1. The problem statement, all variables and given/known data I was helping some chemistry student and there was a homework problem on converting Fahrenheit to Celsius. The first two were 16°F and 16.0°F which were easy, but the last one was 0.0°F which had me wondering about 0 and 0.0. 2. Relevant equations C° = 5/9(°F - 32) or C° = 1.8(°F - 32), but no trouble with actually calculating the answer. 3. The attempt at a solution The problem was with the subtraction 0.0 - 32 (assuming 32°F has an infinite number of significant digits) 0.0 -32.0 -32.0 My thought was that the last 0 in 0.0 is significant even though there's no other nonzero digit to determine whether that last 0 is significant, since the first one is usually placed there by convention. Assuming the last 0 is significant, our answer was -17.8° which I'm sure is right in any case. After thinking about what was significant in 0.0, I thought about 0.01 which has only one sig fig and the last 0 is just a placeholder, which led to thinking that the last 0 in 0.0 is not significant. I looked at my edition of the student's textbook for similar problems and see one with converting 0°F to C°. The answer in the back said -18°C which is what I would get with 0°F having just one sig fig. Now, would putting a .0 on the end to make 0.0 make two sig figs? How should a person look at these numbers?