When chemical reactions occur, the atoms are rearranged and the reaction is accompanied by an energy change as new products are generated ... So much energy is released that the hydrogen gas released spontaneously burns in the air.

I hope you were here for this answer only.

Hii aspirant ,

Coming to your question I would like to compete your question as it seems to me that is your only question so,

A chemical reaction is always accompanied by a definite changes in its appearance .

For eg -there may be achange of the colour or the heat and light may be generated , or many more which changes its appearance .

Hope this helps you ,

Good luck .