Question

Calcium $ + $ Water $ \to $ Calcium hydroxide $ + $ hydrogen
This is a:
(a) Decomposition reaction
(b) Combination reaction
(c) Displacement reaction
(d) Precipitation reaction.

Answer 1

Hint: Calcium hydroxide, also called slaked lime, is obtained by the action of water on calcium. When mixed with water, a small portion of it dissolves, forming a solution known as limewater. The rest remains as a suspension called milk of lime.

Complete answer:
 $ Ca(s) + 2{H_2}O(l) \to Ca{(OH)_2}(aq) + {H_2}(g) $
 $ 1 $ mole of calcium reacts with $ 2 $ moles of water to produce $ 1 $ mole of calcium hydroxide and $ 2 $ moles of hydrogen gas.
The above reaction explains the reaction of calcium with water, when calcium is dropped in water, after a second or so, the calcium metal begins to bubble vigorously as it reacts with the water, producing hydrogen gas, and a cloudy white precipitate of calcium hydroxide.
The reaction is a displacement reaction because according to the reactivity series, calcium comes above the hydrogen. That means calcium is more reactive than hydrogen, so The calcium displaces the hydrogen atom from water forming calcium hydroxide and hydrogen gas, less heat is produced in this reaction due to which hydrogen gas formed does not catch fire.
Option: $ (c) $ Displacement reaction.

Note:
The presence of hydrogen gas can be observed, because during the reaction bubbles formed which indicates the evolution of hydrogen gas. And the presence of calcium hydroxide is demonstrated by the addition of a few drops of phenolphthalein indicator, which turns the solution pink, indicating that the solution is basic.