Snowflake Art

Make some beautiful paper snowflakes using the instructions on the websites listed below. Plain white copier paper works great; you just need to cut it into 8 1/2 inch (22 cm) squares first.

Paper Snowflakes - Free instructions
Make a Paper Snowflake
How to Make Paper Snowflakes

"No Two Snowflakes Are Alike": True or False?

If you look closely at a natural snowflake, you'll see that it is based on a six-sided hexagon, but has a very complicated, delicate shape overall. You've probably heard the statement that "no two snowflakes are alike". Is this really true, and how do we know?

Of course, no one has inspected every snowflake that has fallen and compared it to every other snowflake throughout history. Certainly, if you look at a number of flakes from a single snowfall, you won't find any copies. And, given the number of variations that a flake's shape can take on, it is very, very unlikely that two have ever had exactly the same shape. And snowflake shapes are influenced by the air temperature, humidity, and other factors that also influence the shape. So, while it's not absolutely impossible for two snowflakes to look alike, you shouldn't waste time looking for them!

Snow, and Sleet, and Hail - Oh My!

Graupel and virga, too! During the winter, lots of different kinds of frozen precipitation can fall down from the clouds. They aren't the same thing:

Snow consists of clumps of tiny ice crystals that stick together as they fall.

Freezing rain starts falling as snow. However, on the way to the ground, it passes through warm air, which melts the snowflakes. Then they move through a layer of cold air, which re-freezes them into tiny drops of ice.

Sleet is frozen raindrops that bounce on the ground.

Hail is large ice balls that form in the presence of both rain and ice. The rain coats the ice, making the drops larger. As the ice drops fall, the winds push them back up again, where they get an even heavier ice coating. This can happen several times. Eventually, the hail stones become too heavy to be blown around, and they fall to the ground. They can even get to 6-8 inches (15-20 cm) in diameter!

Graupel, or "soft hail" are little balls of snow surrounded by ice.

Virga is rain or ice that never reaches the ground because it evaporates first. You often see virga streaks below clouds; they are particularly obvious at sunset.

"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.

Remove De Ice

Ice is great if you want to cool a drink, or as a surface to skate on, but it can be really dangerous when it covers streets, sidewalks, and other places where people have to drive or walk. What common substance is spread on roads and walkways? Salt! This is the same old table salt that you might sprinkle over your French fries, although the particles are usually larger. What effect does salt have on ice? Here's an experiment you can try that will reveal its effect!

What you'll need:

Glass

Water

Ice cube

Sewing thread

Salt in shaker

Cut a piece of thread about 12 inches (30 cm) long. Fill the glass about 3/4 full of water and put the ice cube into the water. Lay the thread over the top of the ice cube. Hold the ends of the thread up, and pour salt onto the ice cube, covering the area with the thread. Wait about one minute, then take hold of both ends of the thread. Lift them up. What do you notice?

The ice cube comes out of the water with the thread! The salt melts the ice, which then wets the thread. As the salt continues to dissolve, it moves away from the ice cube, and the ice re-freezes. However, by now the salt solution has covered the string, fastening it to the cube.

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?

Brrrr!!!!

What has the weather been like this winter where you live? Many parts of North America have had a LOT of snow, but others, like New Jersey where Dr. B lives, have had warm temperatures. Maybe you even live in a place that almost never gets snow. This month, we'll explore both snow and ice - how they form and why, how they behave and why, and how we can make them less troublesome when they cover our roads, houses, and yards!

Burning Both Ends

Have you ever heard the expression "burning the candle at both ends"? It means that someone is working very hard, usually without sleep. But I'll bet you have never seen what happens is you actually burn a candle at both ends!

This experiment involves both sharp knives and fire, so definitely have an adult help with it!

What you'll need:
Candle, as described below
Knife
Ruler
Long sewing needle
2 wine glasses
Matches or lighter
Newspaper

First, you have to find a candle that will work well. The best candles for this experiment are straight cylinders about 1/2 - 3/4 inches (1 - 1.5 cm) in diameter. Candles that get smaller near the top won't work as well. With a sharp knife, cut about 1 inch (2.5 cm) off the bottom of the candle, and dig out enough wick that you can light it. Trim the candle so that the bottom looks very much like the top. Measure the length of the candle and make a mark in the exact center. Push a long sewing needle through the middle of the candle from one side to the other.

Spread newspaper or other absorbent paper on the surface you will be working on. Position the two wine glasses next to each other at a distance slightly larger than the candle diameter. Balance the needle on the glass rims, so that the candle acts like a seesaw (see diagram). It is best if the candle is well-balanced enough that it is horizontal, but the experiment will also work if the candle is at an angle. Light both wicks and watch what happens!

With both ends of the candle burning, wax drips from the top and bottom. When a drop falls from the top of the candle, it becomes lighter, and tips upwards. Then a drip falls from the bottom, and that part tilts up. You will see the candle start to rock back and forth, just like a seesaw. The rocking will get stronger and stronger. If the candle isn't perfectly balanced, the rocking may slow down, or even stop, but it will start up again.

So now you know what happens when you burn the candle at both ends!