What type of reaction is Ca 2H2O → Ca(OH)2 H2

The reaction Ca + 2H2O → Ca(OH)2 + H2 is classified as a single replacement reaction (single displacement reaction), where calcium metal displaces hydrogen from water molecules. In this reaction, elemental calcium replaces hydrogen in water, resulting in calcium hydroxide and hydrogen gas as products. This is also simultaneously a redox reaction: calcium is oxidized (loses electrons, going from 0 to +2 oxidation state) while hydrogen is reduced (gains electrons, going from +1 in water to 0 in H2 gas).

This reaction exemplifies the behavior of highly reactive metals with water and is notably vigorous—calcium reacts vigorously with cold water, producing hydrogen gas bubbles and significant heat that can sometimes ignite the hydrogen. The reaction is characteristic of Group 2 alkaline earth metals, though calcium is less reactive than the alkali metals (like sodium or potassium) that react even more explosively with water. The single replacement classification is confirmed by observing one element (Ca) trading places with another element (H) in a compound (H2O), leaving one element free (H2). Understanding this as both a single replacement and redox reaction helps predict that reactive metals will displace hydrogen from water, with reactivity increasing as you move down and left on the periodic table, and that the reaction will produce flammable hydrogen gas requiring proper ventilation and ignition source control.