In order to observe a property of a substance, the substance must interact with another substance, such as when gasoline burns or when sodium metal reacts with water.

This is mostly true for chemical properties. Physical properties however don't require other substances so as to check them out such as changing the state of matter, Turning water into ice does not require other substances, only water, and yet you can check the physical properties with it. Chemical properties are checked with experiments and careful observation of what happens when things are mixed.