If you leave an open bottle of water in the fridge overnight, it will not spoil in the sense that it won't develop harmful bacteria or pathogens that could make you sick. Water itself does not support the growth of microorganisms, so it remains safe to drink.
However, leaving an open bottle of water in the fridge overnight might affect its taste. The water may pick up odors and flavors from other foods in the refrigerator, resulting in a less appealing taste. This can happen because the fridge is a closed environment, and the air inside can circulate and transfer flavors between different items stored in it.
On the other hand, if you leave an open bottle of water out at room temperature, there is a slightly higher chance of microbial contamination compared to the fridge. Bacteria and other microorganisms present in the environment might find their way into the water, especially if it's exposed for an extended period. However, in most cases, the risk of immediate spoilage is low, especially if the water was initially clean and free from contaminants.
If the open bottle of water is capped, it will generally remain safe to drink for a longer time, whether stored in the fridge or at room temperature. The cap helps to prevent the entry of external contaminants, including dust, dirt, and airborne microbes. However, it's still a good practice to consume the water within a reasonable time frame to ensure freshness and taste.
In summary, an open bottle of water left in the fridge overnight won't spoil in a harmful way but may have an altered taste due to potential odor and flavor transfer. If it's capped, the taste is less likely to be affected, and the water will stay protected from external contaminants. Leaving an open bottle of water at room temperature also won't cause immediate spoilage, but there's a slightly higher risk of microbial contamination compared to the fridge.