True or false. a change of color is a sign that a chemical reaction is taking place.

True, it shows that a reactant (s) have changed propertirs and hense there must be a new compound to cause the colour change.

Yes, endothermic means it turns cold. and some chemical changes include change of tempature.

Both models are effective because they show the valence electrons, and these valence electrons indicate the reactivity of an element.