Freezing point depression and organic molecules Do organic molecules reduce the freezing point of water in the same way that inorganic salts do?  I've recently been swimming in lakes with a high organic content (e.g. mostly tannins.  There's a lake in this area called Tea Lake).  I've got an infra-red thermometer and readings taken from the water at this time of year before total freeze up have been around -1 to -2 degrees C.  Are these reasonable (accepting of course possible inaccuracies and calibration issues with the thermometer)?
Thanks,
Phil
To get a freezing point depression of $\pu{−1.86 °C},$ the solution should be made by adding $\pu{1 mol}$ organic substance to $\pu{1 L}$ water. And $\pu{1 mol}$ per liter is much. For glucose, it is $\pu{180 g}$ per liter. It is nearly a syrup. I doubt your lake has such a high concentration whatever the nature of the solute.
For salts, it is less, because usual salts are made of two ions. For example, $\pu{29 g}$ $\ce{NaCl}$ in $\pu{1 L}$ water is enough to get a freezing point depression of $\pu{−1.86 °C}.$ Anyway, $\pu{29 g}$ salt added to one liter is not negligible. Are you sure of your measurements?