Salty Roads

So why does salt cause ice to melt? And is there a limit on how cold it can be for this to work?

Salt melts ice because it dissolves in the layer of liquid water that forms on the surface of ice. When you dissolve a substance in water, the freezing point of the resulting solution is lower than that of just water. This is known as freezing point depression. You may know that pure water freezes at 32^oF (0^oC). If the solution is 10% salt, it freezes at 20^oF (-6^oC). A 20% solution freezes at 2^oF (-16^oC). So, if the air is 28^oF (-2^oC), a container of pure water would freeze solid, but both 10% and 20% salt solutions would remain liquid. At an air temperature of 14^oF (-10^oC), the pure water and 10% solutions would freeze, but the 20% solution would not. What do you think would happen at -6^oF (-21^oC)? All three solutions would turn to ice! If you live in an area with very, very cold winter temperatures, the road crews may not even use salt on the roads. If it's too frigid for even pretty salty solutions to melt, there's no point in using salt at all.

Well, no point in using regular table salt, that is. What we normally call "salt" is a chemical called sodium chloride (NaCl). Chemists frequently use the word "salt", in a more general way, to describe substances that form in certain reactions (the reaction of an acid and a base, to be exact). There are other types of "salts" that can melt ice at lower temperatures than sodium chloride. One example is calcium chloride (CaCl₂) (not all salts are chlorides). Calcium chloride can melt ice down to a frosty -20^oF (-29^oC)!

But why do any of these solutions cause a drop in freezing point? When pure water freezes into ice, the water molecules slow down and line themselves up in a very organized pattern. When another substance is dissolved into the water, the dissolved particles keep the pattern from forming. The water molecules have to slow down even more to make ice; this requires a lower temperature. When a salt, such as sodium chloride or calcium chloride, dissolves in water, it breaks up into two parts: sodium chloride produces a sodium and a chlorine, and calcium chloride forms one calcium and two chlorines. The more particles, the more they intefere with freezing. So, CaCl₂ (three particles) has a greater effect on the freezing point of water than sodium chloride (two particles), and will melt ice at lower temperatures.

Explosive Metals

So, now you know that elements like sodium and potassium are metals. But, you might be thinking, isn't table salt also known as sodium chloride? Is salt a metal? And don't bananas and oranges contain a lot of the nutrient potassium? Do these fruits contain pieces of metal?

Well, no. Chemical elements come in different forms, depending on whether they're pure substances or in compounds, with two or more elements combined. Pure sodium and pure potassium are metals. They're gray and shiny, like most metals. They can be cut with a kitchen knife, which is kind of weird. But they are definitely metals. However, when sodium combines with chlorine (another element), it makes sodium chloride, a compound with properties very different from those of the original elements.

One more thing about the alkali metals, which include sodium and potassium, but also lithium, rubidium, cesium, and francium - they explode and burn when they contact water! This is much too dangerous for you to try on your own, but here's a link to a video in which you can see the Mythbusters playing with sodium and potassium.

Fun Facts 10

Just one more set of facts after this one!

• The radioactive element actinium (Ac, #89) really does glow in the dark!
• Darmstadtium (Ds, #110) actually has a nickname. In Germany, where it was discovered, the emergency telephone number is 110 (not 911, as in the U.S.). Since Ds is element #110, it is sometimes jokingly called policium.
• Sometimes people worry about getting too much sodium (Na, #11) in their diets. But some sodium is essential to good health, because it helps send signals through your nerves.
• Rubies get their beautiful red color from chromium (Cr, #24).
• Metal objects that are going to be used outdoors are often galvanized, or coated with zinc (Zn, #30), which does not rust.
• Promethium (Pm, #61) has been detected in the stars of the Andromeda galaxy.
• Many laundry bleaches contain chlorine (Cl, #17). These are the bleaches that cannot safely be used on colored fabrics.
• Scientists have found rocks that contain a lot of manganese (Mn, #25) on the ocean floor. No one knows how they formed!
• In the future, rubidium (Rb, #37) might be used to make a new type of battery.
• Terbium (Tb, #65) is one of four elements that are named after the small town of Ytterby, Sweden. In the 1800's, rocks from that area were found to contain four unknown elements - ytterbium, yttrium, erbium, and terbium.