I have a small question about everyday life
Thread Content
When heating milk, whether it is heated in water or with steam on a steamer, the milk at the top heats up first, while the milk at the bottom heats up later. What is the reason for this?
Milk with a higher temperature has a lower density, so it floats to the top.
The original poster should take a look at solar water heaters: the milk on the walls of the container heats up first, its density decreases so it rises to the top, while the milk with a higher density remains at the bottom due to its lower temperature.
Heating milk with water is actually a process of heat conduction and convection. During convective heat transfer, the milk near the walls loses density as it heats up and moves upward, while the milk with higher density moves downward, thus creating convection until the temperature becomes uniform.
This question seems similar to “why does the top part of steamed buns heat up first?” I feel that the heat transfer of water vapor relies on phase-change heat transfer; a large amount of heat is released during phase change, and the vapor releases heat as it cools in the air above the milk pack, which is why the upper part warms up first.
The hot ones float to the surface, while the cold ones sink
Hot substances have a lower density, while cold substances have a higher density; therefore, hot milk will float to the surface while cold milk will sink
Milk is a high-molecular-weight organic substance; therefore, its density is low when hot and high when cold
First, cold milk has a higher density and moves downward, while hot milk has a lower density and moves upward. Second, heat transfer via gas is faster compared to heat transfer across the surface of direct milk.
It’s the same principle as having radiators on the floor and air conditioners placed at a high height! Hot gases have a low density. Go up!