The white alkaline solid you're describing is likely calcium oxide (CaO), also known as quicklime.

Here's the reaction:

CaO (s) + H₂O (l) → Ca(OH)₂ (s)

* Calcium oxide (CaO) is a white, powdery solid.

* Water (H₂O) is a clear liquid.

* Calcium hydroxide (Ca(OH)₂) is a white, powdery solid and is also known as slaked lime. It's alkaline (basic) in nature.

This reaction is exothermic, meaning it releases heat.

Let me know if you have any other questions.