If I get paid salary to work 40 hours a week, can my boss demand that I work 48 hours?

Unless you have an employment contract, you are an employee at will. As an employee at will, you may be terminated at any time, for any reason--including not working more than 40 hours. Furthermore, your employer sets--and may therefore change at will--your compensation, unless you have a contract setting it. Therefore, from what you write, yes--if  you don't work the extra hours, your boss may reduce your salary.

However, one thing to verify is that you are actually an non-exempt employee--that  is, that you actually are exempt from overtime. The mere fact that you are paid on a salaried, not hourly, basis does not, by itself, mean you can't earn overtime. To be overtime exempt, your duties and job must meet certain criteria or tests. You can find these tests on the Department of Labor website: in brief, if you're an office work (e.g. no a field sales rep). to be exempt, you need to be a manager; a highly educated professional (high-level computer staff, accountant, etc.); a creative professional (certain graphic artists or designers, for example); or a fairly high-level administrator with a good amount of authority and discretion. If you don't meet one of these qualifications, then even if you are paid a salary, you still should get extra pay (overtime) for working more than 40 hours in a week. It would be worthwhile for you to go to the Deptment of Labor website, check out the tests to be exempt, and compare them to your job, to see if perhaps you  might be owed extra pay.