If you want to melt ice, you have a few ways of doing it quickly. Ice is water in a solid form and when it melts it turns into a liquid, which is actually a transition into a different phase, or what a lot of scientists call a “phase transformation”. To change the water from a solid to a liquid state, you need to provide some energy to the system. Phase transformations are a thermodynamic process, means that the absolute best way to melt ice quickly is to make sure it’s surrounded by really hot air or liquid. This is because a bigger temperature difference between the ice and its surroundings will mean you have more energy available to heat the ice up quickly, and you will have plenty of energy available to cause the phase transformation.
Now, you asked about which liquid, specifically, is the best for melting ice. Let’s say I have a glass of water and a glass of salt water (not to drink!). If I make sure both of these liquids are at the same temperature before I drop an ice cube into each of them, the ice cubes will melt at different speeds even though the liquids were the same temperature in the beginning of the test. That’s because temperature isn’t the only thing to worry about when we want to cause a phase transformation of water!
The glass of water has water in two forms: liquid water and solid water. So when the ice melts in this glass, the most important thing to worry about is the temperature difference. However, the glass with the salt water has different materials: liquid form, and the ice is pure water in a solid form. When an ice cube is dropped into a water-salt mixture, the salt from the water-salt solution is actually going into the outer surface of the ice cube. When salt is mixed with water it will actually lower the temperature needed to change from solid to liquid. This means that a salt-water mixture will be liquid at temperatures where pure water is still frozen (or solid). As the ice cube in the second glass heats up, the outer portion of the ice cube (the part that has salt mixed in) will turn to a liquid before the rest of the ice is warm enough for a phase transformation. As the outer portion is melted, the salt from the water-salt solution can get further into the ice cube, which means more of the ice cube can melt at lower temperatures. This process continues until the entire ice cube is melted, which all started happening earlier that the ice cube in the first glass (the water-water glass) could start to melt.
But there is a tricky catch to all of this: when we drop an ice cube into a glass of liquid, we need to consider more than just how the salt will affect the melting point. We also need to consider how adding salt to water will change its density, because that will change the way that the liquid and the ice cube will transfer heat.
Liquid transfers heat through a method called “convection”, which is basically the liquid mixing itself because of small changes in temperature and density in different areas of the glass. When you add a lot of salt to liquid water, you will make it much more dense, which can have the effect of making it harder to efficiently transfer heat to the ice cube. If it’s harder to transfer heat, then it’s also harder to cause a phase transformation quickly, which means that the glass with pure water will melt the ice faster if you have too much salt in the second glass. So if I want to melt ice quickly and I know that I’m going to be stirring the glass constantly, I should choose a salt-water mixture or add in something else that can lower the melting point of water.
We would need to stir the glass ourselves as a way to make sure we are getting good heat transfer between the liquid and the ice cube, because we know that if we change the density of the liquid too much then it could have a hard time heating up the ice cube. If I want to keep ice in my drink for as long as possible, I might want to stick with just a glass of water!
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