Great question! A Thermos, as you probably already know, is just a special kind of container that insulates whatever liquid it is containing. That means that it is very difficult for heat to flow in or out of the Thermos. However, a Thermos is not a perfect insulator, and there is some heat that does flow in or flow out.
Before we look at the answer to your question, there are a couple of things you should know about heat flow. The first thing is that heat always flows from the hotter object to the colder object. For example, when you put an ice cube into a glass of warm water, the heat from the water melts the ice cube; the ice cube does not make the water hotter! The second thing is that the rate of heat flow between two objects is bigger when the temperature difference between them is bigger. That means that if you put an ice cube into a glass of cold water and a glass of hot water, heat will flow from the hot water to the ice cube faster than from the cold water to the ice cube.
Now letís consider what happens when you have liquid contained in a Thermos, and see if we can come up with a plausible answer to your question. When a hot liquid is contained, heat will flow from the liquid, out of the Thermos, and into the room because the liquid is at a higher temperature than the room. Water boils at 100 degrees C, so letís estimate the temperature of your generic hot liquid to be somewhere between 80 and 100 degrees C. Room temperature is approximately 25 degrees C, which means that there is a difference of 55-75 degrees between the temperature of the hot liquid and the temperature of the room. When a cold liquid is contained in the Thermos, heat will flow from the room into the Thermos. Since water freezes at 0 degrees C, letís estimate the temperature of your generic cold liquid to be somewhere between 0 and 10 degrees C. Then the difference in temperature between the cold liquid and the room is only 15-25 degrees; this is much less than the 55-75 degree difference that we got with the hot liquid!
When we taste the hot (or cold) liquid, imagine that we think that it tastes bad if the temperature has changed by 10 degrees. Since the temperature difference was much bigger that 10 degrees to begin with in the hot case, but not so different from 10 degrees in the cold case, the hot liquid will take less time to change its temperature by this amount than the cold liquid will, and the hot liquid will start to taste bad before the cold will.
All of this is just a guess, but it might be a pretty good explanation for why its harder to keep hot things hot than cold things cold, as you observe.
(published on 10/22/2007)