When is it wrong to build conversion factors from given information?
I have a problem that says "1.00 g of water at 4°C has a volume of 1.00 mL.". Can I write 1.00 g water/4°C as a conversion factor?
No. That conversion factor implies that there is a relationship between g and
°C which does not in fact exist. Suppose you have 2.00 g of water at 4°C.
Then take away 1.00 g of water. Does the temperature change? If changing grams
doesn't change °C;, you can't write a g-to-°C; conversion factor! It's ok
to write 1.00g water/1.00 mL water, though, because mL of water does depend
on g of water.