# Tag Info

10

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, ...

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 ...

7

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 ...

7

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 ...

6

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 ...

5

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 ...

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

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 ...

4

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, ...

4

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 ...

4

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) ...

3

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 ...

3

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: ...

3

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 ...

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 ...

3

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 ...

3

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 ...

2

I would say it is a simple case of heat transfer. The new water (from the mains) is above freezing (usually by 10C or more), so the flow is transfering heat from the relatively warm input water. I would also say, that the opening through the end of the pipe might act a bit as a pressure relief valve, i.e. some freezing of the contents of a closed pipe means ...

2

There is a simpler way to do the calculation, though using it also gives me 7% of the water freezing. The heat needed to warm the water from T degrees below zero is simply: $$E = MTC_w$$ where $M$ is the mass of the water, $T$ is the degrees below zero and $C_w$ is the specific heat of water (assumed constant over this range). The heat released when a mass ...

2

K Libbrecht has a nice paper that answers your question in considerable detail and has some nice pictures-- his homepage: http://www.its.caltech.edu/~atomic/publist/kglpub.htm Scroll down to the article in American Scientist in his publications list "The Formation of Snow Crystals," K. G. Libbrecht, American Scientist 95, 52-59 (2007). View pdf. the pdf is ...

2

the set temperature will determine the final, equilibrium temperature, but the rate of cooling is determined by how fast the air inside the freezer can extract the heat from the new (warmer) food. The cold air molecules will heat each time they collide with the food which is at a greater temperature. If the freezer is empty, that molecules will get cold ...

2

Yes, that is possible. The usual way to cool down object to this temperature, is by putting it in liquid nitrogen. For an example, consider this movie, where it is done with a tulip. The water inside the object is freezing, which makes it breakable (as you can break ice, but not water).

2

The cheap answer to your question is "a gas," probably most specifically helium since it stays a gas longer than anything else as it gets colder. Your question boils down to how the ratio of two forces changes with temperature. First you have the separation forces that push molecules or atoms apart, and then you have the binding forces that pull the ...

2

EDIT: I misread the question I see you asked what kind of material contracts the most when you cool it. In this regard, hardly anything beats the ideal gas, whose contraction is about .1% per degree at room temperature. If you want a material, consider a bunch of balloons mushed together with drops of glue, or something microscopic equivalent. Materials ...

2

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, ...

1

Why laundry dry up also in cold/frost? Probably because, initially, the clothes and the liquid water trapped in the clothes fibres, are both at a temperature well above 0 C. When you have frost, water in the clothes should freeze, And it does, when the temperature of the garment and water trapped within it have eventually reduced to below 0 C ...

1

The explanation is that hot water evaporates, leaving less water to freeze (but see John Rennie's answer for a contact effect which is probably the biggest factor for most of the reports of the effect. Aside from John's effect...) this is the only significant difference, there is no other, despite what you sometimes read. The evaporation effect is not even ...

1

The bit of The Frozen Planet that mentions this is short on detail, but I would guess they are referring to Antarctic Bottom Water; see also Weddell Sea Bottom Water. It's similar in mechanism to the brinicle you mentioned. Water cooled at the surface becomes very cold and very dense (high salinity) so it sinks and forms a layer on the sea bed of the ...

1

Not all snowflakes are symmetrical. One can disrupt the symmetry quite easily by introducing impurities or some mechanical artifact. In nature, snowflakes have plenty of time to form and it is more natural for them to form symmetric shapes because of the molecular structure of water. That is, when there is more time for the molecules to move about and ...

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