* Physical properties are characteristics of a substance that can be observed or measured without changing the substance's chemical composition.

* Chemical properties describe how a substance reacts with other substances or how it changes its chemical composition.

Decay rate refers to the rate at which a substance breaks down or decomposes. This process doesn't change the substance's chemical composition, but rather alters its physical state or form.

Here are some examples to illustrate:

* Radioactive decay: A radioactive element decays into a different element over time. This is a physical change, as the original element's identity changes, but the decay process itself is a physical property.

* Decomposition of organic matter: Organic matter breaks down into simpler compounds due to the action of microorganisms. This is a physical change, as the original molecules are being broken down, but the decomposition process is a physical property.

In summary, decay rate is a physical property because it describes the rate of a physical change (decomposition) that doesn't alter the substance's chemical composition.