"Bridge Ices Before Road"

Have you ever seen this sign on a road? Ever wondered why the bridge ices up first? There are actually two reasons.

The first reason is that the underside of the bridge is exposed to the elements. Cold air gets all around the bridge structure, making any water on the surface freeze. The road surface is protected by the soil underneath. Eventually, the road becomes cold enough for the wet surface to freeze.

The second reason is that bridges and roads are made from different materials. Bridges are made mostly of steel and concrete. Both of these materials conduct heat well. If you heat them, they get hot fast. If you cool them, they get cold fast. So, once the air temperature drops, the bridge starts losing heat, leading to relatively quick formation of ice. The road is made of asphalt, which holds heat in. Therefore, it takes a long time for the road to become cold enought to freeze.

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.

Shrinking and Growing

In the last experiment, the volume of the water increased as it turned to ice, and the volume of the oil decreased, or stayed the same, as it froze. Why do these two liquids act so differently?

Most liquids contract (get smaller), at least a little, when they freeze. Molecules in a liquid move in all directions very quickly. They slow down as they get colder, and get really lazy around the freezing temperature. When the molecules are about to turn into a solid, they begin to crowd together and stick to one another. This arrangement of molecules takes up less space than the liquid form, and the volume of the solid that results after freezing is less than the volume of the original liquid.

Water (H₂O) acts differently because of its molecular structure. A water molecule has its oxygen atom in the center, with hydrogens on either side. However, the molecule is not straight, but rather, is bent at an angle of 105^o. So the water molecules cannot pack tightly like most other molecules can. They end up with a structure that looks like a honeycomb, in a six-sided, or hexagonal arrangement. This structure actually takes up more space than the original water, and so solid water has a larger volume than liquid water.

Here's a video you can watch that shows the motion of water molecules in ice (solid), water (liquid), and steam (gas).

I See I-CE!

Water is an unusual molecule, for reasons we'll talk about soon. One of its weird properties is that it expands (grows) when it freezes. Most liquids contract (shrink). In this experiment, you'll compare the freezing behavior of water to that of olive oil.

What you'll need:

2 empty cans of the same size
marker

water

olive oil

freezer

Fill one can to within 1/2" (1 cm) of the top. Make a mark on the outside of the can to show where the water level is. Do the same thing with the other can and olive oil. Try to get close to the same amount of liquid in each can. Place both cans in the freezer and leave overnight. The next day, check to see whether the liquid has expanded or contracted. What did you find?

To Freeze, Or Not To Freeze?

In the last posting, you learned that some foods shouldn't be frozen, if you want to eat them later. They don't go bad, or become unsafe to eat, but they turn icky and disgusting. In this activity, you can experiment with different foods, to see which kinds of foods you won't want to eat after they've been frozen and thawed.

What You'll Need:
Small paper cups, one for each food you want to freeze
Metal baking pan
Marker
Freezer
Several foods to try freezing, such as lettuce, apples, carrots, milk, tomatoes, bread, cottage cheese, mayonnaise, oranges, pasta (cooked), meat, yogurt, eggs, water, soda, or potatoes

First, clear out some space in the freezer so that the baking pan will lay flat. Place a little bit of each food into each cup and label. Place all the cups onto the baking pan, then put everything into the freezer. After 2-3 hours, take the pan out and put it on a table or counter to thaw. After two hours, inspect the foods and see how they look. Which ones would you still eat? Which ones look really yucky? Even though all of these foods are still safe to eat, it's probably a good idea to throw away anything that doesn't still look right.

What you'll find is that foods with lots of water tend to turn mushy after they're frozen, and that foods that are mixtures of many different things usually separate into different layers.

Lots of Leftovers

Now that Thanksgiving is over, you probably have lots and lots of leftovers. You'll probably freeze much of what you can't eat. Freezing is a great way to save both cooked and uncooked food, as long as your freezer is colder than 0^oF. The U. S. Department of Agriculture has a fact sheet that answers lots of questions about freezing food safely. Here are some of the highlights:

• Almost anything can be frozen, except for eggs in their shells and anything in a can.
• Some foods turn yucky when they're frozen, but they're still safe to eat (there will be an experiment on this)
• Food never "goes bad" in the freezer. It never loses its nutritional value, although it may get dried out and not taste very good.
• Bacteria "sleep" when they're frozen, but become active again when the food is thawed. Remember that freezing does not kill bacteria or other nasty beasties.
• Fresh vegetables should be cooked briefly in boiling water or a microwave oven before freezing. This is called blanching. Vegetables sold in frozen form have already been blanched.
• It's OK to freeze meat in the supermarket packaging, but only if you're going to use it in a month or two. Otherwise, the meat will dry out.
• Freezer burn can occur if food is exposed to air in the freezer. It causes dried-out, grayish spots. They're pretty disgusting, but you can just cut these spots off and use the food as usual. To prevent freezer burn, be sure to wrap food tightly.
• Freezing can cause some foods, particularly meats, to change color, but it doesnt't make them unsafe or less nutritious.
• You should only thaw foods in the refrigerator, in the microwave, or in cold water. Never thaw foods on the kitchen counter, outdoors, or in the dishwasher!
• Once food has been defrosted, it is safe to freeze it again, even if it hasn't been cooked.