An egg that typically sinks in tap water can effortlessly float in saltwater. This common kitchen experiment showcases a fundamental principle of physics that governs why objects either sink or float in liquids. The phenomenon is a demonstration of how the properties of water change with the addition of dissolved substances.
Understanding Density and Buoyancy
The behavior of an object in a fluid, whether it sinks or floats, is determined by two main concepts: density and buoyancy. Density refers to how much mass is packed into a given volume. For instance, a brick is denser than a sponge of the same size because it contains more material in the same volume. An object will sink in a fluid if its density is greater than the fluid’s density, and it will float if its density is less than the fluid’s density.
Buoyancy is the upward force exerted by a fluid that opposes the weight of an object immersed in it. This concept is described by Archimedes’ Principle, which states that the buoyant force on an object submerged in a fluid is equal to the weight of the fluid that the object displaces. A fresh egg typically has a density of about 1.031 grams per cubic centimeter. Since fresh water has a density of approximately 1.0 grams per milliliter, the egg is slightly denser than fresh water, causing it to sink.
How Salt Transforms Water’s Density
Adding salt to water causes a significant change in its density. When salt, such as sodium chloride, dissolves, its particles spread out among the water molecules. This process increases the total mass of the water solution without a proportional increase in its volume. Consequently, the water becomes denser.
The density of typical seawater is around 1.025 grams per milliliter. To make an egg float, enough salt must be dissolved to increase the water’s density beyond the egg’s, usually requiring it to exceed approximately 1.031 grams per milliliter. Once the saltwater becomes denser than the egg, the buoyant force exerted by the saltwater becomes greater than the egg’s weight, allowing the egg to float on the surface.
Real-World Density Applications
The principle of density and buoyancy extends far beyond floating eggs, influencing various real-world phenomena and technologies. Large ships, despite being made of dense materials like steel, float because their overall design displaces a large volume of water. This makes their average density, including the air within their hulls, less than the water they float in, allowing the buoyant force to counteract the ship’s weight.
Another example is the Dead Sea, known for its high salt concentration, which is about ten times saltier than the ocean. This extreme salinity makes its water significantly denser, allowing people to float effortlessly on its surface without sinking. Density differences are also harnessed in tools like hydrometers, which measure the density of liquids, and are also at play when oil, being less dense than water, floats on its surface during an oil spill, aiding cleanup efforts.