Dropping an egg into water is a simple home experiment that demonstrates fundamental concepts of physics. This surprising phenomenon, where an object that typically sinks begins to float, shows how mass, volume, and density are interconnected. Observing the difference between an egg in fresh water and an egg in a saltwater solution reveals the forces that govern whether an object sinks or floats.
The Baseline: Egg in Fresh Water
When conducting this experiment, the first step involves establishing a control: placing a fresh egg into plain tap water. The expected result is that the egg quickly sinks to the bottom of the container. This observation is governed by density, which is the amount of mass contained within a specific volume.
Fresh water has a density close to 1.0 gram per cubic centimeter (g/cm³). A typical fresh chicken egg, however, has a greater density, averaging around 1.03 to 1.08 g/cm³. Since the egg possesses a greater density than the surrounding fluid, the downward pull of gravity is stronger than the upward buoyant force exerted by the water.
The Observation: Egg in Salt Water
The next step involves introducing the variable by gradually stirring table salt (sodium chloride) into the water until it dissolves completely. When the egg is placed into this saltwater solution, the observable result is a significant change: the egg no longer sinks. Depending on the concentration of salt, the egg will either float completely on the surface or remain suspended in the middle of the container. This demonstrates that the physical properties of the water have been altered by the addition of dissolved salt.
The Scientific Mechanism: Understanding Density and Buoyancy
Buoyancy and Archimedes’ Principle
The shift from sinking to floating is explained by buoyancy, the upward force a fluid exerts on an immersed object. This force is quantified by Archimedes’ Principle, which states that the buoyant force on an object equals the weight of the fluid the object displaces. An object floats when the buoyant force pushing it up is equal to or greater than the object’s weight pulling it down.
The Role of Salt
Adding salt directly increases the fluid’s density. When salt dissolves, the sodium and chloride ions fill the spaces between water molecules, adding more mass to the same volume of liquid. As the salt concentration increases, the solution’s density rises, eventually surpassing the average density of the egg (about 1.03 g/cm³). Once the water’s density exceeds the egg’s density, the buoyant force becomes stronger than gravity, causing the egg to rise and float.