How are hot and cold defined?

My teacher asked me this question a few days ago. "How to define hot and cold?" And I said, "The higher the temperature, the hotter you are. The lower the temperature the colder you are." But my teacher said, "That's not 100 percent correct."


So, how do you really define hot and cold?




The concept of temperature arises from the zeroth law of thermodynamics:


If two systems are in thermodynamic equilibrium with a third system, the two original systems are in thermal equilibrium with each other. Basically, if system A is in thermal equilibrium with system C and system B is also in thermal equilibrium with system C, system A and system B are in thermal equilibrium with each other.


From here, many hypothetical scales for temperatures are stablished.


It's said than one body has higher temperature than another when there is a heat exchange from the former to the latter.


But hot and cold are just **sensations or feelings defined by people**. Something is called hot when it's at higher temperature than our body (~37 °C), so a positive heat exchange will take place on contact. The opposite for cold (heat transference from our body to the cold object).


So, you are right, the higher the temperature of an object, the hotter it will be, but the concept hot is just to define the thermal perception.


 6

Hot and cold are subjective terms that can vary from person to person. This means that what I think is warm, you might think is *cold*, meaning there is no true value for these definitions.


If you were a reference point however, we could that boiling water is *hotter* than you, and ice is *colder*. But what about something more subtle like the difference between hot water for green tea (maybe 180 $^\circ$ F ) and coffee (about 198-201 $^\circ$F)? They are both hot to you, but to the coffee, green tea would be cold. Get what I'm saying?


 4

I finally find out the real answer. It is actually the amount of heat absorb or release which determine how hot or cold you are.


I find this answer by a simple experiment. The experiment just simply putting one in cold water and the other one in hot water. Then, put both hands in warm water. You will find the the hand with the hot water will feel cold and the other will feel hot.


It is because 


1) The temperature of the cold water is lower than the warm one. So, when we put the hand in the warm from the cold water, our hand will absorb heat, causing the hand to feel hot.


2) The temperature of the hot water is higher than the warm one. So, when we put the hand in the warm from the hot water, our hand will release heat; causing the hand to feel cold


Thus, we can conclude that the more heat it absorbs, the more hotter it is. Whereas the more heat it releases, the colder it is.


 4

Hot and cold are relative terms, nothing is absolute hot or cold. When you say something is hot then you are defining it as having a higher temperature with a reference body.


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