According to Halacha, is it permissible to open a bag of chips on Shabbat? Does it depend on if it is a small one time bag vs a family sized bag? If not, which melacha would it fall into?
It is not a problem either way.
Rabbi Ribiat's The 39 Melochos (Vol III) writes extensively about food wrappings.
As a background, the potential issues would be any of the following:
- Koraya (Tearing)
- Mechateich (Measured cutting)
- Mochaik (Erasing)
- Boneh (Building)
- Makeh B'Patish (Final hammer blow)
However, in the case of wrappings it very much depends on the type of wrapping.
Potato chips come in thin wrapping. He writes p.833 accordingly:
Paper or plastic wrapping around food or other items that are required for food or physical needs on Shabbos present minimal Halachic problems with regard to tearing and opening them on Shabbos. The wrapping material is ordinarily thin and flimsy and summarily discarded after opening. They may therefore be ripped or opened, providing that no letters or designs will be torn.
As a result, on p.835 he notes that bags of food that are made from thin/flimsy disposable material can be opened in any manner (even non-destructive). As they are only essentially protective coverings, they are not subject to Koraya or Makeh B'Patish. Moreover, opening them in the normal way (e.g. at the top for a one-time use) is also neither makeh b'patish or Boneh or any melocho due to the flimsy material from which these bags are made.
One may open a bag of potato chips, candy (even if the cellophane is stapled shut). One may also tear open a sealed plastic bag containing new stockings, plastic cutlery, paper cups, bowls etc. because all of these types of bags are too flimsy to be reusable and are summarily discarded after use...