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Q & A: Freezing Saltwater

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Q:
what would happen if I put ice in the frezzer with salt on it? P.S. please say why
- Daniel (age 11)
miami,fl,U.S.
A:

Nice question.
Although pure water freezes at 0°C (32°F), water that has salt dissolved in it has to be colder before it freezes. If the water has as much salt dissolved in it as it can hold (that's called a saturated solution of salt), so that any further salt would just come out as crystals, the freezing temperature is around -21 °C, or about -6 °F. If your freezer isn't colder than that, the part of the ice touching the salt will start to melt. If you've put so much salt on the ice that the water can all melt and form a saturated solution, and still leave some salt crystals, then it will all melt. If you've put only a little salt on, it will melt some ice until the salt crystals are gone. Now as more ice melts the solution becomes less salty, more like pure water. So its freezing temperature goes up. At some point its freezing temperature will be the same as the freezer temperature, so the freezing will stop. You'll have some ice left, and some salty water.

What is interesting is that this effect is used all over the place. Often, salt is put on roads to melt ice. If there's a lot of ice, you need a lot of salt. If the temperature drops below -21°C, it won't work at all.

You also wanted to know why it works, why saltwater has to get colder than pure water before it freezes. We've got some other answers on that, which you can find by searching this site for "saltwater". Briefly, the ice is a crystal, an almost perfect array of pure water molecules with almost no salt in it. To make that out of pure water requires limiting the ways the water molecules move around. To make that out of salt water requires BOTH limiting the ways the water molecules move around AND limiting the ways the salt can move around (it's stuck in the liquid, or in separate crystals), which is harder to do.


Adam and Mike W.


(republished on 07/25/06)

Follow-Up #1: salt melting ice

Q:
how come salt water will freeze but salt melts ice?
- Anonymous
A:
Salt water will only freeze if it gets cold enough. For water as salty as it can get, thats -21C. When you put salt on ice it will melt some of the ice but only if the temperature is above -21C. So at any temperature where fully salty water will freeze, salt wont melt any ice.
Now if you have slightly salty water, it will start to freeze just a little below 0C. It wont completely freeze until its down at -21C. Lets say you have some slightly salty water cooled to -10C, so some of it is frozen leaving some saltier water as liquid. If you dumped a little salt on ice sitting at -10C, it would melt just enough ice to leave salt water at the same level of saltiness reached in freezing. Whether youre melting or freezing, the equilibrium at some temperature is reached at the same salt concentration in the liquid.

Mike W.

Lee H

(published on 03/03/07)

Follow-Up #2: salt water freezing

Q:
what subtance in salt water that does not make salt water freeze?
- Anonymous (age 12)
Tulsa,Ok,U.S.A
A:
Its the salt itself- the sodium Na+ and chlorine Cl- ions- that lowers the freezing point. However, any other molecules that dissolve in water do about as good a job. Its just that salt is cheap and lots of it can dissolve in the water.

Mike W

Lee H

(published on 03/03/07)

Follow-Up #3: feeezing rates

Q:
so for science were trying to figure out what freezs faster fresh water or water with salt, do ou have any ideas or answers? please help us figure this out thank you : )
- madisson and tanaya (age 13 and 12)
tulare,california
A:
If the main question you want answered is which freezes faster, say in your freezer, the simplest and most reliable way to get the answer is to do the experiment. Usually, we expect fresh water to freeze faster because it doesn't have to get as cold as saltwater to freeze, but tricky things might happen to those rates in some special circumstances. We say try the experiment.

Mike W.

(published on 08/21/07)

Follow-Up #4: freezing rates

Q:
My son performed this experiment for Science class. He used the same plastic cups, cold filtered water from the frig, added a little salt to one cup, placed them in the freezer at the same time and in the same spot. We checked the cups later (about an hour) and were surprised in our findings. The fresh water had a little rim of dense ice, with a water temperature of 32F. While the salt water had a much thicker layer of less dense ice, with a water temperature below freezing. My son wrote down that the salt water froze faster than fresh water. He, of course, got a "B" on the assignment because that was not the expected outcome. Can someone please explain what happened?
- Lesley S. (mom)
Arkansas
A:
I'll explain what I can, but it's only part of what your son observed.

1. As each cup was partially frozen, the temperature (T) was lower in the salty water. That's because salt water freezes at a lower T, as we've explained in other answers on this site. T doesn't drop much below the freezing T until all the water is frozen. So what your son noticed was the well-established fact that salt lowers the freezing T.

2. The salty water froze faster. That's a little surprising (see above) but not at all impossible. I don't know why it happened. One possible reason might be that as the water started to evaporate (which it will do) a salty crust formed on the edge of the glass. That crust can help wick water up and speed up evaporation. Evaporation itself helps speed cooling. (That's why we sweat when it's hot.) That's just a guess. The point is that freezing rates are complicated things that can show surprising behavior.

3. The teacher down-graded your son for accurately reporting his observations. I don't necessarily understand this educational phenomenon either, but when you think of how poorly trained most of the science teachers in the US are, it's not all that surprising.

Mike W.



(published on 11/25/09)

Follow-Up #5: The freezing and boiling points of orange juice.

Q:
What is the freezing point and boiling point of orange juice?
- Nikita (age 14)
Perth
A:
I've marked this as a follow-up to an old question where we discuss a bit of the basic physics. More discussion can be found on other questions here (e.g. http://van.physics.illinois.edu/qa/listing.php?id=1457) and on Wikipedia (http://en.wikipedia.org/wiki/Boiling-point_elevation, http://en.wikipedia.org/wiki/Freezing-point_depression).

I don't know the sugar and salt contents of your orange juice exactly, so I can't give exact figures. I've read that the initial freezing point is around -1.17C. (table) It's lower than for pure water for the reasons discussed on those other answers. Most importantly, the OJ will only start to freeze at that temperature. As it freezes, the sugars and salts will become more concentrated in the remaining liquid. That lowers its freezing point more. I bet that there will be some liquid left down to somewhere around -20C.

Both the freezing point depression and the boiling point elevation are driven by the same basic physics. To a good approximation they're just proportional to each other.
That freezing point would correspond to an initial boiling point of about 100.32C. Again, as the water boils away, that leaves more concentrated sugars and salts in the remaining liquid. That raises its boiling point.  I bet you'll have to heat it to very roughly 105C to get rid of all the liquid. (Some water molecules would still be left, but bound into crystals with the sugar etc.)


Mike W.

(published on 09/07/11)

Follow-Up #6: Why does salt melt ice?

Q:
I understand sea water freezes at a lower point than fresh water I know it is because of the salt and other minerals can you explain why?
- Michael (age 12)
Gig Harbor, Wa, USA
A:
I've marked this as a follow up to some earlier questions.
The key point is this: at any temperature nature finds the arrangement which can be made by the largest net number of different microscopic (quantum) states. There are two basic ways you can increase the number of states.

1. Directly have some scrambled-up arrangement with lots of states,
2. Find a low-energy arrangement that dumps energy to the surroundings, letting them reach lots of different states.

At low temperatures, (2) tends to be more important. That's why ice forms at low temperature.

With salt or sugar or alcohol or any other solvent in liquid water, freezing some of the water limits the room for the solvent molecules to run around.in the liquid, where they remain. That reduces how many states are available to them. So that makes it harder to freeze. The liquid has to get colder before the process (2) makes up for that. The freezing point is lowered.

Mike W.

(published on 03/05/12)

Follow-up on this answer.