What is a key hazard associated with unstable (reactive) materials?

A key hazard associated with unstable or reactive materials is that they may undergo violent changes under certain conditions. Reactive materials have the potential to decompose, polymerize, or otherwise react in a manner that can result in explosive or violent reactions. These changes can be triggered by various factors such as temperature fluctuations, pressure changes, or the presence of catalysts. Understanding this behavior is critical in laboratory settings to ensure proper handling and storage to prevent accidents.

The other factors listed do not adequately capture the primary hazard associated with reactive materials. For instance, while some reactive materials might be flammable or heavier than water, these characteristics alone do not encompass the broader range of violent reactions that can occur. Additionally, not all reactive materials will necessarily explode upon contact with water, as this is specific to certain substances, rather than a defining characteristic of all reactive materials. Therefore, the most comprehensive and relevant concern regarding unstable materials is indeed their propensity for violent changes under specific conditions.