List four observations that indicate that a chemical reaction may be taking place

For a given reaction to take place, there might be at least one of these (7) signs;

- formation of gas bubbles, formation of a precipitate (creation of a solid from a solution), a change in color, a change in volume (due to change in density), a change in temperature (endothermic or exothermic), change in taste or smell and emission of light or fire ...