Hot tap water often exhibits a milky, opaque, or white appearance when first poured from a faucet. This cloudiness is a common and entirely natural physical phenomenon, not an indication of contamination or a problem with the water supply. The temporary change in clarity is a direct result of how water handles dissolved gases under changing conditions.
The Physics of Dissolved Gas
Water in a natural state or in a municipal system is never purely H2O; it contains various dissolved gases, primarily nitrogen, oxygen, and a small amount of carbon dioxide. These gases enter the water through continuous contact with the atmosphere and are also introduced during the water treatment and pressurization process.
The maximum amount of gas that water can hold in solution is referred to as its saturation limit, a value that changes with pressure and temperature. Under the high pressure of the water mains and at the typical cold temperatures found in reservoirs or underground pipes, water can hold a relatively high volume of dissolved air. This capacity for holding gas is why cold water drawn directly from the tap appears crystal clear.
How Heat Triggers the Cloudiness
The core reason for the hot water’s milky look is the inverse relationship between water temperature and gas solubility, a principle described by Henry’s Law. This law dictates that as the temperature of a liquid increases, the solubility of a gas within that liquid generally decreases. Heating water in a hot water heater or supply pipe dramatically lowers the water’s ability to keep its dissolved gases in solution.
When cold water is rapidly heated, the gas molecules within the liquid gain kinetic energy and become more likely to escape the water phase. Since the water is held within a pressurized plumbing system, the newly liberated gas cannot immediately escape and is forced out of solution, forming millions of microscopic air bubbles. These tiny bubbles scatter light as they travel through the water, creating the characteristic opaque, white, or milky appearance.
The cloudiness clears quickly once the water is poured into an open container because the pressure drops back to atmospheric levels, allowing the trapped bubbles to escape. The bubbles, being lighter than water, rise rapidly to the surface and pop. This process causes the water to clear from the bottom of the glass upward, as the air escapes from the lower layers first.
Distinguishing Harmless Cloudiness from Water Quality Issues
The simple appearance of white or cloudy water should not be a cause for concern, provided it is caused by air bubbles. To confirm the source of the cloudiness, a quick test can be performed by filling a clear container with the water and observing it for a few moments. If the water begins to clear within thirty seconds to a minute, starting at the bottom and moving toward the surface, air bubbles are the confirmed cause.
Cloudiness caused by genuine water quality issues, such as sediment or mineral deposits, behaves differently. Sediment, like sand or rust particles, is heavier than water and will typically settle to the bottom of the glass rather than dissipate or rise. If the water remains persistently cloudy, has an unusual odor, or contains visible particles that do not clear, it may be an indication of sediment accumulation or other contaminants that require further investigation.