Final answer:
The Zippo lighter didn't light with 2-propanol at low temperatures because it didn't produce enough vapor. Adding naphtha, despite its higher flash point, allowed the lighter to ignite, likely because naphtha ignited and heated the 2-propanol enough to allow it to combust.
Step-by-step explanation:
The observations you made concerning the lighting of a Zippo lighter with 2-propanol and naphtha seem counterintuitive given their respective flash points. The flash point of a substance is the lowest temperature at which it can vaporize to form an ignitable mixture in air. At a lower temperature, the substance may not produce enough vapor to sustain combustion, which is why the lighter with 2-propanol did not light at 3-4 degrees Celsius. It's important to note that flash points can vary and depend not just on temperature, but also on other conditions such as pressure and the presence of a spark or flame.
When you added a drop of naphtha, which has a higher flash point than 2-propanol, you introduced a substance that could ignite more readily at the same temperature. This initial burning of naphtha heated the wick and possibly the remaining 2-propanol enough to encourage vaporization and facilitate lighting, even at a temperature below its reported flash point. Thus, the observed behavior isn't necessarily about the flash point being lower, but rather about the naphtha's ability to initiate combustion under the conditions provided, which then supported the combustion of 2-propanol.
Remember also that boiling points, which are the temperatures at which a substance goes from a liquid to a gas, differ from flash points. A substance with a lower boiling point can produce vapors more readily at lower temperatures than a substance with a higher boiling point.