How can we know that a chemical reaction has taken place?

1. change in state CO(g)+H2(g)------>CH3OH(l) 2. change in colour Fe(s)+CuSO4(aq)------->FeSO4(aq)+Cu(s). blue coloured solution changes to a green coloured one. 3. change in temperature heat can be given out as a process. they are exothermic reactions and increase the surrounding temperature. such as Zn+H2SO4------>ZnSO4+H2. heat can be taken in as a catalyst. this is an endothermic reaction. surrounding temperature reduces. such as CaCO3------CaO+Co2 4. evolution of gas. Zn+H2SO4------>ZnSO4+H2. hygrogen is in gaseous state. 5. formation of precipitate. such as Pb(NO3)2+KI-------->PbI+KNO3. pbi is a white precipitate. this is a double displacement and precipitation reaction. any reaction which gives a precipitate is a precipitation reaction.