I would use these writing prompts to have students practice expository writing. I would write at the top of the sheet "Write to explain..." and let the students choose their writing prompt.
Expository Thinking Map
Expository writing worksheet that helps students generate ideas for their paper.
Adventures of a Future Teacher: Expository Writing Anchor Chart elementaryeducati...
Use this rubric to easily and consistently evaluate your students' expository writing. $1
Expository Writing Unit
Expository planning- I wish I could make charts this pretty!
Expository Writing Anchor Chart | Expository vs. Narrative Compositions
Expository Writing Freebie, includes anchor charts, draft book, teacher draft book, and brainstorming pages