Which observation occurs when sodium reacts with water?

When sodium reacts with water, it undergoes a vigorous exothermic reaction, resulting in the formation of sodium hydroxide and hydrogen gas. The heat generated from the reaction causes the sodium metal to melt, transforming it into a liquid state. As this occurs, the sodium rapidly reacts with water rather than simply sitting at the bottom of the container. This results in the sodium appearing to "disappear" as it melts and dissolves (reacts) into the water, producing bubbles of hydrogen gas.

The other observations provided do not accurately reflect the behavior of sodium in this reaction. While sodium does produce hydrogen gas, which may create bubbling, it does not produce a foul smell or black smoke. Moreover, sodium does not sink and remain in solid form; it actively engages with water, leading to the melting and reaction instead. Thus, the observation of sodium melting and disappearing during the reaction is the correct representation of what occurs when sodium comes into contact with water.