Answer:
Magnesium and calcium belong to the second group i. e. alkaline earth metals. They are known as earth metals because they are extracted from the earth. They are very reactive elements. Their reactivity increases when we go from top to bottom because the outermost electrons goes farther from the nucleus i. e. atomic radius increases so less energy is needed for its removal.
Answer 1)

+

----> 2 HCl
This is a pure
chemical reaction that is taking place.
In this reaction the two chemical species which are Chlorine and Hydrogen undergo addition reaction and form two moles of hydrochloric acid. No new species is formed except the combination of reactants which gives the product.
Answer 2) H + H ----> He + n
This is a nuclear reaction.
As the reactant species is undergoing a reaction which is resulting in formation of new species completely different than the elements in reactants.
Also there is a neutron which is generated after the reaction this is clear indication of nuclear fusion reaction. Hence, it can be called as nuclear reaction.