Calcium starts floating because the bubbles of hydrogen gas formed during the reaction stick to the surface of the metal, reducing its density and causing it to float.
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When calcium reacts with water, it forms calcium hydroxide and hydrogen gas. The hydrogen gas produced adheres to the surface of the calcium, creating buoyant bubbles that make the metal float on the water’s surface. The formation of hydrogen gas during the reaction is exothermic, and the released heat contributes to the buoyancy. The hydrogen bubbles displace water and reduce the overall density of the calcium, causing it to rise and appear to float. This phenomenon is characteristic of the reactivity of certain metals with water and showcases the displacement of water by the evolved gas during the chemical reaction.