Why does black tea go cold faster than slightly warmer tea with milk in?

Answer 1

Good question. I don't know if it is true, but if it is, I can imagine several possibilities:

1. If the black tea starts out cooler, why wouldn't it cool to room temperature faster?

2. One way warm liquids cool is evaporation. It could be that the black tea evaporates faster. (There is a classic trick in which an insulated pail of hot water cools faster than an identical pail of cooler water because of this. In that case, there is so much evaporation that by the time the hot water cools to the temperature of the cooler water, there is less water left to cool)

3. This leads to the possibility that there is just less fluid in the container of black tea.

4. Another way that liquids transfer heat is by convection. The less viscous the fluid, the faster the fluid moves for a given temperature differential. If the added milk makes the tea more viscous, it would cool more slowly.

5. Yet another method of heat transfer is radiation. Black tea is transparent while tea with milk is opaque. The transparency of the black tea might let the radiation from the warmer middle of the container escape from the black tea container while the radiation from the middle of the milk tea container gets trapped. It is also the case that, in general, light colored surfaces don't radiate as well as dark

Which, if any, of these is a significant effect in your particular circumstances is not clear.

Certainly one of the factors is the container. There is a world of difference between a tall container and a short (height affects convection rate), an insulating container (where evaporation becomes critical) and a conducting container with large surface area (where conduction through the container wall becomes dominant), etc.