Question

Why do alkali metals float on water?

Answer 1

Hint: Whenever a substance or an element floats on water if and only if the density of water is greater than the density of the element. While floating on the surface of the water the element will face buoyancy. Metals having a density greater than water will sink down.

Complete answer:
Mostly alkali metals when added into the water they generally produce a large amount of heat. This is because of a large amount of energy released when it reacts with water. Thus the reaction of alkali metals with water is exothermic in nature. But when we placed a liquid drop of sodium- potassium alloy on the water it seemed floating. The reason behind this is buoyancy. When alkali metals react with water they form hydroxides. Also there is a release of hydrogen gas and a surplus amount of energy. The buoyancy of alloy and the gas produced slow down the reaction and hence the reaction does not produce a large amount of heat at a time. There is also a formation of an inert atmosphere during the reaction which prevents the ignition of the hydrogen gas evolved during reaction. Thus the alloy of sodium-potassium becomes less dense than that of water and hence it floats on water.

Note:
As we go down in the alkali metals group the reaction rate of metals with water increases rapidly. This is because of an increase in hydration energy. Thus the reaction becomes more exothermic in nature. But when we take alloy, then sometimes it floats on the surface of the water. Potassium is the most reactive metal. Therefore lithium, sodium, potassium metals can float on the water surface for a very short span of time.