Here's why:

* Alkalinity: Alkalinity refers to the ability of a substance to neutralize acids. In chemistry, this is often measured by pH, where a higher pH indicates a more alkaline solution.

* Alkali Metals: Francium is an alkali metal, a group of elements known for their strong tendency to lose an electron and form positive ions. This makes them highly reactive and readily form hydroxides (like NaOH, sodium hydroxide) which are very strong bases.

* Reactivity: Francium is the most reactive of all the alkali metals, meaning it readily forms hydroxides and is extremely basic.

Important Note: Francium is a highly radioactive and rare element, making it difficult to study and not directly involved in everyday chemistry.