Why does ice make crackling sound when you pour liquid over it?

It's used because it depresses the freezing point of water and thus causes ice to melt, provided the air and ground are not so cold that the freezing point depression that salt can produce is not enough.

I'd actually be interested to know whether in Canada they bother to salt the roads. I would expect there must come a ground temperature at which it is no longer effective.

 

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Well, it doesn't lower the temperature of the water - but it does reduce the freezing point of water (which is why it's useful for melting ice on roads.)

Fun observation - when we get takeout margaritas they make the margarita and then put it in a plastic container with lots of ice. By the time we get home the container is coated in ice. The alcohol lowers the freezing point of water, and when they add ice (that is at the temperature of the icemaker, probably -10C or so) then the ice cools the liquid down to below 0C, thus allowing ice to form when water condenses on the container.

 

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They do.

Wholly aside from what it does to the freezing point of water, salt also acts as a very good grit, providing extra traction. And it has the advantage over sand/gravel that it is easily dissolved and carried away by weather.

 

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Having thought a bit more about this I realise I may have been misleading wegs a bit over this experiment trying to freeze salty water. I'm getting terribly rusty on this stuff.

The experiment probably won't prove anything. This is because, when the salty water freezes, the salt will be excluded from the ice structure and will be left outside it. So the ice that forms will be essentially pure water ice. However, what may happen is that the veins of salt that get left behind when all the water turns to ice may run through the ice cube, effectively splitting the ice up into smaller units, or greatly pre-weakening it. If this occurs then you won't get the cracking, because the thermal stresses won't be so big in these smaller units of ice.

These things always get more and more complicated when you think about them.................

 

Sorry typo, it should be -4°.

Like it says on that graph I mention.

 

I've done a bit of further reading on this and there is yet a further complicating factor when salt water is frozen.

At first, pure water ice freezes out of the mixture, leaving behind a stronger salt solution. However, there is a eutectic point, at a concentration of 23.3% salt by weight. This eutectic mixture will finally freeze, in the form of separate ice and salt crystals, forming together, at -21.1C. This is shown in this diagram:

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A yet further twist is that the salt crystals that form from the eutectic mixture will not be the standard anhydrous NaCl that we all know and love, but a hydrated form: NaCl.2H2O. I never knew this existed!

I'm unsure whether the average domestic freezer gets cold enough to freeze the eutectic composition: it looks borderline to me.

Thanks wegs: there is quite a lot to this subject, it seems.

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You can't really compare cavitation in liquids to what happens in solids when they warm, however cavitation depends on nucleating centres and water traps small air bubbles when it freezes (warm water has dissolved gas in it after all).

I haven't found much about what role if any is played by air bubbles in warming ice, but ice sheets are known to emit sound because of the trapped air bubbles and the weight of the ice.

 

About -10F. But air temp is critical, as is traffic volume (traffic heats and abrades).
Any Provinces in Canada with residential roads - and afaik all northern and bordering States in the US - at least salt the bridge decks and higher speed intersections to dry them in advance of a predicted cold snap.

Salt makes a big difference during the transitions from cold and damp to ice on the road (would-be ice runs off, under the effects of grade and traffic) On sidewalks and the like it will roughen the surface even if it doesn't get rid of the ice completely - safety.

It can get pretty sophisticated - there is more than one kind of "salt"; you don't want the stuff washing into nearby lakes and rivers; if you salt a deeper snow in the evening and don't get a good dry, a hard overnight cold snap can freeze the badlands you created into something even a plow won't take out completely; different road surfaces and sun exposures affect drying; and so forth. The road and airport crews in the northern US often carry serious expertise in management.

 

Interesting. The eutectic point is -21.1C which is about -6F I think. Maybe the "salt" they use has other things in it that depress this point a bit further.