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Thursday, 16 February 2017

How does salt lower the freezing point of water?

When the winter season arrives you might have probably observed the highway department spreading salt on the road to melt the ice.

And the better simple example where you will be observed this type of condition is at our homes. When we are preparing to make ice cream at our homes we will use salt as one of the ingredients to lower the temperature.

In the above scenarios, we are using salt.

But did you know the reason behind it?

Why are we doing that?

How does it happen?

When you are making ice cream, the temperature around the ice cream mixture needs to be lower than 32 F if you want the mixture to freeze.Salt mixed with ice creates a brine that has a temperature lower than 32 F. When you add salt to the ice water, you lower the melting temperature of the ice down to 0 F or so. The brine is so cold that it easily freezes the ice cream mixture.

But what about the places like Antarctica, Greenland, and Canada the fresh water in the air freezes to snow and falls onto the land without a melting season to get rid of it. Over time, this snow builds up and compacts into a glacier.

The typical freezing point of fresh water is 0° Celsius [32° Fahrenheit]. Generally, water molecules are composed of hydrogen and water molecules and they have bonded together into a crystalline structure of ice. Eventually, the molecules move so slowly that they can no longer escape the intermolecular attractions between water molecules. As a result of these forces, a lattice of water molecules form and water becomes ice.

During this phase change, water molecules enter and leave the solid at the same rate.  Salt disrupts this equilibrium by simply being present. With the addition of salt, fewer water molecules are present at the interface between liquid and solid. In other words, salt particles block the water molecules from re-entering the solid phase, so more water molecules are leaving and less are entering the solid.

When the temperature lowers even further, the water molecules leaving the solid phase will slow down even further and the rate will eventually match the rate at which water molecules can find the solid in the presence of salt. When the rate at which water leaves the solid balances the rate at which water molecules enter, a new (lower) freezing point is established.

On average, seawater in the world's oceans has a salinity of about 3.5 percent (35 g/L, or 0.600 M). In another word, every kilogramme of seawater has approximately 35 grammes (1.2 oz) of dissolved salts-predominantly sodium (Na+) and chloride (Cl-) ions are present.As a result of these salts, seawater is denser than both fresh water and pure water. Therefore the freezing point of seawater decreases as salt concentration increases.

Although the saltiness of ocean water varies, this lowers the freezing point of ocean water to about -1.8°C or 28.8°F. So ocean water will freeze.

Another factor that affects the freezing of ocean water is its ocean current. Ocean current can be described as thermal convection combine to create large-scale flows of ocean water. This constant motion of the ocean water helps keep the water molecules from freezing into the somewhat stationary state of ice crystals.As a result, only really cold areas, such as the North Pole or the South Pole, usually get cold enough for ocean water to freeze.

More significantly, the ocean currents continuously pump warm water from the equatorial regions to the colder ocean regions.

Read More: salt lower the freezing point of water

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