I'm a little confused by what exactly determines if a reaction is reversible or irreversible. I thought I have heard that if the delta G of a reaction is negative (which means it's spontaneous), that it's then irreversible, but that doesn't make a whole lot of sense when I think about it because the vast majority of reactions don't have a delta of zero and there are a lot of reversible reactions. Obviously there are reactions like combustion reactions that are essentially irreversible, but outside of these few reactions I'm not sure how to judge what factors are important. I get that changing the temperature can change whether something is endergonic or exergonic, but when asking if a reaction is reversible, I'm assuming they don't mean that you can change the temperature. Is there a litmus test that one can when looking at a reaction to determine reversibility?