Water, a fundamental compound composed of two hydrogen atoms and one oxygen atom (H2O), does not inherently “expire” or go bad in the way perishable foods do. The molecules themselves remain stable over time. However, the perception of water “going bad” often arises from changes in its sensory qualities or from external factors that can affect its safety and suitability for consumption.
Why Water Seems to “Go Bad”
The stale taste commonly associated with water left out is primarily due to its interaction with the surrounding air. When water is exposed, it absorbs carbon dioxide from the atmosphere, which then reacts to form a small amount of carbonic acid. This slight increase in acidity can subtly lower the water’s pH and alter its taste. Temperature also plays a role, as room temperature water often tastes different from chilled water, influencing the perception of freshness.
Beyond chemical changes, airborne particles and bacteria can also contribute to changes in water’s taste or appearance. While these may subtly alter flavor or introduce a faint film, they do not necessarily make the water unsafe. These sensory shifts are generally cosmetic, indicating a change in quality rather than true spoilage.
Environmental Factors and Contamination
External environmental factors can significantly degrade water quality and safety over time. The material of the container plays a role, particularly with plastics. Many plastic bottles are made from polyethylene terephthalate (PET) or high-density polyethylene (HDPE). These plastics can leach chemicals, such as bisphenol A (BPA) and phthalates, into the water, especially when exposed to heat or direct sunlight. Their leaching rates increase sharply at temperatures above 35-45°C.
Exposure to light can promote the growth of algae, particularly in water that contains some nutrients, turning it green. Warm temperatures also encourage the proliferation of bacteria in water, especially if containers are not clean. The “danger zone” for rapid bacterial growth in water is typically between 20°C and 45°C. Open containers allow airborne contaminants like dust and microbes to enter the water.
Best Practices for Water Storage
Proper water storage is crucial for maintaining its quality and safety. For tap water, use clean, food-grade containers made from materials like PET, HDPE, glass, or stainless steel. Store these containers in a cool, dark place, ideally between 10-21°C (50-70°F), and away from direct sunlight or chemicals, as vapors can permeate plastic. It is recommended to replace stored tap water every six months to ensure optimal freshness.
Commercially bottled water often carries an expiration or “best by” date, which refers primarily to the degradation of the plastic bottle itself rather than the water. Chemicals can leach from the plastic, affecting taste and purity, though the water may remain safe. Therefore, storing bottled water in optimal conditions, similar to tap water, helps preserve its quality.
Water should generally not be consumed if it exhibits a strong, unusual odor, an odd color, or contains visible particulate matter, as these can be signs of contamination. While water itself is resilient, thoughtful storage practices are essential for ensuring its long-term quality and safety for consumption.