in a physical process like that is equal to the force times the distance over which the force is applied, and you don't have to push ...


8

The answer to this question is "probably not". The reason for this is quite interesting. Ice skates have such low friction because a layer of water forms in between the ice and the blades. In order for this to happen, you need a substance that will turn from solid to liquid when it's compressed, which (according to thermodynamics) is the same thing as ...


7

Ice coming from the freezer will typically be around -19 deg. celsius, and can only be stored for a limited time at room temperature. As soon as the ice is heated to 0 deg. or above, the ice will melt into liquid water. Liquid water coming into contact with ice will be cooled, and if cooled below 0 deg. it will also freeze. The answer to your question is ...


7

In the absence of salt, the ice and water at 0C are in equilibrium, so unless you add or remove heat nothing changes. However when you add salt it reduces the freezing point of the water. This means the ice and salt water are no longer in equilibrium, and the result is that the ice starts to melt. Melting the ice requires heat. Specifically it requires the ...


6

Most materials contract on cooling. The notable exception to the rule are some phase transitions and water. But even ice contracts on cooling. Water expands on cooling only between $0^\circ\text{C}$ and $4^\circ\text{C}$ (including phase transition). This corresponds to the part of the graph below, in which density rises with temperature (note suppressed ...


6

When water freezes, you get ice. Ice, like many solid materials, forms a crystalline structure. In the case of water, the crystalline structure may be attributed to the hydrogen bond, a special kind of an attractive interaction. So a big chunk of ice will have a crystalline structure - preferred directions, translational symmetry, and some rotational ...


6

Short answer is it can't form when the temperature of the water is above the freezing point. As @Krazer and @tpg2114 have pointed out the temperature of water on surfaces will frequently be lower than the air temperature. I'm answering just to clarify that the wet-bulb temperature is only indirectly relevant. The wet bulb temperature is not (...


5

I'm answering my own question. Apparently this is one of those rare cases when the physicist must doubt what he observed -- or what he thought he observed -- and believe the numbers his theory yielded instead. From further experiments I've noticed that the ice tends to form thin plates inside the supercooled water once the crystallization process starts --...


5

It may not be the answer you are looking for but I recommend you get a thermos or a well insulated flask. These are what mountaineers use and you do not have to change the chemical composition of water this way.


5

If you decrease the pressure, the freezing point of water will increase ever so slightly. From 0° C at 1 atm pressure it will increase up to 0.01° C at 0.006 atm. This is the tripple point of water. At pressures below this, water will never be liquid. It will change directly between solid and gas phase (sublimation). The temperature for this phase change, ...


5

Suppose you have a lump of ice, and you want to melt it to water completely, you will heat it. As the temperature of the ice reaches $0^\circ \mathrm C$, temperature of the ice will stop rising, and all the heat will be used to convert the ice to water. While this is happening the ice and water will simultaneously exist in equilibrium(because all the ice ...


5

Can this frozen form freeze further? Or can it become more solid? (for example, by exposing to colder temperatures and/or a higher pressure). Can ice freeze further by transforming into a different crystalline form? The ice will remain solid while lowering temperature or pressure but might change in state, or phase, as you mention. But you should use ...


4

You can get localized soft tissue damage rather like a burn from sustained contact with moderate amounts of cryogenic liquids, and large amounts can freeze flesh solid---which is really bad. Small amounts will dance on your skin because of the vapor barrier that develops as they vaporize. Treat cryogenic materials with respect. Think about what you're ...


4

Liquid nitrogen will not moisten human skin, so short contact with small amount of it should not be too harmful -- it would just float on a evaporated portion of itself like water over very hot pan; yet of course putting a hand into a container with it is not a good idea. From what I have heard, the biggest problem is when you pour it on your shoes, because ...


4

While this may be true, a better option is actually to spray your plants with water. When water freezes, it releases heat (a little counterintuitive, I know, but that's why you have to put water in a cold place to freeze it - you have to take away heat). So, if your plants have a thin layer of water on them that freezes, it actually helps keep them warmer. ...


4

If the temperature is not much below freezing, the rate of heat transfer from your plants (and particularly from the earth around their roots) is low, if there is a lot of water present, the high heat of fusion means that it will take a long time to actually freeze much of it. So maybe the plant makes it through the night without too much damage. Note that ...


4

It is called sublimation. It is how ice cubes disappear in the freezer. Snow and ice sublime, although more slowly, below the melting point temperature. This allows a wet cloth to be hung outdoors in freezing weather and retrieved later in a dry state. I .... Sublimation is the process of transformation directly from the solid phase to the gas ...


4

Adding salt to water makes it freeze at a lower temperature. This fact is being used in two different ways in the two scenarios you mention. Dissolving sodium chloride in water is slighly endothermic, but this effect is small and to the best of my knowledge isn't important in the drink cooling process. Putting salt on the highway is quite straightforward: ...


4

As we also all know that salt water has a freezing point much lower than 0$^\circ$C (and a boiling point higher than 100$^\circ$C). This is because of changes in the entropy of the solution (see the wiki for more details). The cells in poultry (and the "stuff" in between the cells) don't contain pure, distilled water (which freezes at 0$^\circ$C), but "...


4

The density of water rises from 0 to 4 degrees celsius. So you would have a gradient of watertemperatures. On the bottom 4 degrees celcius and colder water above it. This is also the reason why fish survive a winter, they just dive to the bottom where the temperature stays warm longer. Also the density of ice is smaller than water so it will float.


3

yes, hydrogen (and anything) evaporates/sublimes in vacuum


3

Water is very odd in that it expands when it freezes - almost everything else contracts. I don't know what material has the largest volume change on freezing. But among liquids - organic solvents, with much weaker bonds between molecules than water, tend to have much larger expansivities. There is a very odd material (zirconium tungstate) that shrinks as ...



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