Effect of temperature on optical rotation of sugar solution How does temperature affect the specific optical rotation of sugar solution at constant concentration and why?
If, for example, the temperature is increased, will the optical rotation for a given path length and concentration increase or decrease, and why?
 A: The temperature changes it's optical activity.  This depends on the wavelength of light used and the substance.  From my own personal experience, a 633nm laser will result in little change in optical activity.  A green (546.1nm) has a much more easily measurable change (if you are doing a lab).
We have the equation
$$[\alpha]=\frac{\alpha}{l c}$$  where $[\alpha]$ is the specific rotation (at a specific temperature and wavelength), 
$\alpha$ is the observed rotation, $l$ is the path length (through the sugar water), and $c$ is the concentration (in this case the sugar).  
I'm not exactly sure why temperature changes the specific rotation, but my best guess (from experiments that I have done) is that temperature changes the polarization of the substance.  (Should make sense if you think of how water interacts with light at different temperatures)  Our above equation is sometimes written as 
$$[\alpha]^T_\lambda = \frac{\alpha}{lc}$$
Where $T$ is temperature and $\lambda$ is the wavelength.  So the only thing that can change in here is the original polarization of the substance.
A: The temperature associated with a measurement of the specific or molar rotation of a given substance must be specified. Thermal volume changes or alterations in molecular structure (as induced by a temperature change) are capable of producing detectable changes in the observed rotations. 
