Sorting All Sorts of Trash

In some communities, people must separate their recyclable materials into different categories, such as glass (clear or colored), paper, cardboard, aluminum, plastic, etc. Other communities use single-stream recycling, in which everything is put into one large container; sorting occurs later.

If you watched the recycling center videos, you saw that they use some pretty cool methods for separating trash into different types. Sometimes, it's very simple, with a trained sorter picking items out by hand. Other separation methods use magnets or flotation. In this experiment, you will investigate different items that might be in the trash, and will try to figure out ways to separate them all.

What You'll Need:
Pieces of different plastics about 1 inch (2.5 cm) square (find plastics with different numbered recycling codes to ensure that they're different)
Aluminum foil (1 inch or 2.5 cm pieces)
Aluminum can
Steel can
Newspaper (1 inch or 2.5 cm pieces)
Copier paper (1 inch or 2.5 cm pieces)
Magnet
Knitted wool or acrylic item (like a hat or mitten)
Hair dryer
Large bowl
Water

1. Place the magnet near all the different kinds of trash you've collected. Which kinds are attracted to the magnet? Could you use a magnet to remove these from the recycling stream?
2. Rub the knitted item on your hair to give it a static charge. Are any of the trash items attracted to the charged knitwear?
3. Place all of the pieces of trash on a table and aim the blow dryer at them. Turn the dryer on. Which types of trash get blown about, and which ones stay put.
3. Fill the bowl with water and place all of the pieces of trash in the water, one at a time. Which ones float, and which ones sink?
4. Using what you have found, can you figure out a series of sorting techniques that will let you separate all of your trash into different categories? You can use techniques more than once if that helps. If you can't, which things are hardest to separate?

Don't Be Dense!

Here's a quick experiment you can perform to prove to yourself that hot air is less dense than cool air.

What You'll Need:
Dry, empty 2-L soda bottle
Balloon
Hair dryer

Stretch the mouth of the balloon over the top of the soda bottle, and let the balloon hang down over the side. Heat the bottle with the hair dryer. What happens to the balloon?

Pretty quickly, you'll notice that the balloon starts to inflate. Now, you're not blowing into it. The hair dryer isn't blowing into it. There's no chemical reaction going on to create gas. Why is the balloon inflating?

The collapsed balloon, bottle, and the air inside the bottle weigh a certain amount, and there's a certain volume of air in the bottle. If we knew what the exact numbers were, we could calculate the density. When you heat the air, you make it less dense. You can't make it weigh less, because there are the same number of molecules in the cool air as in the hot air. The only way to change the density is to make the volume larger. Since the balloon is free to inflate, that's where the extra volume goes, and you see it begin to grow larger!

Full of Hot Air!

Colorful hot-air balloons are certainly beautiful, as they float through the sky. They stay up as long as the air inside them remains hot. If you're watching the balloon from a distance, they may seem very quiet, and much of the time, the balloons do just float along silently. But every now and then, the balloonist has to turn on the burner to heat the air again, and that's quite noisy if you're in or near the balloon!

Hot-air balloons rise because hot air is less dense than cooler air. Density is the weight of something divided by its volume. A hot-air balloon typically holds about 100,000 cubic feet (2800 cubic meters) of air. Cooler air (68^oF, 20^oF) weighs about 7500 pounds (3400 kilograms); hot air (250^oF, 120^oC) weighs about 5600 pounds (2500 kilograms). That's a difference of 1900 pounds (900 kilograms)! When the air in the balloon is heated, the whole thing becomes lighter, and it rises into the air.

You may notice that you usually don't see hot-air balloons during the day. They're most often spotted in the early morning or just before sunset. That's because it's safest to take off when the air is at its calmest. Also, the air tends to be cooler at these times, so the difference between the cool air and the heated air is greatest.

Another thing about hot-air balloons - you can't really steer them! Balloon flights aren't used to get from one place to another, because where the balloon goes depends on which way the winds blow. Balloons are followed by chase vehicles, cars or trucks that meet the balloon when it lands, driven by people who help the balloonists pack the thing up and take it back to the starting point.

Have you ever been to a balloon festival? As beautiful as one or two hot-air balloons are, it's really amazing to see over one hundred of them take off at once. Some famous balloon festivals are the Quick Chek Festival of Ballooning in Readington NJ (where Dr. B took the photo above) and the Albuquerque International Balloon Fiesta in NM. You can look for a balloon festival near your home here.

Hee-Hee... Helium!

What's in those balloons that float up into the sky? Helium! What's so special about helium, that it makes balloons lighter than air?

It's all about density. Density is defined as the weight of a substance divided by its volume. For example, the density of iron is 491 pounds per cubic foot (a cube 12 inches on each side), or 7870 grams per liter. It's pretty obvious that, if you had a piece of iron smaller than a cubic foot, it would weigh less. But the ratio of the weight of the smaller piece to its volume would be the same as the larger piece.

If you mix substances with two different densities, the more dense one will sink, or looking at the the other way around, the less dense substance rises. If you dropped a chunk of iron (whether large or small) into water, it would sink. But if you put a piece of wood (most kinds, at least) into water, it floats because its density is less than that of water. Helium gas is less dense than air (0.011 pounds per cubic foot or 0.18 grams per liter for helium versus 0.078 pounds per cubic foot or 0.078 grams per liter for air), so balloons filled with helium float!

Shrinkies

OK, so you can't really shrink people like they show in movies. But here's a fun experiment you can do with something that actually does shrink.

What you'll need:
Foam plastic plates
Scissors
Markers
Foil
Oven set to 350^o
Oven mitts

Cut out small round or rectangular pieces from the flat bottom of the foam plates. Draw pictures on them with the markers. Arrange them on a piece of foil so that they are not touching, and heat in the oven for 4-5 minutes, or until they have shrunk. Take the foil out of the oven carefully using the oven mitts and let the shrinkies cool. You will see a miniaturized version of what you originally drew!

So, why does this kind of shrinking work? The foam has a Swiss-cheese-like structure, with lots of air holes. When you heat the piece of foam, the air bubbles collapse, and you end up with a solid piece of plastic. You have actually changed the density of the plastic, by decreasing the volume, keeping the mass (weight) the same.

Incredible Shrinking Movies

One science theme that shows up in movies from time to time is the ability to shrink things down to a tiny size. The 1957 movie, The Incredible Shrinking Man, didn't use computer graphics because there were no computers at the time! In 1966, Fantastic Voyage told the story of medical professionals who were miniaturized and injected into the bloodstream of a patient so that they could perform surgery on a blood clot. More recently, in Honey, I Shrunk the Kids , a scientist invents a shrinking machine which puts his children, and his neighbor's children, in danger (although they're all OK in the end).

So, what about the science behind these movies? It's not so good - in fact the science is very wrong. It's true that, at normal size, there's a lot of space between the molecules of our bodies. But you can't actually make the molecules smaller; they'd just have to get closer together, and that would start to change the way our bodies work. In science, there's a Law of Conservation of Mass, which means that you wouldn't get any lighter, even if you could get smaller. So you would just become more dense. Just imagine a group of normal-weight people swimming through your blood vessels! These movies may be a lot of fun, but they really are incredible!

Swimming in the Sea

If you've ever gone swimming in the ocean, you may have noticed that it's a lot easier to float in salt water than in fresh water (like a pool or lake). Why?

Earlier, in "Sinking Soda?", you saw that the density of an object determines whether it floats or sinks. But it turns out that the density of the liquid matters, too. Obviously, salt water has lots of salt (or sodium chloride) dissolved in it. In fact, sea water is usually about 3.5% salt. This means that every gallon of water has about 15 teaspoons of salt dissolved in it. This makes sea water heavier than fresh water. In other words, sea water is more dense.

Fill two drinking glasses with about 1 cup of water (make sure the glasses are big enough first!). Add one teaspoon of salt to one of the glasses and stir until it dissolves. This makes the water about as salty as seawater. Take two eggs, and carefully place one in each glass. What do you see?

Ship Shape

Density isn't the only property that affects whether something floats or not. If you drop an iron nail into water, it will sink. But a huge ocean liner made of steel (which is mostly iron) floats. How can that be? And what experiment can you do to figure out what shapes float?

Well, you could heat the iron very hot then pound it into different shapes, but there must be an easier (and safer) way! You can use clay (but not Play-Doh) instead. Roll some clay into a ball, then drop it into a bowl or bucket partly filled with water. What happens? Now flatten the clay and fold up the sides to make a little boat. Does this float?

Once you have a boat design that floats well, try filling it with "cargo", like marbles or paper clips. How much can it hold before it sinks? Experiment with other designs, and see which one holds the most cargo.

Sinking Soda?

Here's an easy experiment that illustrates the effect of density on floating. Get two cans of soda - one diet and one regular (with sugar). It's best if the two cans are the same size. Find a bowl or other container that is large enough to hold both cans and fill it about 3/4 of the way with water. Place both cans in the water and watch what happens, but be careful that the water doesn't overflow.

Both cans hold the same (or very nearly the same) volume of soda. Does one can feel heavier than the other? If you have a kitchen scale, weigh them. What can you conclude about the density of diet versus regular soda? And which floats better - the more dense or the less dense soda?

Don't Be Dense!

Before we start experimenting with flotation, let's have a few words about density, because it will be very important in the upcoming posts. Talking about density always reminds Dr. B of a silly riddle: "Which weighs more, a pound of iron, or a pound of feathers?" Most people say, "Of course, iron is heavier, so a pound of iron weighs more!"

But think about this carefully - a pound is a measuret of weight, so a pound of iron and a pound of feathers weigh exactly the same. Now, a pound of iron is certainly smaller than a pound of feathers , but the weight is identical. When we use the word "heavy", we often really mean that something has a high density. Density is the weight of an object divided by its volume (size). One cubic foot of iron (a cubical box with each side a 1-foot square) weighs almost 500 pounds, so its density is about 500 pounds/cubic foot (abbreviated as cu ft). Two cubic feet of styrofoam weighs just about 4 pounds, so its density is 4 pounds/2 cu ft, or 2 pounds/cu ft. What is the density of water, if 4 cu ft weighs 250 pounds? Check back in a few days for the answer and for the first experiment on floating.