Final answer:
In a chemical reaction, a change in properties always occurs as the reaction proceeds. This can include changes in color, temperature, mass, or volume. However, not all of these changes will occur in every reaction.
Step-by-step explanation:
In a chemical reaction, the event that always occurs as the reaction proceeds is a change in properties. This can include a change in color, temperature, mass, or volume, depending on the specific reaction. However, not all of these changes will occur in every chemical reaction.
For example, a change in color occurs when there is a formation of a new compound that has a different color than the reactants. A change in temperature can happen if the reaction is exothermic (releases heat) or endothermic (absorbs heat). A change in mass or volume may occur if there is a formation of a gas or a solid precipitate.
Therefore, while all of these changes are possible, the specific change that occurs in a chemical reaction depends on the reactants and conditions of the reaction.