#1 He doesn't drop only a chunk of metal. It's in a small container that is made of water soluble material, such as polysacharide. If it wasn't in this container, it would have lit on fire in the air or explode immediately on contact with the water. For the sake of an increased visual effect and the safety of handling the metal it is put in said container.
+4 1. stonelocomotief commented 12 years ago on video How alkali metals react with water and air.