why does ice melt faster in salt water
Wow, where to start? First, I assume that by "water and ice" you meant "water and salt", otherwise it doesn't connect with the rest of your discussion. Let's start with the obvious parts based on familiar facts. If salt tended to stabilize ice more than water, by the fancy mechanism you describe, adding salt to ice would help keep it frozen. Somebody better tell the Highway Department, because they've been using salt to
ice all these years. Working our way toward slightly less familiar facts, the mechanism you describe would only work if the salt was actually in the ice, helping line up those water molecules. However, virtually none of the salt goes in to the ice. You can check this by partially freezing some salt water. You'll find that the non-frozen liquid is saltier than what you started with, because salt was from the ice. I'm trying to see if there is any grain of truth in what you've written. It is true that dissolving table salt in water will lower the temperature of the water, because it takes energy (more precisely, enthalpy) to pull those ions apart.
That could help freeze the water. However, that effect amounts to only about 0. 9 K cooling for 1 M salt concentration. (That cooling is temporary and has no effect on the freezing time for the salt water if you let it stand at room temperature before putting it in the freezer. ) The same 1M NaCl lowers the freezing point of water by about 3. 7 K, so it still needs to cool than plain water before it freezes, even if you didn't let it reach room temperature first. So I have to conclude that everything you wrote about the science of water and salt is incorrect. Is it possible that your glass of salt water froze first? Maybe- freezers don't cool evenly, some spots already have frost, some are in good contact with metal, so it takes a lot of care to check whether the result depended on details of placement, etc. However if you got the water really salty, stirring in NaCl until no more would dissolve, the water wouldn't freeze until it was cooled to -21. 1 C. Ordinary home freezers don't usually get quite that cold, so unless you have a very good freezer on the coldest setting you would have seen it freeze.
For anybody who wonders who is right: do the experiment yourself. Mike W. (published on 09/11/2009) Answer 1: It's actually a little difficult to answer with certainty without running an experiment. When ice melts in water, heat from the water is being transferred, or conducted, to the ice until the ice warms up enough to melt into water. This process cools down the water immediately around the ice, and so more heat will be conducted in from the surrounding water, and some of the water will move because cold water is slightly denser than warm water. Heat flow in a stationary medium is influenced by the thermal conductivity of the material. The higher the thermal conductivity is, the faster heat can flow, and the faster the ice will melt. According to the thermal conductivity of saltwater decreases as salt is added, which suggest that the saltwater will melt the ice more slowly.
But water is not stationary, as it cools down it gets denser and falls away from the ice, so warm water can flow in without the heat flowing through the cold water. This process is called heat convection. Saltwater changes in density a bit faster as it cools down than freshwater which means it might move faster when you put the ice cube in, which helps melt the ice, but it also is more viscous so it's harder which makes it harder to move which slows down the melting process. This was found out from. Since there are two factors working in opposite directions I don't know whether there will be more or less convection in the saltwater vs. freshwater. This could be calculated but it's tricky. If there is less convection in saltwater then it will almost definitely take longer to melt the ice than freshwater. If there is more convection in salt water then the time to melt the ice may be closer, or the saltwater may even melt it faster.
My guess would be that the higher thermal conductivity of the freshwater will melt the ice more quickly regardless of convection. But I think the best way to find out is to run an experiment. Measure out a quantity of salt and water and combine them. Keep track of how much salt there is, so you can compare saltwater with different salt concentrations. You can keep track of it by weight with a kitchen scale or by volume with measuring cups and spoons. Fill two identical glasses with the same volume of water, one saltwater, one tap water, and put in two identical ice cubes from the same batch of ice. Time how long it takes each ice cube to melt. For fun, see if you get different results with different concentrations of salt but the same volume of water, what happens when you change the volume of water, or what happens if you try using distilled water instead of tap water. Long answer but hopefully it answers the question!
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