Unearned Revenue has a normal balance of a?

Credit. Unearned Revenue is a Liability and like all Liabilities it has a Credit Balance.

I decided to add this as I have been asked "why" is Unearned Revenue a liability isn't it Revenue?

Yes and no. The key Word here is "Unearned". Because of the fact that it is unearned, the company (although has received money) is liable for that in some form. For example, if a person pays a business $5,000 in advance for painting their house, the company now is liable for that amount, meaning they have to do one of two things.

1. Complete the job and "earn" the money

or

2. Refund the money and not do the job

Until this is done, the money received in advance for the job is listed as Unearned Revenue and categorized as a liability.