An element A is soft and can be cut with a knife. This is very reactive to air and cannot be kept in open air. It reacts vigorously with water. Identify the element from the following

Na (sodium) possesses all the above properties. It is too soft and can be cut with a knife. Na is a highly exothermic reaction producing a lot of heat. It also reacts with water and burns due to the vigorous formation of hydrogen gas. That is why sodium is stored in kerosene oil to prevent any reaction.