How are physical and chemical changes different

Explanation:

Physical changes do not change the properties of the initial substance. Physical changes are temporary. For eg if you boil water, it turns into vapor. It's chemical composition is still H2O but now its just in gas form instead of liquid but overtime that gas will condensate into water. SO physical changes are reversible most of the times.

Chemical changes change the properties of initial substance. Like burning wood, you can't really reverse this reaction because you can't go from coal to wood, its permanent change. So chemical changes are permanent changes. Like souring milk, acid-base reactions, rotting banana etc.

nuclear fission produces elements that are heavier than helium.

the elements that are used in nuclear fission and their products are much heavier than helium.