Under what conditions is water considered unstable?

Water is considered unstable outside the pH range of 4 to 9 because this range is close to neutral pH, where the properties of water remain most consistent and predictable. In more acidic or basic conditions, water can undergo significant changes in behavior and chemical reactivity.

At extreme pH levels (either highly acidic or highly basic), the stability of water is compromised due to the increased reactivity with other species. For example, at low pH, the concentration of hydronium ions increases, while at high pH, hydroxide ions predominate. This can lead to increased rates of hydrolysis or other side reactions that can destabilize the typical behavior of water as a solvent.

While other conditions, such as temperature or the presence of oils, can affect the physical state or solubility of water, they do not alter the intrinsic stability of water itself in the same fundamental way that extreme pH would. Thus, focusing on pH is critical for understanding the chemical stability of water in various environments.