1) Boiling point - It is easy to observe the boiling point of a substance because it is only a physical process, raising the temperature enough to get to its boiling point.
2) Flammability - Observing the flammability of a substance has the effect of decomposing the substance because of the combustion, therefore its identity is not preserved.
3) Density - We can observe the density of a substance without changing its identity since it is just the relation between mass and volume.
Reactivity - The way to observe the reactivity of a substance is by undergoing a reaction with it, resulting in products that differ from the initial substance, so it is not possible to observe reactivity without a change in the substance.
Final answer: Boiling point and density can be observed without a change in the identity of a substance