Question

why does a piece of calcium float when put in water ? Does magnesium react with cold water, why or why not ?

Answer 1

Why does a piece of calcium float when put in water ?

The reaction of calcium with water is not very violent. SO, the heat released is not sufficient for the hydrogen to catch fire. Calcium starts floating because the bubbles of hydrogen gas which are formed during the reaction stick to the surface of the metal and hence caclium floats on water.

Does magnesium react with cold water, why or why not ?

Very clean magnesium ribbon has a very slight reaction with cold water.

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Answer 2

Calcium starts floating because the bubbles of hydrogen gas which are formed during the reaction stick to the surface of the metal and hencecalcium floats on water.
magnesium react with hot water...because reaction with magnesium and cold water is very slow. so increase in temperature leads the reaction to occur fast