# What Makes a Hot Air Balloon Float?

It’s one of those basic rules we grow up hearing: hot air rises and cool air sinks, but why is that? If you’ve seen our piece on buoyancy, you know that density is one of the key factors in whether an object sinks or floats in a given substance, but a substance’s density doesn’t have to remain the same. The secret is in the name itself: hot air balloon. By altering the temperature of the air inside, we decrease its density, allowing it to float. Since setting off a hot air balloon would be too expensive and a floating lantern is a fire hazard, we showed this same effect by altering the temperature of a helium balloon to prevent it from floating in air.

This all ties back to something in physics called the Ideal Gas Law, which can be expressed as PV=nRT; pressure x volume = number of moles of a substance x the gas constant x temperature.

In the case of a hot air balloon and the helium balloon, “n,” “R,” and “P” are not changing, we can focus on volume and temperature. By decreasing the temperature of the helium balloon, the volume must also decrease. Since density is described as mass divided by volume, as volume decreases, density increases. With enough drop in volume, the helium balloon becomes too dense to float.

After being removed from the liquid nitrogen, the balloon slowly heats back up to room temperature. As it heats up, it returns to its original volume and floats back to the ceiling.

The reverse is true in a hot air balloon, with its density starting equal to the air outside it. As the air inside it is heated, its density decreases to the point where it has so much buoyant force lifting up on it that it can lift not only the balloon but the basket and passengers as well. When it’s in the air, the operator has to make sure the air stays at a consistent temperature for them to maintain altitude, with an increase or decrease in temperature will lead to an increase or decrease in altitude respectively.

# DIY Density Column

We know that density depends on mass and volume of a substances. We can easily see differences of density when comparing anything to the air in our atmosphere. But what about common liquids compared to each other? Lucky for you, that’s exactly what we did. In this DIY experiment we poured seven liquids into one container to see what would happen.

The seven ingredients that we are using are honey, corn syrup, maple syrup, whole milk, dish soap, water, and vegetable oil. Just to spice things up we added a bit of food coloring to the water. If you pour them in order of most dense to less dense, they do a really good job in staying separated. However, the food coloring that was added to the water turned out to be more dense than water and the dish soap and started mixing in with the milk layer!

Ideally, if we mixed up the density column and incorporated all of the ingredients together, after a few hours we would be able to get the layers back. However, the dish soap will react to the milk and oil, making separation impossible for every single layer.

We tried this experiment with just seven liquids. Can you think of other liquids to try adding? When you do this at home, try dropping in different household items like paper clips, bottle caps, or ping pong balls to see how their densities compare. For more all things density, check out Will it Float?

# Will it float? Experimenting with Density

Have you ever wondered exactly why a boat floats or why rain falls down and not up? It all has to do with density! Density is a measure of how much mass there is in a given volume, or rather how much stuff you have in a certain place. It determines whether something sinks or floats when compared to something else.

Since, we are talking about sinking and floating, let’s use the most obvious example we have, water! Water has a density of 1g/cm3. That means if anything is less dense, it will have less stuff in that same volume. A great example of that is ice which has a density of 0.9167g/cm3, which is why your ice floats in a glass of ice water. Moreover, air floats on top of water because it’s density is roughly 0.001225g/cm3.

Here are two experiments that clearly shows a difference in density. For the first experiment we will use these two silver spheres have the same mass, but very obviously different volumes. When I drop them in air, it is hard to tell the difference in densities, but when I move it to water the difference is clear! The smaller sphere has the same amount of stuff as the larger sphere, but in a smaller space, therefore it is more dense and will sink. The larger sphere floats because it’s mass is not large enough to displace the same volume of water.

Now take two cans, one of Coke and one of Diet Coke. These clearly have the same volume, but do they have the same density? It’s pretty hard to tell in air, so let’s take it to water. Drop them both in at the same time and watch what happens! The can of Coke will sink to the bottom, while the can of Diet Coke will float. So what’s the difference? A can of coke has about 39 grams of sugar which is about 10 teaspoons full. Where as a can of Diet Coke doesn’t have any. Instead, Diet Coke has an artificial sweetener in it called aspartame, that has the same amount of sweetness in it as regular Coke, but only uses as much as a few drops of it. So regular Coke will sink because it quite literally has more stuff in it.

So, things don’t float or sink due to how much they weigh, but rather it’s due to how much matter they have packed into a certain space. Can you think of other examples of things that share a characteristic but have different densities?

Heat rises, boats float, clouds hang listlessly in the sky, and icebergs aimlessly bob around the ocean. All of these things happen because of the same scientific property: density!

Density is a rather simple property. It depends on two things, mass and volume. Mass is basically how much something weighs, but something still has the same amount of mass weather it is on Earth, Jupiter, or out in space where its weight would change dramatically. Volume doesn’t have anything to do with how loud something is, but rather with how much space it takes up. Mathematically, an object’s density is equal to its mass divided by its volume.

The demonstration here using the pipette in a bottle of water is a great way to buoy your own understanding of density. The pipette is carefully balanced to barely float in the water. However, by squeezing the bottle, water is forced up into the pipette. This makes the pipette heavier, which raises its density. Releasing the bottle down the opposite. For another cool example of density, click here!

By simple comparing the density of different things, we can tell if they would float or sink in water or air or anything else that has a known density. This is incredibly useful and can lead to some fun facts. For example, Saturn is less dense than water. That is to say it would float in a bathtub, but it might leave some rings! Red giant stars, like the famous Betelgeuse, are actually less dense than air! They are incredibly heavy–Betelgeuse weighs in at about 18 suns worth of mass–but they take up so much space that they are essentially GIANT balloons!

The sun would not float like a balloon now, in fact it is currently much more dense than water. However, given 5 billion years it will enter the red giant phase of its own life and grow in volume by about 800 million times! This will decrease its density so much that it goes from 40% higher than water to lower than air! As you can clearly see, density is not necessarily a constant property of something, it is actually something that can change.

One of the most commonly accepted consequences of density is that heat rises, but that isn’t technically true. It would be more correct to say that heat rises on Earth. This is because as things warm up, their volume increases which lowers their density. However, if we leave the friendly confines of Earth, this is no longer true! In the microgravity environment aboard the International Space Station, there isn’t really an “up” or “down”, gravitationally speaking. That makes it hard to decide what “heat rises” even means! Fire gets equally confused. Check out this comparison between fire on Earth and aboard the ISS from NASA.

# Discovering Density with Liquid Nitrogen

Why does water form lakes and oceans underneath a vast expanse of airy sky? The answer to that, and many more questions, is density! Density is most easily described mathematically by the following equation:

This means that density depends on only these two quantities: Mass, or the amount of matter that something is made up of, and volume, a measure of how much space it takes up.

Most of us have heard of density in relation to things floating in water, but it also works for air. Seeing whether or not something floats in another thing is a great way to compare the density of two substances. Here, we can see that a balloon filled with nitrogen is more dense than air. Air is mostly made up of nitrogen, so it is mostly the additional weight of the balloon itself that causes it to fall to the ground. Alternatively, the helium balloon floats upwards as it is so much less dense than the surrounding air that the comparatively heavy rubber balloon is dragged up along with it. These two objects have the same volume, but their densities differ because of their mass.

The other quantity that we can look at is volume. If the volume becomes smaller, but contains the same mass, the density will increase. Alternatively, increasing the volume will decrease the density. In this demonstration, we take advantage of a property of gases described by Charles’s Law, which states that if we change the temperature of a gas, its volume will change proportionally. By lowering the temperature of the helium balloon with liquid nitrogen, we decrease its volume. This raises its density to be above that of air…until it warms up and expands back to a low enough density to again take flight!

This concept can help to explain many things, even if they seem odd at first. Clouds are made of water, but hang in the sky! It might seem like this is just because clouds are light, but the average cumulus cloud weighs over 1 million pounds! That is definitely not light, but they are light for how big they are, and they are big. Clouds are made up of water, but this water is very spread out. The volume is so big that this 1 million pounds of cloud actually weighs less than an equal amount of air would, even though we always think of air as being pretty light!

However, as this air rises it cools and gets closer together just like the balloon dunked in liquid nitrogen. Eventually, these water molecules band together as they get cold. When enough of the molecules stick together, they get to be more dense than the cloud around it, and plummet back to earth as precipitation.

By the way, the average person is a little less than a thousand times more dense than air. This means if you were 10 times taller, wider, and thicker but not any heavier, you would float like a balloon! Please don’t try this at home.

Written By: Scott Alton

# Tabletop Rockets Science

Temperature is a measure of energy. Adding energy to a substance makes it hotter; removing energy makes it colder. Warm, energetic molecules move faster and farther, spreading out over a larger volume of space.

This balloon has been cooled to hundreds of degrees below zero (Fahrenheit), condensing the gas molecules inside. At room temperature, the condensed gas spreads out and expands, stretching the balloon back out to its original size!

We can make a gas less dense by heating it up. Less dense substances float in denser substances. This is how hot air balloons work! The warm gas inside is thinner and lighter than the air outside, so the balloon rises up through the thicker, heavier air around it. In this experiment, we’ll harness the temperature-dependence of density to turn ordinary tea bags into miniature rockets.

Step 1: Cut the staple, string, & folded paper away from the top of the tea bag. Step 2: Empty & unfold the bag to form a cylinder. Step 3: Ignite the rocket from the top.

Tea bags work well for this demonstration because they’re light, flammable, and conveniently shaped.  Emptying and unfolding the bag yields an open-ended cylinder. As the delicate paper burns, the air inside the cylinder heats up and becomes less dense. At the same time, some of the tea bag is converted to smoke, leaving a super-light skeleton of ash behind. Takeoff occurs when the structure becomes so light– and the air inside so thin– that the rocket is, overall, less dense than the air around it.

WARNING: flaming tea bags follow unpredictable flight patterns. If you try this experiment at home, be sure to choose a non-flammable setting, and keep a fire extinguisher handy.

# Stacking Liquids with the Density Column

Why do these liquids stack so cleanly? For the same reason that helium floats on air, and air floats on water: it’s all about density. Density, or mass per volume, measures how much stuff is squeezed into a given space. The higher the mass-to-volume ratio, the denser the object. A ten-gallon bucket of rocks occupies the same amount of space as a ten-gallon bucket full of air, but it contains more mass– it’s more dense. Unsurprisingly, the bucket of rocks also feels heavier. Mass and weight are closely related! Denser substances are heavier by volume, so they sink beneath less dense substances. We can layer household substances of different densities to get a crazy stack of liquids!

The corn syrup is the most dense of our layers because there’s more “stuff” squeezed into a cubic inch of corn syrup than a cubic inch of maple syrup, whole milk, or any of the other density column ingredients. Weigh a fluid ounce of each liquid and you’ll find that corn syrup is the heaviest. It makes sense that it should sit at the bottom of the stack!

Trying this at home? For best results, pour slowly, and avoid letting the liquids run down the sides of the container. Adding your liquids in order from densest to least dense will also help to keep the layers cleanly separated.

For more info on density and buoyancy, check out our Coke vs Diet Coke experiment here!

# Paperclip Science!

Today is National Paperclip Day! Yes, even those simple bits of bent wire have their very own day. Being a science camp, we decided to celebrate in the only way that made sense: Paperclip science!

Lets start with a simple question. How can you tell if something will float? The most common and simple answer is density. Density measures how heavy something for a certain volume of it. It can be a little tricky to think about, so to make it simple, consider a two liter soda bottle. If we fill it with water, it will weigh two kilograms (about 4.4 pounds). Any substance that would make the bottle heavier than that will sink, and anything lighter will float. Paperclips are made of steel wire. If we filled the bottle with steel, it would weight a little north of 15 kilograms (or just over 34 pounds!). Paperclips should not float!

These two metal spheres have the same mass, but very different volumes, resulting in very different densities. The larger one is hollow.

But we see them floating at the top of the water when carefully placed. As Eric mentioned in the video, this has to do with surface tension. We talked a fair amount about surface tension in our Teacher Appreciation Day post. For a more in depth discussion of this, check out that post here. In short, water molecules hold onto each other tightly. Its what pulls water into droplets, allows you to slightly overfill a glass of water, or pile drops of water on a penny.

Water on a penny. The last drop is just too much for the surface tension to hold.

When the paperclip is carefully placed on the water, the surface tension bends and cradles it. The paperclip is still made out of steel, so it still should sink if you think about its density. However, there is another way to think about buoyancy. It’s called Archimedes’ Principle and has to do with displacement.

If you have a full bathtub, and then you get in it, the water will spill over the rim. This is displacement. When something goes into the water, it moves this water out of the way. The water doesn’t compress. Instead, it is lifted up. When you put something in the water, the force pushing up on it is the weight of the water that it pushed out of the way.

The less dense ball, when pushed underwater, displaces a mass of water greater than its own. Since buoyancy is stronger than gravity for this object, it is launched out into the air!

This fits perfectly with the density explanation as well. If we put the bottle of steel in the water from before, it will displace 2 kilograms of water, but weigh over 15 kilograms! As it weighs much more than the water it is displacing, it will sink.

However, the surface tension changes things for the paperclip. Above is a picture from beneath the floating paper clips. This is also how water striders walk on the water. The water bends, and displaces more water than the paperclip normally would.

A paper clip weighs about half of a gram. With the surface tension bending the water, it displaces more than half of a gram of water, allowing the paperclip to float, delicately, on the surface.

Soap is a surfactant. It greatly reduces the surface tension of the water. With the surface Note that another object like a ping pong ball would still float. It is held up by the fact that it is less dense than water, and does not require the aid of surface tension.

This same phenomenon is what causes this to happen when soap is added to some milk and food coloring!

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