When alkali metals react with water, they produce a metal hydroxide and hydrogen gas.

Alkali metals, located in Group 1 of the periodic table, are known for their high reactivity, which stems from having a single electron in their outermost shell. This characteristic makes them eager to engage in chemical reactions, particularly with water. Upon contact with water, a displacement reaction occurs. This reaction is a type of redox (reduction-oxidation) process, where the alkali metal displaces the hydrogen in the water molecule, resulting in the formation of a hydroxide of the metal and the release of hydrogen gas.

The general equation for this reaction can be represented as:

In this equation, $M$ denotes the alkali metal. The hydroxide produced is a strong base, which contributes to making the solution alkaline, hence the name ‘alkali’ metals. The reaction is also exothermic, meaning it releases heat; in fact, it is so exothermic that the generated hydrogen gas often ignites, leading to a fiery explosion.

The reactivity of alkali metals increases as one moves down the group in the periodic table. For instance, lithium, which is positioned at the top of the group, reacts with water relatively slowly, whereas potassium, as well as rubidium and cesium found further down the group, react much more vigorously. This trend can be attributed to the increasing atomic size and decreasing ionization energy as one moves down the group, which makes it easier for the outermost electron to be lost, facilitating the oxidation of the metal.

In laboratory settings, these reactions must be conducted with caution. Alkali metals are typically stored under oil to prevent their reaction with moisture in the air. When demonstrating the reaction with water, it is crucial to take safety precautions, such as using small amounts of the metal and maintaining a safe distance. The reaction produces a caustic solution and flammable hydrogen gas, both of which pose hazards.

In summary, the reaction between alkali metals and water serves as a captivating demonstration of the reactivity of these elements. It vividly illustrates the principles of redox reactions and the trends observed in the periodic table.