Scalding, random thought that came into my head

[al78]

For some inexplicable reason, the physics behind scalding came into my head and it raised a question, specifically regarding steam scalding. If you put your hand in steam that is just above the boiling point, you will get scalded, but if you put your hand in an oven at 200C, you will suffer no ill effects (providing you don't touch any surface or liquid), because air does not transfer heat well. Since steam is a gas, and with gases, I'd have thought its ability to directly transfer heat to a solid surface would be similarly lacking as for air if there was no condensation (which is unrealistic I know), am I correct in thinking that the primary reason steam scalds the skin is because of condensation and latent heat transfer, and the actual temperature of the steam is a minor effect (at the temperatures we deal with in the kitchen at least)?

The same question applies to cooking food by steaming. Is the primary mechanism the temperature of the steam cooking the food by raising it to near the boiling point of water or it is the steam condensing onto the food and releasing latent heat?

 

[Bletchleyite]

The answer regarding the oven is that when you open the door, the heat all escapes, and it's only the objects that stay hot. You must recall how uncomfortable it is when you open the door and all that heat goes in your face despite being dispersed and mixed with cooler air. If there was a way to put your hand in the oven with the air remaining hot, you'd still burn.

 

[Iskra]

I should imagine that some of the water droplets in the steam sticks to you too, whereas air doesn't, thus you feel it more as you are actually in contact with it.

 

[al78]

The answer regarding the oven is that when you open the door, the heat all escapes, and it's only the objects that stay hot. You must recall how uncomfortable it is when you open the door and all that heat goes in your face despite being dispersed and mixed with cooler air. If there was a way to put your hand in the oven with the air remaining hot, you'd still burn.

The only discomfort I receive when opening an oven door is if fat has spat out of the baking tray during cooking leading to smoke production, or the air coming out of the oven is very humid. That in my face (eyes specifically) is uncomfortable, but the temperature of the air in my face isn't, because by the time it gets to my face, it has entrained and mixed with cool ambient air through convection.

This addresses the oven question:

Why is my hand not burned by the air in an oven at 200 °C?

I have this problem from University Physics with Modern Physics (13th Edition): The inside of an oven is at a temperature of 200 °C (392 °F). You can put your hand in the oven without injury as ...

physics.stackexchange.com

Combination of mixing with ambient air, air having a low heat capacity, and air having poor thermal conductivity. The latter two are why you can put your hand in air at a temperature of 80C, but don't try it with water at the same temperature.

I am curious if the scalding effect of steam is largely due to the temperature, latent heat of condensation, or are they equally contributary?

When I say steam, I mean the proper definition of steam (water in gaseous form above the boiling point), not the visible plume you see coming out of a boiling kettle, which is not just steam, but suspended water droplets, equivalent to fog.

 

[Domh245]

Latent heat IIRC - my thermo lecturer in second year pointed out that this is the same reason that it can be 1° and not feel unpleasant, but as soon as it's 1° and there's moisture in the air it feels far colder.

 

[eMeS]

If my memory is correct, the latent of steam is a massive 240cal/gm. (Sorry if it's in old units...but then so am I.)

 

[al78]

Latent heat IIRC - my thermo lecturer in second year pointed out that this is the same reason that it can be 1° and not feel unpleasant, but as soon as it's 1° and there's moisture in the air it feels far colder.

I don't think that is to do with latent heat. If you are standing outside on a dry day and the sun is shining, your body is absorbing incoming solar radiation which acts to warm you. If you are standing outside on a humid day at the same temperature, there is a good chance it is cloudy (or even raining, getting wet cools you), so you don't get the warming from the incoming solar radiation. Personal comfort also depends on what you recently ate or drank, what clothes you are wearing, how physically active you are, and your gender.

 

[najaB]

Since steam is a gas, and with gases, I'd have thought its ability to directly transfer heat to a solid surface would be similarly lacking as for air if there was no condensation (which is unrealistic I know), am I correct in thinking that the primary reason steam scalds the skin is because of condensation and latent heat transfer, and the actual temperature of the steam is a minor effect (at the temperatures we deal with in the kitchen at least)?

Latent heat IIRC

Indeed. Steam at 200°C contains a lot more energy than air at 200°C - this is because it takes a lot of energy to get water molecules to turn to gas, that energy doesn't disappear it turns into the kinetic energy of the water molecules.