Why does steam at 100 degrees burn more than water at 100 degrees?

In order to continue enjoying our site, we ask that you confirm your identity as a human. Thank you very much for your cooperation.

• Answered by:
  
  Narayan R. from Kolkata
• 
  Like

• Answered by: R. K. from Chennai
• 
  Like

• Answered by: Mohit K. from Varanasi
• 
  Like

• Answered by: Sindhuja from Bangalore
• 
  Like

Get 5 credit points for each correct answer. The best one gets 25 in all.

What we define as "hot" or "cold" is the transfer of energy -- how much (quantity) and how fast (rate of transfer) -- and how it raises our temperature. The more energy that is transferred from the object quickly, the hotter the object feels.

First, steam is in a vaporized phase -- which is why it has more energy. At 100 Celsius, water can exist both in gaseous and liquid phases. However, to vaporize liquid water, an energy input is required. This energy (called vaporization energy) is specific to each material, but if added, won't raise the temperature, but will simply vaporize the liquid into a gas. So, by vaporizing 100C water, you have water vapor at 100 degrees. Similarly, you can condense this vapor, by removing that same amount of energy required to vaporize it. In that case, you'd recover water at 100 degrees.

When you touch something hot, it will transfer heat to you until the temperatures have equalized. So when you touch hot water, the water will simply transfer whatever energy it needs to reach your hand's surface temperature (which won't happen, you'll take your hand out much sooner). However, when you touch steam, it will also transfer the condensation energy to you -- which is actually a lot of energy. This energy drastically raises your hand's the temperature, and you feel it as "hot."

Consider the simple heat transfer equation: the heat transfer rate $H$ is $$H=kA \dfrac{T_\text{hot}-T_\text{cold}}L$$

$L$ is unimportant for our case. What is important is $k$, the thermal conductivity constant -- this constant depends on the material. The higher this constant, the faster heat gets transferred, so more heat gets transferred, and your hand's temperature increases.

Next, $A$ represents the contact area between the surfaces. As @Wrzlprmft points out, steam can more easily enter skin pores. This will ensure more heat is transferred, since the total contact area is greater.

We can also maximize heat transfer by increasing the temperature difference, $T_\text{hot}-T_\text{cold}$. The greater this difference, the greater the heat flow. Note that as heat flows, this difference will shrink. In the case of water, $T_\text{hot}$ gets lowered and $T_\text{cold}$ gets higher. However, with vapor at 100C, the condensation energy leaves the vapor first, without changing the gas's temperature, so $T_\text{hot}-T_\text{cold}$ shrinks more slowly; $T_\text{hot}$ does not change, and thus, heat transfer is faster. Furthermore, the condensation energy is, for lack of a better word, quite large, which means that a lot will be transferred at that high rate.

TLDR: The reason steam feels hotter, is that it can transfer more energy to us faster (that is, without decreasing its temperature by transferring condensation energy), whereas water cannot. Our feeling of what's hot is determined by how much energy and how quickly an object transfers that energy to raise our temperature.

Edit: I forgot to mention that unlike water, steam can be packed tightly because it is a gas. Depending on how compressed the steam is in a given volume, you may experience 100C steam to feel warmer or colder than 100C water. For the purpose of my answer, I assumed the steam to be dense and tightly packed -- which can eventually make up for steam's lower thermal conductivity constant.

Steam burns are more dangerous than water burns because more heat is transferred due to the additional release of latent heat of condensation.

To vaporize a liquid, energy as heat must be transferred to the substance in order to break the intermolecular bonds so that the substance becomes gaseous. In the case of pure substances, the temperature remains constant until the liquid has completely vaporized. The heat added during vaporization therefore does not result in an increase in temperature, since it is used to break the intermolecular bonds (hydrogen bond). The heat to be added for the complete vaporization of a certain amount of liquid is also referred to as heat of vaporization or, more generally, as latent heat.

Water requires a very large amount of heat to vaporize. For example, to vaporize 1 kg of water, a heat energy of 2257 kJ is required. If we compare this amount of heat with the heating of water from 20 °C to 100 °C, only 336 kJ is required. Thus, more than 6 times as much heat is needed for vaporization as was necessary for heating! The transferred heat of vaporization cannot simply have disappeared due to the conservation of energy. Rather, this enormous amount of energy is stored as internal energy in the gas phase.

Water needs a multiple of the amount of heat for vaporization compared to heating up to boiling temperature!

During condensation, i.e. when gaseous water liquefies on a cold object, the previously absorbed latent heat is released again. The emitted heat from the substance is absorbed by the cooler object. The (internal) energy of the water decreases and the intermolecular bonds can form again, resulting in the liquid state. In the case of condensation, one also speaks of heat of condensation, which is also a form of latent heat. The amount of heat of condensation is the same as the heat of vaporization.

The energy absorbed by a substance during vaporization in the form of heat of vaporization (latent heat) is released during condensation in the form of heat of condensation (latent heat)!

This understanding now also explains why steam burns are generally much more painful and dangerous than water burns. If the relatively cool skin comes into contact with water vapor (steam), the water condenses there and heat of condensation is released and transferred to the skin. As already explained, due to the large amount of latent heat involved, there is a huge amount of thermal energy transferred. Thus, in contact with steam, significantly more heat is transferred to our skin than in contact with liquid water, although the temperature is the same in both cases (100 °C).

Steam burns are more dangerous compared to water burns because additional latent heat is transferred in the form of heat of condensation!

Note that the human perception of warm or cold is not based on temperatures, but on heat flows (transferred heat per unit time). Thus, although the temperatures are identical at 100 °C in both cases, condensation results in a much larger heat flow. This greater heat flow not only causes a psychologically warmer perception, but also leads physically to more dangerous burns. More information on the perception of warm and cold can be found in the article Why does metal feel colder than wood.