When sodium is added to water, it undergoes a vigorous reaction, known as the sodium-water reaction. This reaction produces sodium hydroxide and hydrogen gas. The hydrogen gas produced during the reaction is highly flammable and can ignite spontaneously in the presence of oxygen, resulting in a fire.

The reaction between sodium and water is highly exothermic, meaning that it releases a large amount of heat. This heat can also be sufficient to ignite the hydrogen gas, causing a fire.

It's important to note that this reaction is highly dangerous and should only be conducted in a controlled laboratory setting by trained professionals.