This is is the common name for the aqueous cation h3o+, formed when a water molecule combines with the proton from a hydrogen atom.

Answer is: hydronium ion.
Chemical reaction: H⁺(aq) + H₂O(l) → H₃O⁺(aq).
Hydronium ion is positive ion present when an Arrhenius acid, for example hydrochloric acid (HCl(aq)) is dissolved in water, hydrochloric acid in solution give up a proton (a positive hydrogen ion, H⁺) to the surrounding water molecules (H₂O): HCl(aq) + H⁺(aq) → Cl⁻(aq) + H₃O⁺(aq).

Yes, endothermic means it turns cold. and some chemical changes include change of tempature.