Which of the following is an example of a metal hydroxide formed when alkali metals react with water?

When alkali metals, such as sodium (Na) or potassium (K), react with water, they produce a metal hydroxide and hydrogen gas. The reaction typically involves the alkali metal displacing hydrogen from water, resulting in the formation of hydroxides.

In this case, sodium reacts with water to form sodium hydroxide (NaOH) and hydrogen gas:

[ \text{2 Na (s) + 2 H}_2\text{O (l) → 2 NaOH (aq) + H}_2\text{(g)} ]

Sodium hydroxide is a strong base and is a prime example of a metal hydroxide that is produced from the reaction of an alkali metal with water.

The other options represent different compounds that do not fit the criteria of being a metal hydroxide formed by alkali metals reacting with water. Potassium chloride (KCl) is a salt, magnesium hydroxide (Mg(OH)2) is formed from magnesium, which is an alkaline earth metal, and calcium chloride (CaCl2) is also a salt and derived from calcium, another alkaline earth metal. Thus, the formation of NaOH directly correlates with the reaction of an alkali metal with