Pressure and Fluids Demonstrations

See also: Gases, Fluids and Surface Tension

Pressure is the force applied per unit area, and in fluids it plays a key role in explaining behaviour and movement. This category examines how pressure changes with depth, how it relates to density, and how fluids respond to forces. Exploring pressure and fluids helps explain natural systems and engineering designs alike.

Demonstration	Materials	Difficulty	Safety	Summary
Air Cannon Smoke Rings	★★☆	★★☆	★★☆	A large trash can fitted with a flexible plastic membrane can be used to create giant smoke rings. Striking the membrane sends a vortex of air through a hole, which can be made visible with smoke to demonstrate air movement and Bernoulli’s principle.
Air Pressure Breaks a Ruler	★☆☆	★☆☆	★★☆	A ruler placed on the edge of a table with covered by a piece of paper can be broken by a swift strike.
Alka-Seltzer Rocket	★★☆	★☆☆	★☆☆	A plastic film canister filled with water and alka-seltzer creates carbon dioxide gas, building pressure until the lid pops off and launches the canister like a rocket.
Ammonia Fountain	★★★	★★★	★★★	Dry ammonia gas is collected in a sealed flask. When a small amount of water is injected, the ammonia rapidly dissolves, creating a partial vacuum that pulls water up into the flask. An indicator shows the resulting alkaline solution.
Balloon Bed of Nails	★☆☆	★★☆	★★☆	A single nail easily pops a balloon because all the pressure is concentrated at one point. But when many nails share the load, the pressure is spread out, allowing a balloon (and by analogy, a person) to withstand much more force before popping.
Balloon in Syringe Boyle's Law	★★☆	★☆☆	★☆☆	Using balloons inside a syringe, this experiment shows how gases expand when pressure decreases and contract when pressure increases, illustrating Boyle’s law.
Balloon Volume and Temperature	★☆☆	★☆☆	★☆☆	This experiment demonstrates how temperature affects the volume of gas inside a balloon. By placing balloons over bottles in hot water, a refrigerator, and a freezer, students observe how gases expand when heated and contract when cooled, illustrating Charles’s Law.
Bernoulli’s Principle Blowing Up Bag	★★★	★☆☆	★☆☆	A long plastic bag is inflated with a single quick breath. The fast jet of air lowers pressure at the mouth of the bag and sucks in surrounding air, so room air rushes in with your breath - an application of Bernoulli’s principle.
Boiling Water at Room Temperature	★★☆	★☆☆	★★☆	By pulling back the plunger on a water-filled syringe with the tip sealed, the pressure inside is reduced. This causes the water to boil at room temperature, demonstrating how boiling depends on pressure as well as temperature.
Boiling Water in a Vacuum Chamber	★★★	★★★	★★★	When water is placed in a vacuum chamber and the air is pumped out, the reduced external pressure lowers the boiling point. The water boils at room temperature, and because the energy for vaporization comes from the liquid itself, the remaining water cools noticeably.
Bottle Rocket	★★☆	★★☆	★★☆	A plastic soda bottle partially filled with water is pressurized with air using a pump. When released, the escaping air and water propel the bottle upward, demonstrating Newton’s Third Law of Motion and the relationship between force and acceleration.
Carbonated Drink Shake Up	★☆☆	★☆☆	★☆☆	This demonstration explores what happens when a shaken soda can is opened. Shaking introduces bubbles that cling to the can’s inner surface. Opening the can releases pressure, causing these bubbles to expand and force liquid out in a foamy burst. By snapping the sides of the can before opening, bubbles are dislodged and float to the top, reducing the mess.
Cracking an Egg Underwater	★☆☆	★★★	★★★	When a raw egg is cracked open underwater at depth, the water pressure holds the egg white and yolk together in a jelly-like sphere. It resembles a floating sea creature and demonstrates how pressure and buoyancy act on fluids without a shell.
Egg in a Bottle	★★☆	★☆☆	★★☆	A peeled hard-boiled egg is placed on the neck of a bottle. When lit matches are dropped into the bottle, the heated air expands and some escapes. As the air cools, it contracts, lowering the air pressure inside. The higher outside air pressure pushes the egg into the bottle.
Floating Ping Pong Ball	★☆☆	★☆☆	★☆☆	A ping pong ball can be suspended in the air stream of a hair dryer. This demonstrates Bernoulli’s principle, which explains how differences in air pressure keep the ball floating and stable in the moving air.
Jumping Coin with Bernoulli’s Principle	★☆☆	★★☆	★☆☆	This demonstration shows how a coin can appear to jump into a cup under its own power. By blowing in a specific way, air pressure differences created by the Bernoulli principle cause the coin to lift and move into the cup.
Lying on a Bed of Nails	★☆☆	★★☆	★★★	The bed of nails demonstrates how spreading force over a large surface area reduces pressure. A person can safely lie on many nails without injury because the force of their weight is divided among the nails.
Magdeburg Hemispheres with Suction Cups	★★★	★★☆	★★☆	Two suction cups are pressed together to create a low-pressure cavity between them. The higher outside air pressure pushes the cups together so strongly that students find it difficult to pull them apart, illustrating how pressure differences create large forces.
Make a Barometer	★★☆	★☆☆	★☆☆	A simple homemade barometer can be made using a balloon, a jar, and a straw to measure changes in air pressure, allowing students to observe weather patterns and make basic predictions.
Make a Vacuum Cleaner	★★★	★★☆	★★☆	A simple vacuum cleaner model is built using a plastic bottle, a motor, and basic craft materials. A propeller powered by a motor creates suction that pulls in small debris, showing how air pressure and airflow work in real vacuum cleaners.
Marshmallow in a Vacuum	★★★	★★☆	★★☆	Placing marshmallows inside a bell jar and changing the air pressure demonstrates how gases expand and contract, showing the effects of vacuum and differential pressure on porous materials.
Mass of Air	★★☆	★☆☆	★☆☆	Two balloons are balanced on a yardstick, and when one balloon is filled with air, it tips the balance, proving that air molecules have mass and are pulled down by gravity. A balloon can also be weighed empty and full on a precise scale.
Pascal’s Principle With Syringes	★★☆	★★☆	★★☆	Using syringes connected with tubing, students can demonstrate Pascal’s Principle: when pressure is applied to a confined fluid, the pressure increase is transmitted equally throughout the fluid. This allows a small force applied on a small piston to generate a larger force on a larger piston.
Ping Pong Funnel Blow	★☆☆	★☆☆	★☆☆	When a ping pong ball is placed inside a funnel and air is blown through it, the ball remains held in the funnel rather than being blown away. This demonstrates how differences in air pressure can keep the ball in place even when the funnel is inverted.
Ping Pong Vacuum Cannon	★★★	★★★	★★★	A sealed tube is evacuated and a ping pong ball is placed near one end. When the near seal is ruptured, atmospheric pressure accelerates the ball down the evacuated tube with negligible air resistance, allowing extreme speeds that can puncture cans and thin plywood. This demonstration highlights gas pressure, Newton’s laws, and the role of drag.
Pythagoras Cup	★☆☆	★★☆	★☆☆	The Pythagoras cup looks like a normal drinking cup, but if filled past a certain level, it uses a siphon to empty itself completely. This ancient invention shows the principles of siphoning and was originally designed to teach moderation.
Shaken Soda Can Race	★☆☆	★☆☆	★☆☆	When two identical soda cans roll down an incline, the unshaken can reaches the bottom first. Shaking one can creates foam and bubbles that change the distribution of mass inside, altering its moment of inertia and causing more energy transfer to the fluid. This reduces the shaken can’s rolling speed.
Simple Accelerometer	★☆☆	★☆☆	★☆☆	This demonstration uses a cork suspended in water inside a jar to show how acceleration affects fluids and objects. Instead of moving opposite to acceleration as many students expect, the cork moves in the same direction as acceleration because of pressure differences in the water.
Soda Can Crusher	★☆☆	★★☆	★★☆	A small amount of water in an aluminum soda can is boiled to fill the can with steam. When the hot can is quickly inverted into cold water, the steam condenses, the internal pressure drops, and outside air pressure crushes the can.
Standing on Balloons Without Popping	★☆☆	★★☆	★★☆	A student standing on a single balloon will pop it, but standing on a board supported by many partially inflated balloons spreads their weight and allows the balloons to hold without bursting.
Tornado in a Bottle	★★☆	★☆☆	★☆☆	This classic experiment demonstrates how tornadoes form by swirling water between two connected bottles. Adding food coloring, glitter, or small objects makes the tornado more visible and models debris caught in real tornado winds.
Upside Down Water Glass	★☆☆	★☆☆	★☆☆	By filling a glass completely with water, covering it with a piece of stiff paper, and inverting it, the paper stays in place and holds the water inside the glass.
Vacuum Power	★★★	★★☆	★★☆	By attaching a vacuum cleaner to a plywood board with weatherstripping, the suction creates a strong seal against a flat surface. The reduced air pressure between the board and the surface allows normal atmospheric pressure to hold the board firmly in place, demonstrating how pressure and surface area combine to create strong lifting forces.
Water Bottle Pressure Demo	★☆☆	★☆☆	★☆☆	By making holes at different heights in a water-filled bottle, students can observe that water squirts out more strongly from the lower holes, demonstrating that pressure increases with depth.

Materials
★☆☆ Easy to get from supermarket or hardware store
★★☆ Available in most school laboratories or specialist stores
★★★ Requires materials not commonly found in school laboratories

Difficulty
★☆☆ Can be easily done by most teenagers
★★☆ Available in most school laboratories or specialist stores
★★★ Requires a more experienced teacher

Safety
★☆☆ Minimal safety procedures required
★★☆ Some safety precautions required to perform safely
★★★ Only to be attempted with adequate safety procedures and trained staff