What’s an example of a simple experiment that represents a chemical reaction. Why is this an example of chemical reaction?

the density(ratio between mass and volume as m/v) decreases from top to bottom in periodic table group as the size increases and nuclear charge decreases. but in periods density increases from left to right as atom size decreases and nuclear charge increases.

distance, luminosity, brightness, radius, chemical composition, and temperature

its first and third is your answer i did the qustion the fourth isnt right