I seem to naturally skip the girl part.

I think you're looking at events in a manner that is too concrete. As usual, I’m advocating for mindset change. Rather than view 'chem prac partner' and 'lunch date' as two distinct events, look for opportunities to either meld the two into one, or simply think of one as an extension of the latter. I think you know this already, but in such a situation like the one you’ve described, you already have a valid reason for asking. When you’re given the opportunity to work with someone, there will always be an opportunity to take things outside the context, but still keep it familiar. What is appropriate depends largely on the context… if it was an ongoing assignments, you could offer to discuss details of the project over lunch; or grab a bite to eat together and continue the discussion later (if the class structure spanned lunch). This approach can also remove yourself from any potentially damaging ‘rejection’ situations, as the work excuse allows you to keep things ‘professional’ and maintain some dignity in the event things turn sour.

Jam it back in, in the dark.