In liquid water, all of the water molecules are constantly moving very quickly. The motions that are most important for liquid water are rotations (the water molecules spin) and translations (the water molecules move from place to place).

To freeze into ice, the water molecules need to lose these motions to become "stuck" into place in a specific pattern, called the ice crystal structure. As liquid water is made colder and colder, the motions of liquid water molecules become slower and slower. When water is at its freezing point, the liquid water molecules that "crash" into the surface of ice are moving slow become "stuck" in place. This is how ice forms at low temperatures.

As salt is added to water, the salt water solution is now composed of both water molecules and dissolved salt ions. As a result, these salt ions "replace" some of the water molecules. For example, one gallon of pure water contains many more water molecules than one gallon of ocean water, because that gallon of ocean water now also contains many salt ions.

Salt water freezes more slowly than pure water because many of the water molecules that would be "crashing" into the surface of the ice in pure water are replaced by these salt ions. The salt ions still "crash" into the surface of the ice like the water molecules do, but the salt ions do not become "stuck," so they end up slowing the growth of ice.

To understand the process of freezing, it’s important to realize that there are two processes at play: water turning into ice and ice turning into water. Even when water is freezing into ice, there are still “ice molecules” that are turning into liquid water. Salt messes with the balance between the freezing and the melting process. One way to think of this is that when a liquid water molecule hits the ice, it becomes part of the ice. But if there’s salt on the ice, the salt will sort of “block” the liquid water molecules from becoming part of the ice. This slows down the freezing process because less water per time can be incorporated into the ice. Meanwhile, the melting rate of the ice is unchanged by the salt. Since the ice/water mixture now has a higher melting rate than its freezing rate, the ice will melt. This also causes the freezing temperature of the water to go down so that it can be liquid at lower temperatures than pure water. If salt is added to ice, it will often melt the ice by slowing the freezing rate.

The easiest way to think of this is that the salt interferes with the ice forming a crystal. The salt likes (or at least is very energetically stable) being mixed with the water. To freeze water with salt, it's actually kindof a two-step process: getting the salt out of the way, and then forming the structure of the crystal. So it takes extra "coldness" to force the salt out of the way to form crystals. This image should help explain it:

Salt lowers the freezing temperature of water. Water freezes less readily because it requires even colder temperatures in order to freeze.

You are asking two questions in one! So let's break up your question in two parts:

1. Why salt addition in pure water makes the salty solution freeze at an even lower temperature?
2. Does it mean that the salty solution will freeze slower than the pure water?

Part I: we can add a lot of salt or a little salt. If you add a lot of salt, then it starts to matter what kind of salt you added. So for now, let's see what happens when you add a bit of salt.

- First of all, it turns out it doesn't matter what kind of salt you add! For example, you can add sodium chloride (table salt) or potassium chloride (also found in the supermarket) or sodium iodide. Because they each have one positive and one negative charge they will behave identically. - Secondly, we will assume that your solution is "ideal". This means that the interaction between the salt and the water is the same as between the water and the water.

So, then what difference does adding salt make?

By adding salt, we actually decrease the fraction of water. This is what matters. Now, in the same space, instead of having say 100 water molecules we only have 90, so if we want to make ice fewer water molecules are available to escape to the solid ice phase. We need to cool more to make them escape to the solid phase.

This simple but amazing property is what keeps sea water still liquid when the temperatures fall below 0 C, keeps fish alive in arctic waters, keeps the streets of Boston without ice in the winter so cars and people drive and walk without slipping.

Part II : Part I had to do with a static view of freezing. As the scientists call it, "an equilibrium" view. But you asked how fast. That depends on how fast we cool the system. Again, for dilute salty solutions that are ideal, since water and salt interactions are assumed to be the same, it should be exactly as fast in the case of pure water and salty water. But, in the case of salty water, more cooling will need to get done to get to lower temperatures.

For example, pure water freezes at 0 C. Upon addition of 10g of sodium chloride in 100g of water, the freezing point will decrease from 0 C to -5.9 C. So, if I place it in my freezer at -3C, the salty solution will NEVER freeze no matter how long I wait.

When a liquid (like water) cools past its freezing temperature, its molecules no longer have enough energy to move freely against each other. Instead, the molecules become locked in place, as a solid. The temperature where a particular material freezes (or melts, depending on the direction) depends on the forces between the molecules and the size of the molecules – the stronger the forces are and the bigger the molecules, the more energy is needed to escape the solid state and become a liquid.

Water is peculiar because it has a rigid solid structure that is less dense than its liquid state (which is why ice floats in liquid water). This is due to the partial negative charge on water and partial positive charge on hydrogen. This polarity allows for hydrogen bonding, where hydrogen associates strongly with the oxygen on other molecules in a network. When you freeze water, this network becomes rigid and locked in place. If you add salt (NaCl) to the water, the ions disrupt the formation of this network because they are too big to be included in it. This lowers the freezing temperature of the water – you have to remove more thermal energy from the solution before you can lock these ions in solid water.

The Celsius temperature scale was defined to match the phase transitions of pure water, with 100°C at water’s boiling point and 0°C at its freezing point. Fahrenheit was also designed with water in mind, but using salt water instead – 0°F is when seawater begins to freeze. The salt content in the water lowers its freezing temperature by 32°F (17.8°C)!

This is a general phenomenon too – dissolving a small amount of any solid in any liquid will decrease its freezing point and increase its boiling point, regardless of what the solid and liquid are. These effects are called the colligative properties of solutions.

The properties of solutions are different than those of pure liquids, and these effects are called colligative properties.

Interestingly, these properties don't depend on the type of salt, but rather the number of salt molecules that are dissolved. When you add salt ions to water, it stabilizes the liquid water. In other words, mixing ions takes work (you have to move them around the whole container), but it happens without you doing work because the end solution is more stable, so the stability gives you the energy to make it happen spontaneously, that is to say, without you doing any work.

(If the solution wasn't more stable, it wouldn't dissolve, in the same way that you can't go up a hill spontaneously -- you have to do work -- but you can go down a hill spontaneously).

Because the solution is more stable than the pure liquid, it is harder to turn it into a had or into a solid. For this reason, the boiling point increases and the melting point (freezing point) decreases.

Note from ScienceLine moderator:
A scientists from NOAA sent the following answer-correction (Answer #9) for any misinformation that the answers above could have. We thank this scientist for his time.

Answers #10 and #11 are from UCSB scientists attempt at clarifying any misinformation in the previous answers. We also thank these scientists for their time and patience while reviewing the question and all the answers here.

1) Fresh water does indeed freeze at 32 F / 0 C
2) Ocean water does freeze at _about_ 28.4 F / -2C
3) It is an outright error to say sea water freezes only at 0 F
4) Fahrenheit's experiment was different.

Since the freezing point always depends on how much salt is present, our operational sea surface temperature analysis includes the effect. It isn't a huge effect -- less than 0.1 K per 0.1% salt change -- but it does affect the analysis and oceans.

The exact freezing point for any solution - in other words, a liquid mixture of substances instead of a single substance like pure water - will depend on both the concentration and the nature of the solute (the substance dissolved in the solution). For aqueous solutions - solutions made by dissolving something in water - typically, the more concentrated the solution, the lower the freezing point because the solute molecules disrupt the ability of water molecules to interact with one another and to form organized structures as the temperature is lowered. We call this phenomenon "freezing-point depression". Since the extent of freezing-point depression - the number of degrees by which the freezing point of the solvent drops - depends on the concentration and nature of the solute, the freezing point of seawater is NOT uniform across different seas and oceans! This is because different oceans and seas have different salinities (a measure of the percentage of salt, which is one of the ways to express the concentration of salt in the waters). This map from NASA ( measure of salt ) shows the differences in salinities across the earth's waters. When we speak of the freezing point of seawater, that number is either an average or a generalization (maybe a number that applies to the middle of the range of salinities, for instance), and not a set figure for all seawater from anywhere on Earth. However, as the NOAA scientist stated, the differences are not large, so the generalized number of 28.4F can be used in most applications.

Fahrenheit's experiment was not done with seawater. The "salt" he used to assign the number of 0 degrees Fahrenheit was ammonium chloride, which has chloride but not sodium; he saltiness in seawater comes predominantly from chloride and sodium, not chloride and ammonium, even though there are other salts dissolved in seawater - the term "salt" in a variety of scientific branches can refer to one of many different substances that are both ionic and crystalline, not just sodium chloride (table salt). Furthermore, as the NOAA scientist has clarified, the amount of solute Fahrenheit put into his experimental mixture was much higher than the salt content in seawater. Therefore, the number Fahrenheit assigned as 0 degrees F can NOT be used in relation to seawater - seawater is simply not that salty. Answer 6 from the UCSB link was correct except the seawater, and I suspect that it was a typo where the poster meant to say "salt water".

Hope this helps!
Best,

The question on ScienceLine was about the rate of freezing, not the freezing temperature. As the NOAA scientist says in answer 8:
1) Fresh water does indeed freeze at 32 F / 0 C
2) Ocean water does freeze at _about_ 28.4 F / -2C

The seawater will freeze at a temperature that depends on the salt content of the seawater, so that's why the NOAA scientist says it's approximately 28.4 deg F.

Sea ice that floats on seawater doesn't have much salt in it:

National Snow and Ice Data Center

I don't think there's a simple answer to your question about adding more and more salt. Some articles talk about temperature AND pressure, such as this one:
Algorithms

My favorite is this one, which talks about the thickness of the 'mushy layer' in ice as the salt concentration increases:
mushy layer

We present new experimental results relating to the growth and evolution of sea ice. These show, in particular, that brine initially remains trapped in the interstices of the sea ice, only draining into the underlying ocean once the depth of the sea-ice layer exceeds a critical value. A general theory for convection within mushy layers is applied to develop a hypothesis for when brine drainage occurs, which is tested against the experimental results.

The main point is that the water-ice freezes, but the salt doesn't freeze.

You can also check out this link for 'maximum freezing point depression NaCl' from a google search. It has the logo for NSF, the National Science Foundation, at the bottom, so I think it's a good link:

here.
I think it's talking about putting salt on icy roads in cold weather.