Density and Buoyancy Demonstrations

Density is a measure of how much mass is contained in a given volume, and buoyancy explains why objects float or sink. This category explores the principles that connect mass, volume, and displacement. Understanding density and buoyancy helps explain natural phenomena and practical applications in science and engineering.

Demonstration	Materials	Difficulty	Safety	Summary
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.
Sinking Ice Cube	★★☆	★☆☆	★★☆	Two beakers contain clear, colorless liquids - water and ethyl alcohol. An ice cube floats in the water but sinks in the alcohol because of the lower density of ethanol.
Archimedes’ Principle	★★☆	★☆☆	★☆☆	This demonstration shows that the buoyant force on an object immersed in a fluid equals the weight of the fluid displaced. By comparing aluminum and brass masses of equal weight, the difference in apparent weight loss reveals the effect of density on buoyant force.
Bubbling Lava Lamp	★☆☆	★☆☆	★☆☆	Two versions of a homemade lava lamp can be created using common household materials. In one, baking soda and vinegar react to produce bubbles that carry colored water through oil. In the other, antacid tablets release carbon dioxide bubbles that produce the same effect.
Cartesian Diver	★☆☆	★☆☆	★☆☆	A weighted pen lid or condiment packet placed in a sealed water-filled bottle floats or sinks depending on the pressure applied to the bottle.
Density Tower	★☆☆	★★☆	★☆☆	Different household liquids are carefully poured into a jar to form distinct layers, each floating above the denser liquid below. This experiment visually demonstrates how density determines whether substances sink, float, or layer.
Floating Egg	★☆☆	★☆☆	★☆☆	This experiment demonstrates how adding salt to water changes its density. A raw egg sinks in plain water but floats in saltwater because the denser solution supports the egg.
Foil Boat vs Foil Ball Buoyancy	★☆☆	★☆☆	★☆☆	A foil boat floats while a foil ball sinks because shape determines how much water is displaced. The foil boat’s open, wide form pushes aside more water, creating a buoyant force that balances its weight. The tightly packed foil ball displaces less water, so the buoyant force is too small to keep it afloat.
Hot Air Balloon	★☆☆	★☆☆	★☆☆	This project demonstrates how hot air can make a lightweight plastic bag rise, using a hair dryer. The heated air fills the bag, making it less dense than the surrounding cooler air, which causes the bag to lift into the air.
Lake Stratification (Overturn) in a Jar	★☆☆	★★☆	★☆☆	Two jars of water at different temperatures are stacked mouth to mouth. With warm water on top the layers remain separated, but flipping the pair places cold water above warm and triggers rapid mixing that models seasonal lake overturn.
Sinking and Floating Soda Cans	★☆☆	★☆☆	★☆☆	This demonstration compares the buoyancy of regular and diet soda cans. Regular soda sinks in water because dissolved sugar makes it slightly denser than water, while diet soda floats because it uses a much smaller mass of artificial sweetener. Adding salt to the water increases its density so that even the regular soda can floats.
Sinking Orange Floating Peel	★☆☆	★☆☆	★☆☆	Two oranges are placed in water—one with the peel and one without. Surprisingly, the heavier orange with its peel floats while the lighter, peeled orange sinks. The porous peel traps air, reducing the orange’s density and allowing it to float.
Snowstorm in a Jar	★★☆	★☆☆	★☆☆	Layer a water–paint mixture beneath baby oil and start an acid–base reaction with pieces of effervescent tablet. Carbon dioxide bubbles lift painty water droplets up through the oil; when bubbles burst, the droplets fall like snow.
Water Salinity and Density	★☆☆	★★☆	★☆☆	Salt water and fresh water can form distinct layers due to differences in density. By testing with an egg and layering colored water, students can observe how salinity affects whether water sinks, floats, or mixes.

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