In ordinary air, sodium metal reacts to form a sodium hydroxide film, which can rapidly absorb carbon dioxide from the air, forming sodium bicarbonate. ... In a comparatively dry atmosphere, sodium burns quietly, giving off a dense white caustic smoke, which can cause choking and coughing. Sodium reacts both with air and water. It is a highly reactive metal. When kept in open, it readily combines with oxygen present in air to form its oxide. ... In order to preserve sodium metal, we generally keep it under kerosene so that neither air nor moisture may come in its contact. Sodium reacts with the oxygen in air to form an oxide called the sodium oxide, and traces of yellow sodium peroxide. Therefore, the sodium metal should be immersed in kerosene oil, if it is kept open it reacts vigorously with oxygen and the moisture that is already present in the air and thus catches fire.