The question of whether gold flakes float presents a scientific puzzle, as the metal is known for its extreme heaviness, yet tiny specks are often observed resting on the surface of water. A “gold flake” refers to a very small, thin particle of gold, not a substantial piece of metal. This apparent contradiction is resolved by examining the physics of density and the powerful effect of water’s surface tension, which governs behavior at a microscopic scale.
The Science of Density and Sinking
The expectation that gold should immediately sink is based on its high density. Pure gold has a density of 19.32 g/cm³, making it over 19 times heavier than the same volume of water (1 g/cm³). This significant difference ensures that any large, three-dimensional piece of gold, such as a nugget, will overcome the water’s buoyant force and sink quickly. The principle of flotation states an object floats only if it is less dense than the fluid it displaces, which solid gold violates. This density-driven sinking is why gold panning works, allowing prospectors to separate the heavy metal from lighter sand and gravel.
The Role of Surface Tension in Floating
The phenomenon of gold flakes floating is due to the influence of surface tension on extremely small particles, not a change in gold’s density. Water molecules cohere strongly, creating a dynamic film or “skin” on the liquid’s surface. For a very small and flat particle, often called “flour gold,” the downward force of gravity is less than the upward force exerted by this surface film. The flake rests upon the surface without breaking through the molecular barrier.
The ability of a gold flake to float is enhanced by hydrophobicity, meaning gold naturally repels water. This prevents the water from easily spreading over the gold’s surface, keeping the flake from being completely wetted and pulled beneath the surface layer. The flat shape of the particle maximizes its contact area, distributing its minimal weight over a larger section of the surface film. This state is temporary; any disturbance, such as vibration or a contaminant, can easily disrupt the surface tension and cause the flake to sink instantly.
Practical Implications of Particle Size
The size and shape of the gold particle are the determining factors in whether it will float or sink. Gold found in nature ranges from large nuggets to fine dust. The floating behavior is almost exclusively relevant to the smallest particles, known as “flour gold,” which can be microscopic in size. This phenomenon is a major challenge for gold prospectors, as this fine gold, or “fines,” may float away and be lost during the panning or sluicing process.
To overcome this surface tension effect and successfully recover the fine gold, prospectors often add a small amount of a surfactant, such as dish soap, to the water. These agents reduce the cohesive forces between water molecules, effectively weakening the surface tension film, which allows the gold to wet fully and sink. The same physical principle is at play with decorative edible gold flakes used in beverages, which are intentionally hammered to an extreme thinness. Their light weight and large surface area allow them to remain temporarily suspended or appear to float in a liquid until the surface tension is disturbed.