Q. What kind of reaction is Ca(II) HCl → CaCl2 H2

The notation "Ca(II)" in this equation appears to be indicating calcium in the +2 oxidation state, though it's unusual formatting. Assuming the intended reaction is Ca + 2HCl → CaCl2 + H2, this is a single replacement reaction (single displacement) where calcium metal displaces hydrogen from hydrochloric acid. In this reaction, elemental calcium replaces hydrogen in the acid, forming calcium chloride salt and releasing hydrogen gas as products. This is also a redox reaction, with calcium being oxidized from 0 to +2 and hydrogen being reduced from +1 to 0.

The reaction is characteristic of how active metals react with acids—following the general pattern: metal + acid → salt + hydrogen gas. Calcium is a relatively reactive alkaline earth metal that readily reacts with hydrochloric acid in a vigorous, exothermic reaction that produces bubbling as hydrogen gas is evolved. The balanced equation requires a coefficient of 2 before HCl because one calcium ion (Ca2+) needs two chloride ions to form the neutral salt CaCl2, and this also provides the two hydrogen atoms needed to form one H2 molecule. If the intended equation was actually Ca(OH)2 + 2HCl → CaCl2 + 2H2O, that would be a different reaction type—a double displacement acid-base neutralization—so clarifying the correct reactant is important for proper classification.