What is in salt that makes ice melt?

Answer 1

When the salt is spread onto the road, the water is already

frozen. Salt, however, is hygroscopic (NOT hyDRoscopic, as many

wrongly term). This means that it will actually absorb water out of

the air and the surrounding environment to, in essence, dissolve

itself. When this happens, it lets off a small amount of

heat. Salt is in a very rigid crystal form and it takes energy to

get the ions into this very organized form. When the salt

dissolves, the crystal lattice breaks and releases its energy in the

form of heat. This heat is actually melting the ice, which

dissolves the salt more and lets off more heat, hence making salt a

very effective chemical to melt ice on the road and keep it melted!

Answer 2

Salt and Ice

How does salt "melt" snow and ice? As we know, pure water freezes at 0 °C. Any impurity that is added to the ice interferes with the freezing process, and water does not become solid ice until the temperature drops even lower. As a result, adding any dissolved impurity such as salt lowers the freezing point of water. Instead of freezing at 0 °C, a mixture of 20% salt in water stays a liquid until the temperature is as low as -16 °C.

-- answer by Divine Crow. (Credits to NELSON: Science Perspectives 9)

The thorough answer requires a bit of thermodynamics.

Equilibrium occurs when the chemical potential of a substance is the same in two different phases. Other ways to state this is that a process where the change in Gibbs free energy is zero is in equilibrium or that the fugacity of the substance is the same in both phases.

I will describe it in terms of "fugacity". Freezing occurs when the fugacity of the solid (ice) is equal to the fugacity of just the water in the liquid phase. The fugacity is a function of the chemical potential but roughly the fugacity of a substance (assuming no phase change) decreases with decreasing temperature, decreases with decreasing pressure, and decreases with decreasing concentration. For very small changes, the changes are linear. As salt dissolves into liquid water, it lowers the concentration of the water in the solution, for example, the water may go from 1.000 mole fraction water down to 0.98 mole fraction with the salt in the solution making up the other 0.02 of the mole fraction. As the concentration of the liquid water drops, so does its fugacity, so it is no longer in equilibrium with the solid at that temperature. Remember fugacity goes down with temperature so the liquid will be in equilibrium with ice at a lower temperature.

The mechanism that we actually observe is that once salt in placed in contact with ice, it begins to interact with the water molecules bound up in the solid ice and starts to form a tiny amount of water/salt liquid solution - pretty close to saturated with salt. The water in this liquid has a much lower fugacity than the water still bound up in the ice so there is a driving force that encourages more water molecules to let go of the solid ice and try to dilute the liquid solution. As this continues, we eventually see some liquid start to form at a macroscopic level where we can observe it with the naked eye - so apparently the salt is melting the ice.

Actually anything that will dissolve in water can lower the freezing point and melt the ice - but a lot substances that dissolve quite well in water at higher temperatures won't dissolve very well in freezing cold water, so it is more difficult to get the concentration high enough to do much good. Salt dissolves very well even in very cold water, so it is fairly effective at melting the ice - plus it it comparatively cheap and readily available compared to alternatives.

Also realize that all the salt is doing is lowering the freezing point. If it cold enough the ice will stay as ice and the salt can't help it melt.

Answer 3

Most answers to this question incorrectly focus on how salt prevents water from freezing from liquid to solid, not on how it "makes" the ice turn into liquid.

Heat makes ice melt. It turns out that ice is melting and refreezing at any given temperature, and melts altogether above 0°C. Melting absorbs heat, freezing release it. Salt interferes with freezing by disrupting the formation of ice crystals and prevents the water that has melted from refreezing, all the way down the lower freezing temperature set by the salt-water solution. Thus is continues to absorb heat and remains a liquid down to the colder temperature, which is useful for clearing roads or making ice cream.

-- Andrew

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the salt rocks, like on ice cream, which it is salt on ice cream, and the salt rocks on ice cream makes the ice melt fastest

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Salt and Ice

How does salt "melt" snow and ice? As we know, pure water freezes at 0 °C. Any impurity that is added to the ice interferes with the freezing process, and water does not become solid ice until the temperature drops even lower. As a result, adding any dissolved impurity such as salt lowers the freezing point of water. Instead of freezing at 0 °C, a mixture of 20% salt in water stays a liquid until the temperature is as low as -16 °C.

-- answer by Divine Crow. (Credits to NELSON: Science Perspectives 9)

Answer 4

Salt helps ice melt because the freezing point of water is 32 degrees Fahrenheit (0 degrees Celsius. Dissolving salt in water to freeze you have to make it colder than 32 degrees Fahrenheit (0 degrees Celsius).

Answer 5

The crystalline structure of salt molecules interferes with the crystalline structure that water (H2O) molecules try to achieve as the water attempts to "freeze" (expel heat to cool from a liquid state to a solid state). Another "catalyst" is beet juice -- interrupts the attempted solidification of water.

Answer 6

Salt lowers the freezing point of water through a process called freezing point depression. When salt is added to ice, it disrupts the ability of water molecules to form solid ice crystals, causing the ice to melt at a lower temperature than it would otherwise. This is why salt is often used to melt ice on roads and sidewalks during winter.

Answer 7

Salt. Sodium chloride.

Any solute lowers the freezing point of a liquid.

Answer 8

The salt release the heat of dissolution.