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.

for step 7 table c on the lab "newtons law of action"   the average time it took the car to travel .25 meters   was 1.92 seconds.

the average time it took the car to travel 0.50 meters was 2.61 seconds

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