Question

Why does Calcium float in water?

Answer 1

Hint: Start the question by reacting 1 mole of calcium and 1 mole of water. One of the products is a gas which is responsible for this phenomenon.

Complete answer:
As we know, Calcium (atomic number – 20) is an Alkali Earth metal or Group-2 element. Group 2A metals have lower solubility in water than Group 1A. Reaction with alkali metals or group 1A metals is very vigorous and dangerous. Therefore, the reaction of calcium in water goes faster in warm water. Reaction of alkali metals with metals is used to measure the reactivity of these elements.
When Calcium reacts with water it gives calcium hydroxide\[[Ca{{(OH)}_{2}}]\]and hydrogen \[\left[ {{H}_{2}} \right]\]. The reaction is given below –
\[Ca\text{ }\left( s \right)\text{ }+\text{ }2{{H}_{2}}O\text{ }\left( l \right)\to Ca{{\left( OH \right)}_{2}}\left( ppt \right)\text{ }+\text{ }{{H}_{2}}\left( g \right)~~~~~~~~\]
This is an exothermic reaction, i.e. heat is released.

As the reaction proceeds, bubbles of \[{{H}_{2}}\] (g) and a white precipitate of \[Ca{{\left( OH \right)}_{2}}\]are formed. The bubbles formed stick to the surface of calcium metal and make it lighter. Therefore, Calcium floats on water.

Note: Since the reaction of calcium and water is less violent, the heat released is less. Hence, hydrogen does not catch fire. Group 2 metals give a more vigorous reaction with water as we go down the group. Beryllium is an exception. Beryllium has a strong resistant layer of oxide on its surface which lowers its reactivity at ordinary temperatures, hence, making it unreactive with water (it reacts with steam at temperatures above \[700{}^\circ C\]).