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We did a little kichten experiment today where we put ice cubes in glasses of water. We had one glass with normal tap water and 5 glasses which increasing concentrations of salt water. My hypothesis was that the ice in the salt water would melt faster because the salt lowers the melting point of water.

What I observed however was that the ice in the water without any salt melted much faster then in the salty water. Can anyone explain why this happened? How is this experiment different from salt that is spayed on roads during the winter to melt snow and ice?

Edit: some more details The glasses were all the same size and all had the same amount of water (around 150ml). All glasses were filled from the same can with tap water to make sure they had the same starting temperature. The first glass had no salt, the second one table spoon of salt, then two table spoons etc. After mixing the salt by stirring slowly we added one ice cube of 20 grams to each glass and left the glasses in the kitchen at room temperature.

After about 40 min the ice in the glass without water had melted completely while all the others were all still about half the original size.

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  • $\begingroup$ Please provide more details. What was the temperature of the water in each of the glasses before you added ice? What mass of water was in each of the glasses before you added ice? How much ice did you add to each glass? Was the ice temperature the same for each glass that you added the ice to? Are all the glasses identical? Was this experiment performed at room temperature, inside the refrigerator, or elsewhere? How long did it take for melting in each glass, and at what salt concentration? $\endgroup$
    – David White
    Commented Jan 21, 2017 at 19:48
  • $\begingroup$ If you google ice melting in salty water, you will run into many pages with the correct explanation, which apparently you cannot find here. $\endgroup$
    – MariusMatutiae
    Commented Jan 21, 2017 at 19:50
  • $\begingroup$ Maybe the convection currents were different in the two cases? For ice in fresh water I would imagine that the cold water from the melting ice cube would rapidly sink to the bottom of the glass and a significant convection current would be set up as the ice cube continues to melt. In the case of an ice cube in salt water, since salt water is heavier than fresh water, it will tend to sink to the bottom of the glass and may tend to block the flow of the cold, fresh water from the melting ice cube. Maybe putting some food coloring in the water will reveal if the circulation patterns are different $\endgroup$
    – user93237
    Commented Jan 21, 2017 at 19:53
  • $\begingroup$ @Sam: It would probably be better to put coloring in the ice so that you can see where the meltwater goes. In any case, I think you have a point about the convection pattern. $\endgroup$
    – Olin Lathrop
    Commented Jan 21, 2017 at 20:13
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    $\begingroup$ I don't want to make this an answer as I'm just reasoning it out, not 100% certain, but I would guess, ice melts faster immersed in water than in air because the heat transfer is greater. Ice is more buoyant in salt water, so less ice is in contact with the water. If you want a variation of this test, try it with different water temperatures. As the water you put the ice-cube in gets colder, you'll probably see an evening out and eventually, if cold enough, the salt water will probably melt the ice-cube faster. I also like @SamuelWeir and Olin's colored dye suggestion. $\endgroup$
    – userLTK
    Commented Jan 21, 2017 at 22:10

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There’s a great answer to this here: https://mirjamglessmer.com/2013/09/04/ice-cubes-melting-in-fresh-water-and-salt-water-post-24/. In summary, when the ice is placed into salt water, the cold melted water from the ice is less dense than the salt water so it stays at the top with the ice, preventing warm salty water from further melting the ice cube. When ice is placed into freshwater, the melted water from the ice sinks (since it is more dense than the warm freshwater), and more warm freshwater can surround the ice, further melting the ice.

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  • $\begingroup$ On a side note, salt water’s higher density is also the reason why you float without any effort in places like the Dead Sea or the Great Salt Lake. $\endgroup$
    – Nold
    Commented Apr 16, 2018 at 0:39

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