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!

Floating Paper Clip

The results of the "Ship Shape" experiment might be confusing to you. Clay is denser than water, and sinks when it's shaped into a ball. But if you make a boat shape, it floats! In this experiment, you'll see that even things that are the same shape sometimes float, and sometimes don't.

Drop a paper clip into a glass of water. The clip sinks. But now bend the end of the clip upwards so that you can use it as a handle. Carefully place the clip on the surface of the water, so that it floats (you may have to try a few times). Take a careful look at the water around the floating paper clip. Here's the puzzle: the density of the metal in the paper clip didn't change, and neither did the density of the water. So why does the clip sink in the first case and float in the second?

The answer is that the very top (surface) of water is different. The surface molecules crowd together, creating a "skin" that is actually denser than the rest of the water. This effect is called "surface tension". If you look at the water around the floating paper clip, you'll see that it doesn't look flat - the paper clip has caused the skin to stretch. Surface tension isn't a very strong effect, so it only helps lightweight things to float, like water bugs, leaves, or pieces of paper.

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.

Floating Away

On hot summer days, few things feel better than floating through cool water, whether you're swimming, are using a swim ring, or are in a boat. But did you ever wonder why you can float in water, but a rock can't? Why can't you float through the air (that would be cool!) but a balloon can? This month, we'll be looking at floating. You'll learn what floats and what doesn't, and why.