Answer
The density doubles.
Work Step by Step
Density is mass/volume, so if an amount of air is compressed to half its volume, the density will be twice as much as it was before.
This is discussed on page 270.
You can help us out by revising, improving and updating this answer.
Update this answerAfter you claim an answer you’ll have 24 hours to send in a draft. An editor will review the submission and either publish your submission or provide feedback.