Step-by-step explanation:
A property which causes change in chemical composition of a substance is known as a chemical property. For example, reactivity, toxicity, combustion etc.
Whereas a property which causes no change in chemical composition of a substance is known as a physical property. For example, mass, volume, density etc.
Sodium is a very reactive metal and it reacts readily with oxygen, water etc. So, when sodium is added to water then occurrence of an orange flame represents its reactivity.
Thus, based on given observation it can be concluded that chemical reaction takes place when sodium is added to water.