Which event must always occur for a chemical reaction to take place

Change in chemical composition of reactants is an event which must always occur for a chemical reaction to take place.

Explanation:

A chemical reaction is defined as a reaction in which there will always occur change in chemical composition of reactant species.

For example,  

Here, chemical composition of both and HCl changes which leads to the formation of silver chloride and nitric acid. Hence, it is a chemical reaction.

Therefore, we can conclude that change in chemical composition of reactants is an event which must always occur for a chemical reaction to take place.

out of these options:

ability to burn paper

pure sodium reacting violently with water

the boiling point of water

helium does not tend to react with anything

i would say this is a physical property:

helium does not tend to react with anything

why? :

i said helium does not tend to react with anything because well, it reacts to ! but ability to burn paper, pure sodium reacting violently with water, & the boiling point of water are chemical or

physical properties: the properties of a substance that can be observed without changing the chemical makeup of the substance

chemical properties: properties of a substance relating to the chemical nature and reactivity of the substance

it's c. elements are placed in rows based on how may valencies electrons have.