Boiling tap water before freezing it often results in ice that is remarkably clear, a noticeable difference from the cloudy cubes typically made with unboiled water. This simple kitchen trick reveals a fascinating process rooted in the physics and chemistry of water. The fundamental explanation lies in how heat alters the water’s composition before it is solidified. Understanding this requires a closer look at what makes ordinary ice opaque and how boiling removes those clouding elements.
The Components That Cloud Ice
The familiar white cloudiness in the center of a standard ice cube is caused by microscopic imperfections trapped inside, primarily dissolved air (oxygen and nitrogen) naturally present in tap water. As the water begins to freeze, the water molecules form a solid structure that cannot incorporate these dissolved gases.
The freezing process typically starts from the outside edges of the tray and moves inward. This directional freezing pushes the dissolved air molecules and other impurities toward the center of the cube. Once concentrated, the air forms millions of tiny, trapped bubbles that become visibly opaque.
These air pockets scatter light passing through the ice, creating the cloudy appearance. Secondary factors, such as mineral deposits like calcium and magnesium found in hard water, also contribute to this opacity. These minerals are forced out of the growing ice crystal structure and become highly concentrated in the final, cloudy section.
How Boiling Removes Dissolved Gases
Boiling water is an effective form of “degassing” because the solubility of gases in liquid decreases dramatically as the temperature rises. When water is heated to its boiling point of 212°F (100°C), the energy input forces nearly all the dissolved oxygen and nitrogen molecules to escape the liquid and bubble out into the atmosphere.
The mechanical action of vaporization aids this process, as the rising steam bubbles carry the dissolved gases with them to the surface. By the time the water cools, its gas content is far lower than when it came out of the tap. This pre-treatment step removes the main source of imperfections that later cause light scattering.
While a single boil removes a substantial amount of air, a second boil after the water has cooled slightly can increase the degassing efficiency further. This simple thermal process addresses the primary issue leading to cloudy ice, leaving the water in a purer state for crystal formation.
The Physics of Crystal Formation in Purified Water
When degassed water is frozen, the absence of trapped air and concentrated impurities allows the water molecules to align themselves into a highly uniform, dense crystalline lattice. This ordered structure is the physical characteristic of clear ice. The water molecules, H₂O, link together in a precise, hexagonal formation that is structurally sound and free of internal flaws.
In this pure, orderly structure, light can pass through the ice with minimal interference. There are no internal surfaces or air bubbles to deflect or scatter the incoming light rays, resulting in the transparency perceived by the human eye.
The crystal formation in degassed water mimics what occurs in nature, such as in lakes that freeze slowly from the top down. By removing the dissolved gases through boiling, one achieves a clarity that is impossible to reach with standard tap water.