Successive half-lives of a decomposing substance are measured as 120 s, then 120 s again, then 120 s once more. What does this pattern indicate about the decomposition?

Core Concept

Constant successive half-lives are the signature of first-order decay. Because t(1/2) = 0.693/k contains no concentration term, every halving takes equally long no matter how dilute the sample becomes. Zero-order half-lives shrink as the reaction proceeds, while second-order ones lengthen. The unchanging 120 s intervals therefore pin the order at exactly one.