10 Simple Experiments for Density and Buoyancy and Air Pressure

Develop an understanding of air pressure, buoyancy, and density using a series of hands-on labs.

When I’m teaching a science concepts like air pressure and density my goal is to help kids build mental models of what’s going on. Whenever possible I try to start with something they can touch and feel and experience. Here’s a simple sequence we did in my classroom. I hope you can see how students’ understanding builds.

1. Air is Stuff: Air Pressure Experiment with Water

This activity is a good place to start. When you try to pour water into the jug, it won’t go in. This is a concrete way to show that air is stuff. This always surprises and puzzles kids and encourages them to play. And when they’re intrigued, kids engage with difficult material more easily.

This is where we begin our study of buoyancy. Can you see where this will lead?

If you don’t get the idea that air is stuff, you won’t believe that it has weight. And if you don’t believe that air has weight, you won’t see how it can produce pressure. And if you don’t understand how air produces pressure, you won’t be able to see how it creates buoyancy. And if you don’t understand how buoyancy works, then it’s tough to grasp the concept of density. Sure, you can memorize the formula for density, but what does that tell you about density? BTW what IS the formula for density? And will it be on the test?

2. Matter Presses: Understanding Pressure

Once we proved to ourselves that air is stuff, we’ll play with the concepts of weight and pressure. This activity is free on my website. If you’re interested in a copy, you can sign up here.

This is a super simple activity to show kids how the weight of an object (our body) doesn’t change as you change your position (squatting, sitting, standing on tiptoe), yet its pressure does. It’s a concrete way for kids to feel the connection between the concepts of weight and pressure.

We’re just getting started on our investigation into density, buoyancy, and air pressure. These three concepts are related, and it’s helpful to study them together. In this activity, kids see how pressure comes from weight. We’ll continue that line of thought in the next couple of activities.

3. Streamlines: Water Pressure Experiments with a Water Bottle

Have you tried this experiment? It’s easy, a little messy, and super fun. Plus, kids find it intriguing, so that’s a huge point in its favor. 

How many observations can you make? Note how the lower streams are shooting farther than the upper ones. What could you conclude from that?

Click here (or on a pic) for middle school labs on this topic.

This is a visual example showing how pressure comes from weight. The greater push comes from the taller column of water. Kids can prove this to themselves by comparing bottles of different diameters and heights. It’s easy to conclude that it’s only the height of the water that changes the shape of the squirt.

This activity gives good evidence that the water sitting above the hole produces the pressure. This is a direct correlation to air pressure, which comes from the weight of Earth’s air sitting on top of you. 

The difficulty with understanding air pressure is that we ignore the surrounding air. We rarely think of air as sitting on us. It’s invisible so we forget it’s there.  Time to roll the tape from activity #1. Air is stuff. It’s always there and we need to remember this to understand air pressure.

If you climb a mountain to a place where there’s less air above you, there’s less pressure. And vice versa, the lower you go, the higher the pressure. We call sea-level standard pressure, but if you go below sea level (into a cave for instance) air pressure increases. 

[Students may know that air high in the atmosphere is thinner than that near sea level. While that’s important, it’s a separate issue and we don’t deal with it yet.]

This is part 3 of our conceptual journey—we’ve determined that air is stuff and we’ve connected weight to pressure. The definition of stuff is that it has weight and takes up space. And if air has weight, it must be able to produce pressure by sitting on stuff. 

And what keeps air sitting on Earth? The same force that keeps every other substance sitting on Earth… gravity! Just because it’s light and thin and invisible doesn’t make it immune to gravity. Gravity gives air its weight and air’s weight produces pressure. It’s that simple. The complicated part is that we haven’t trained our brains to think in those terms. We forget that air is there and we forget that air is stuff. So it’s helpful to refer to experiments that kids have completed—like trying to pour water into a sealed bottle (experiment #1). The water won’t go in because the bottle is already full… of air.

And this is our job as teachers—to help kids think like scientists.

4. Nature Abhors a Vacuum: Playing with Suction Cups

Now that we’re beginning to get an idea of where air pressure comes from, what if we could change it? What if we could change the pressure around an object? How would that affect it? In this activity, we play with suction cups. Their shape allows them to trap some air and then change their volume. 

If their volume increases but the amount of air inside stays the same, the pressure will drop. Now the inside pressure is less than the outside pressure. It’s this small difference that makes suction cups stick. The higher outside air pressure is pushing them against the surface, keeping them attached.

Click here (or on a pic) for middle school labs on this topic.

This is a good activity to delve into the idea that pressure can come from two different sources. We’ve already looked at what causes the outside, or atmospheric, pressure (air’s weight). 

And now we’re looking at the pressure which comes from the air pushing against the sides of the container. All gasses exhibit this pushiness. This is a more common understanding of air pressure and one that confuses kids when they’re learning about atmospheric pressure.

5. Nature Abhors a Vacuum: Playing in the Tub

Who hasn’t tried this? Umm, a lot of kids apparently. Part of our job as science teachers is to help kids play with materials so they can discover concepts on their own. Play builds a library of phenomena and experiences that kids can refer to when unpacking their understandings.  Here they see how they can lift a full, upside-down cup and it doesn’t empty. It remains full until the rim of the cup breaks the surface of the water. They can use a bottle of any shape or size and see the same results.

What keeps the water in the cup? 

Water seeks its level by falling to the lowest point. But for water to leave this cup, a vacuum would have to form in the space since there’s no way for air to enter. The surrounding air pressure pushes on the surface of the water and holds the water in the cup. 

What if the cup were very tall, wouldn’t the pressure from the water in the cup overwhelm the atmospheric pressure? Yup!

Normal air pressure is about 15 pounds per square inch. For a one inch column of water to weigh 15 pounds, it would need to be about 32 feet high. 

Above 32 feet a vacuum would form and the water would not stay higher than that. This is the basis for early barometers. These were made with mercury because it’s super dense and therefore short enough to fit inside a room. Making a water barometer is a cool experiment if you have the time and space for it.

Do you see the barometer here? The sealed tube of mercury is inverted into an open dish of mercury, just like the experiment we did with the cup and water. As the room’s air pressure rises and falls because of changing weather, the height of the mercury will rise and fall.

(Click the image to go to the full painting)

6. Determining Density: An Experiment for Kids

This is the classic way to find the density of an object. While you can use anything that sinks, I prefer polymer clay. It’s sold under brand names Fimo and Sculpey, but there are off-brands too. The beauty of this clay is that it doesn’t dry out, doesn’t leave a residue, and you can use it in water. 

But why clay? By using clay, you can show that density is a quality of a substance. It doesn’t change if you have more or less of the substance. Kids can calculate the density for two or three different-sized lumps to prove this to themselves.

Click the image to go to the lab directions.

7. How do Boats Float? A Buoyancy Lab

You can understand floating and sinking in two ways: 

First, you can look at the way pressure changes with the depth or height of a fluid. As we saw in Activity #3 above, the pressure in a fluid depends on how deep the fluid is. The deeper you are, the higher the pressure is. So, if you’re standing in water, the pressure at your feet is higher than near your head. This difference in pressure causes a force that pushes you upward.

Do you float? It depends. You also have a downward force (your weight) so these two forces work against each other and the larger one wins. 

Another way to look at sinking and floating is to realize that water holds up the water above it. If you could remove a chunk of water and replace it with another object of identical size, will that object float or sink? It depends. If the object weighs more than the same volume of water, then it will sink. If it weighs less, it will float. And if it weighs exactly the same, it will neither float nor sink but stay where you put it.

Click here (or on a pic) for middle school labs on this topic.

It’s this second idea that we’re exploring here. We’re determining how much water an object displaces and whether that amount of water weighs more or less than the object. The cool thing about this procedure is that you can use it with floating objects. Here the boat displaces an amount of water. If we collect and weigh this water, we see that it weighs more than the entire boat. Here we're using polymer clay which is cool because it won't float if it's a solid ball, but it does float if its shaped like a boat. You could also use a square of foil to shape an aluminum foil boat but it's a little less forgiving when trying to reshape it multiple times..

So the weight of the boat (a downward force) is less than what the water can support (the upward force) and the boat floats. If we loaded the boat with weights, it would still displace the same amount of water. When would it sink? At the point when its weight increased beyond the weight of the displaced water. 

I like this setup because it’s simple and cheap to make and is easy to store.

8. Air Is Compressible: How to Deflate a Marshmallow

This activity uses two different pumps—one that pumps air into a bottle and one that pumps air out of a bottle. Can you think what beverage you might use each for?

I love using marshmallows for this since they’re soooo visual. This always draws a WOW from kids and they want to do it over and over. When you pump air in, the marshmallows contract and when you pump the air out, they expand. The marshmallows fatigue over time, but you can use them a few times for sure.

Here we’re back to exploring the idea that air pressure is a function of how much gas is inside a confined space. If you add more molecules to the space, the pressure goes up and if you take some out, the pressure drops. This doesn’t explain surrounding (ambient) air pressure or why that rises and falls, but it’s an important part of understanding.

Click here (or on a pic) for middle school labs on this topic.

9. Out with a Bang: Heat Causes Expansion

This is another not-to-be-missed activity that your students will want to try over and over. It’s simple and quick. I let them do it themselves, though I supervised closely.

Add a centimeter or two of water to an empty can. Place it on a hot plate until the water is at or near boiling. Using tongs, remove the can and invert it into a bowl of water. BANG! The can collapses instantly.

What’s going on? As you heat the water, it turns to gas and drives out much of the air that was filling the can. Since the water vapor is hot, it doesn’t take much to fill the can. When you place the can into the water, it cools and the water vapor condenses. The pressure in the can drops dramatically (since it’s sealed and no air can get in) and the higher outside air crushes the can.

Sometimes the can doesn’t get crushed, but fills with water. Can you see why? Here, the air pressure pushes water into the can until the air pressure inside and outside are equal. It’s the same explanation but with a different outcome. And if this happens, you can reuse the can for another try!

10. Local Pressure: Heat Causes Expansion

This is the last in our lineup. Here we add some very hot water to a milk jug and swirl it around to heat the plastic. Next we dump out the water and cap the jug and wait. Before long the jug implodes. It’s not as dramatic as the previous demo but it gets the point across. I appreciate doing different setups that focus on the same concepts. It helps solidify ideas.

Plus, we’re scientists, we repeat stuff.

As much as possible, we begin with concrete experiences that kids use to construct their understanding based on what they’re seeing. A sequence like this forms the basis of our comprehension and gives us something to discuss and return to again and again.

Click here (or on a pic) for middle school labs on this topic.