# Does salt water melt faster or slower than pure water?

I am trying to conduct an experiment on the melt time time of two ice cubes with a different salt concentration, one with none and one with a lot. Does salt water melt faster than pure tap water? (The salt will be dissolved in the water) f anyone has an answer to this, please answer it fairly simply. I've been trying to research it to make my hypothesis but there is nowhere that has a simple answer or an answer at all. Thanks, Charlotte (age 14)

• Due to freezing point depression the 'salty ice' is likely to melt faster than pure ice. My guess anyway... – Gert Feb 10 '20 at 22:55
• When you freeze salt water, you get fractional crystallization, and fresh water ice results. Your assumption of salt water ice is mostly invalid. – David White Feb 11 '20 at 3:52
• – anna v Feb 11 '20 at 7:05

This question is an interesting one as it focuses on how fast a phase transformation occurs - something no textbook that I have addresses! A number of factors probably play a role here.

Thinking about one, qualitatively, I would expect the farther your system (ice in this case) is from its transition temperature, the faster the system will want to move to the new phase. Anyway this seems logical.

Let us assume we have a pure water ice sample that melts at 32F and a salt water ice sample that melts at 28F. If we take these two cubes from the same initial temperature (for example, let's just say 20F), and place them at the same temperature (for example, room temperature which we'll take to be 72F), then the salt cube is 4 degrees farther from its melt temperature than the pure water ice cube. I would then expect, all other things being equal, that the salt cube would melt faster.

Good luck and if you try the experiment, let us know how it goes! (If you try the experiment, remember, you'll need to control the size of your samples. It won't do to measure how fast they melt, if one cube is substantially different in size than the other.)

your question and your research is very interesting and I suggest you go forward in your own way and try to analyse and interpret the results on your own!

However, in the experiment you are suggesting here there are some problems.. Let's look at them! I hope this answer is simple enough. At the end, I will suggest you some more experiments that you can do in your kitchen !

Solubility

When you mix salt and water, something happens: the salt molecules you add will go around looking for water molecule and become friends. This will make both water and salt happy!

If you add too much salt, some salt molecules will remain alone and they will be sad and will not mix with water! This tells us that there is a magic number (called solubility) that tells you how much salt you can add to water before they become sad. For water and salt this magic number is approximately 350g in one liter.

So the first thing you need to make sure is that you have enough salt in your water for your experiment to be right (if you put very little salt you will not see any effect as the salt is too little) but also not too much that the salt molecules will be sad!

In more serious terms: water molecules will dissolve the salt in two ions (called Na+ and Cl-). That's what dissolving something in water means. In order to do that, you need to have enough water to dissolve all ions, or some of them will stay in their "salt" form and they will not dissolve. Notice also that solubility depends on the temperature and each material has its own!

Freezing point

You know for sure that water becomes ice at 0 C (32F). This particular temperature is called the "freezing point" of water. What happens at the freezing point? Water molecules, which were running around everywhere in your container, will start feeling cold and move slower and slower and slower until they stop where they are and become "frozen"!

If you add salt to water, because water and salt now are friends, they will try to run together as long as possible, they don't care about the cold anymore! Until it's really cold, then they stop!

So they will not freeze at the freezing point but at a lower temperature (which depends on the amount of salt you put inside). For example, sea water which is a bit salty freezes at -2C (28.4F). But if you add more and more salt the temperature can go as low as -20C (-6F)!!!

So you have to also make sure your freezer can become cold enough to freeze your salty ice at the amount you decided to use!

In more serious term: because of very interesting effects that have to do with entropy and with the way molecules bind, adding salt to water leads to a (sometimes drastic) decrease in freezing temperature. It's as-if water and salt will try to be together as much as possible and you need much lower temperatures to force them apart.

Does salty ice exist

This is the biggest problem you have to face. Water forms ice, you know this. I told you how water molecules start getting cold and stop. The problem is that salt molecules don't feel that cold and they will keep running!

This is a big problem because as you make your salty water colder and colder, eventually (but still at a colder temperature than pure water) the water molecules will start slowing down but the salt will keep running and at some point, the water will be left behind and the salt will have ran so far away that the water and the salt are not close together anymore.

The result is that water will freeze but salt will be still salt, it will be unfortunately "kicked out" of the ice.

So if you do your experiment, you won't be able to tell the water-ice from the salty-ice apart because.. they are both the same ice!

(there is a secret trick to freeze salty water: you need to make it go so cold and so fast that even the salt stops running! But you need to do it in a lab with professional scientists ;) )

In more serious terms: water will start freezing very very locally, forming small ice crystals which will then expand. The salt will be "kicked-out" of the ice crystal. This is why you can make drinkable water (and even beer!) from icebergs that float in the sea! This is because, when forming ice, water molecules now prefer to be together in a more ordered and "slow" form rather than to bind to ("play with") salt.

Maybe a better experiment?

Maybe a better experiment will be: why don't you study the freezing point of salty water?

You can take 10 glasses of water. You add no salt in glass number 1, a bit of salt in glass number 2, a bit more in glass number 3 and so on (but remember: never more than 350g/1 liter, which would be approx. 50g in a glass of water and that is approximately 8 teaspoons of salt maximum).

Then you mix it a lot with a small spoon (really a lot!), then you take an ice-stamp and put the water in glass number 1 in the first stamp, the salty water of glass number 2 in another stamp and so on. Remember to marky how much salt you put in each glass and which glass you put in what stamp of course!!!)

Finally, put the stamps in the freezer and wait some time. Then check: did all of them freeze? If not, at what salt amount did it stop freezing? Write down your results and explain it!

Then, do another experiment: take all of the ice cubes that formed (remember which is which), wash them quickly with fresh water from your sink, put each of them in a glass and wait until they melt (you can count the time, but they will all melt in the same time I predict! Why? Remember what we discussed before!)

Then.. drink the water! Does it feel salty? Why / why not?

Another thing you could try, is freeze water and then add salt on top: does it change the time it takes for the ice to melt?

Let us know what your results are!

The problem is interesting and relevant, for example in connection with Arctic sea ice or the ice at the Thwaites glacier.

I suppose that a lot will depend on whether you stir or not. If you do not stir, the melting will create fresh water that will stay on top of the heavier salt water. Then I would not expect the difference to be very large.

Sorry for an answer that is not really an answer. This is not my area of expertise. Good luck!

• Measuring the melting speed may prove difficult too. You need to start from identical initial temperatures, e.g. - 18 C (freezer temperature). Heat capacity of the ices then comes into play. – Gert Feb 10 '20 at 22:59

Adding salt to water will lower the freezing point of the water, I would expect the same is true for ice that contains salt, i.e. the melting point is lower when the ice contains salt.

• "When seawater freezes, however, the ice contains very little salt because only the water part freezes. It can be melted down to use as drinking water." oceanservice.noaa.gov/facts/oceanfreeze.html – anna v Feb 11 '20 at 7:04