Final answer:
Physical properties that can be observed without changing the substance's identity include melting temperature, boiling temperature, conductivity, and flexibility. Reactivity towards water is a chemical property as it changes the substance's chemical makeup.
Step-by-step explanation:
From the options provided in the question, the ones that would be considered physical properties are:
A. melting temperature
B. boiling temperature
D. conductivity
E. flexibility
All these properties can be observed or measured without changing the identity of the substance. Reactivity towards water (option C) is a chemical property because it involves a change in the substance's chemical identity when it reacts with water.