Answer:
Your question is not specific enough so, I've given you a list of things that occurs, when a reaction has taken place.
It could be in the form of:
Heat or light given off or a colour change.
Hope this helps!
Dobereiner's triads were an early attempt to sort the elements into some logical order by their physical properties. The appearance and reactions of the elements in a triad were similar to each other. He showed that in each triad the mean of the lightest and heaviest atomic weights approximated the atomic weight of the middle element.
Answer:
The value is 
Explanation:
Generally the reaction between oxygen and magnesium is
Generally from the law of mass conservation in a chemical reaction we have that
Here 
is the mass of magnesium
is the mass of oxygen
is the mass of magnesium oxide
So
=> 
=>
Joules? i m not sure about this E Q U A S I O N
