# 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?

• This question (and its answer) might be helpful as well: chemistry.stackexchange.com/questions/10227 – Michiel Nov 20 '14 at 6:56
• Um, I guess you are asking the wrong people about the wrong question. The right question would be how your teacher defines that, and the right person to ask is your teacher, he/she gets payed for that. Nevertheless, you are right to ask for a second opinion here. ;-) – Gyro Gearloose Dec 27 '15 at 17:18
• The kinetic energy of the substance's molecules also determines how hot or cold the substance is. – Romulo Ramirez Dec 11 '16 at 17:12
• For life, boiling is clearly too hot and freezing is too cold. Actually, you get protein denaturing much above $37^oC$ and metabolic processes slow down much below that. Is it a chemistry teacher, a physics teacher or a biology teacher, or a culinary arts teacher? – Joseph Hirsch Dec 11 '16 at 18:21

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.

• As an interesting little addition: the fact that your body temperature is the reference for hot/cold also means that if the outside air is above 37$^o$C, blowing air over your skin with a fan actually makes you feel MORE warm instead of less (as people are accustomed to, because the outside temperature is almost always cold with respect to our body temperature) – Michiel Nov 20 '14 at 6:54
• @Michiel it's more complicated than that. If the outside temperature is above 37, you are typically all sweaty, and blowing air over your skin makes the sweat evaporate more quickly, resulting in a cool feeling. The effect you mentioned does occur, but only when the air temperature is much higher than 37 Celsius and the evaporation effect cannot compensate for the heating by the new air. It is well observable in dry saunas, for example. – rumtscho Nov 20 '14 at 9:09
• @rumtscho Good point, I indeed ignored the evaporative cooling effects of sweat in my comment. – Michiel Nov 21 '14 at 7:07

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?

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.

• You don't recognize the difference of "heat (energy) absorbed" and "temperature change". Think you got a piece of hot potato in your mouth. Case one: you rest it on the tip of your tongue. The heat exchange is minimal, but the temperature change will burn your tongue. Case two: you flush the hot bit with lots of saliva. The heat exchange will be much greater than in case one, but the temperature of the potato will decrease rapidly and the temperature change inflicted on your tongue will be much less. – Gyro Gearloose Dec 27 '15 at 18:05

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.