Gases and Their Effects on Objects

The Phenomenon: A Puffy Bag of Chips

This effect is not limited to chip bags. People who live at high elevations sometimes notice that sealed plastic bottles brought from lower areas look crushed or dented. Scuba divers are taught never to hold their breath while rising to the surface because the air in their lungs expands dangerously. All of these events involve gases interacting with objects — and scientists use particle models to explain exactly why they happen.

• What do you think caused the chip bag to expand even though nobody opened it or blew air into it?
• If the gas particles inside the bag didn't change in number, what did change about them?
• How could you build a model to show what's happening to the gas inside the bag at different altitudes?

What Scientists Know: Gas Particles and Pressure

To explain the puffy chip bag, we need to understand some key ideas about gases and the tiny particles that make them up. Scientists use a particle model of matter — the idea that all matter (solids, liquids, and gases) is made of extremely small particles that are too tiny to see. In a gas, these particles are spread far apart and move around freely in all directions at high speeds.

Let's Investigate: Modeling Gas Behavior

Developing and Using Models (Science Practice)

Scientists develop and use models to describe phenomena that cannot be directly observed. Since we can't see individual gas particles, we build models — diagrams, physical demonstrations, and computer simulations — that represent particle behavior and help us explain effects we can see.

Investigation question: How does changing outside pressure affect a sealed, flexible container filled with gas?

Materials for a classroom version:

• A large syringe (without a needle) with the opening sealed
• A marshmallow placed inside a sealed syringe or vacuum chamber
• A balloon placed inside a bell jar connected to a vacuum pump
• Drawing materials for particle models (before and after)

Procedure: Seal a marshmallow inside a syringe and pull the plunger back (reducing pressure inside). Observe: the marshmallow expands. Push the plunger in (increasing pressure), and it shrinks back. Students draw particle models showing before and after — labeling particle spacing, direction of particle motion, and the forces acting on the marshmallow's surface.

What you would observe: When outside pressure decreases, the gas trapped inside the marshmallow's tiny air pockets pushes outward more than the atmosphere pushes inward. The marshmallow puffs up. When pressure increases again, the outside air squeezes the gas pockets smaller.

What We Discovered: Explaining the Effect

Now that we have investigated how changes in outside pressure affect a sealed container, let's connect the evidence back to our anchoring phenomenon — the puffy chip bag. The key insight is that gas particles exert pressure by colliding with surfaces, and the observable effect on an object depends on the balance between internal and external pressure.

When a chip bag is sealed at a factory near sea level, the air pressure inside the bag equals the air pressure outside. The gas particles inside push outward on the bag's walls, while the atmosphere pushes inward with an equal force. These forces are balanced, so the bag holds its normal shape. But when you carry that same sealed bag to a mountaintop — where the atmosphere is thinner and exerts less pressure — the inside pressure hasn't changed, while the outside pressure has dropped. The inside gas particles now push outward more than the atmosphere pushes inward, so the bag swells.

This same principle explains many other observable effects. When you squeeze an inflated balloon, you increase the external force on the gas inside, compressing the particles into a smaller space. When you heat a sealed container, the gas particles move faster and collide more forcefully with the walls, increasing internal pressure. In each case, we can use a particle model to show the spacing, motion, and collisions of gas particles — and then use that model to explain and predict what we observe.

Investigation Data: Marshmallow in a Syringe

The data table above shows a clear pattern: when outside pressure decreases, the gas inside an object pushes outward and the object expands. When outside pressure increases, the gas is compressed and the object shrinks. This is a cause-and-effect relationship that the particle model explains beautifully — changes in the balance of particle collisions on either side of a surface cause observable changes in the object's shape or size.

Patterns and Connections: Cause and Effect

The crosscutting concept at work in this lesson is Cause and Effect. Scientists look for cause-and-effect relationships because they help us explain why things happen — not just describe what happens. In our investigation, the cause was a change in the balance between internal gas pressure and external atmospheric pressure, and the effect was an observable change in the object's shape or size.

This same cause-and-effect pattern — where changes in gas pressure produce observable effects on objects — appears across many areas of science. Let's look at how it connects to different situations.

Notice the pattern: in every case, the observable effect on the object is caused by an imbalance between the gas pressure inside and the pressure outside. Scientists design tests to identify what causes the pressure change — whether it's altitude, temperature, or the amount of gas — and then use that information to predict the effect. This ability to identify cause-and-effect relationships is one of the most powerful tools in science.

Real-World Connections and Engineering

Understanding how gases affect objects isn't just a science concept — it's knowledge that engineers and designers use every day to solve real problems and keep people safe.

Airplane Cabin Pressurization

At cruising altitude (about 35,000 feet), the air outside an airplane is so thin that humans couldn't breathe, and sealed containers would expand dangerously. Engineers solve this problem by pressurizing the cabin — pumping air into the airplane to maintain a pressure similar to what you'd experience at about 6,000–8,000 feet. This keeps passengers comfortable and prevents dangerous pressure imbalances. The airplane's fuselage must be engineered strong enough to hold this higher pressure inside while flying through the low-pressure atmosphere outside.

Scuba Diving Safety

Deep underwater, the weight of the water above creates enormous pressure. A scuba diver's lungs contain gas that is compressed by this pressure. If the diver rises to the surface too quickly, the gas in their lungs rapidly expands as the water pressure decreases — which can cause serious injury. Divers are trained to ascend slowly and breathe normally so that expanding gas can escape safely. This safety rule comes directly from understanding how gas pressure affects the body.

Packaging Design Challenge

Engineers who design food packaging must consider pressure effects. A chip bag filled at a factory near sea level and then shipped to a store in Denver, Colorado (altitude 5,280 feet) will expand. Packaging engineers solve this by leaving extra room inside the bag, using flexible materials that can stretch without bursting, or filling bags with nitrogen gas at a controlled pressure. This is a real engineering design problem where understanding gas behavior leads to better solutions.

Key Vocabulary Review

• Gas — A state of matter in which particles are spread far apart and move freely in all directions, filling any container they are in.
• Particle model — A scientific model that represents matter as being made of tiny particles too small to see, showing their spacing, motion, and interactions.
• Gas pressure — The force that gas particles exert when they collide with surfaces, such as the walls of a container. More collisions or harder collisions mean higher pressure.
• Air pressure (atmospheric pressure) — The pressure exerted by the weight of the atmosphere on everything at Earth's surface. It decreases at higher altitudes.
• Balanced pressure — When the gas pressure pushing outward from inside an object equals the atmospheric pressure pushing inward, resulting in no change in shape.
• Unbalanced pressure — When the pressure on one side of a surface is greater than on the other, causing the object to expand, compress, or change shape.
• Model — A representation (drawing, diagram, physical object, or computer simulation) that scientists use to describe, explain, and predict phenomena.
• Cause and effect — A relationship in which one event (the cause) makes another event (the effect) happen. Scientists test cause-and-effect relationships by changing one variable and observing the result.

Practice: Test Your Understanding

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