Why can steam at 100 degree celsius burn worse than boiling water at 100 degree celsius?
- When hot water is in liquid form and in contact with your skin thermal energy is transferred from the hot water to our skin.
- When water is in gas form (steam) in order to turn into liquid form it must release its latent heat of vaporization to the object upon which the steam condenses.
- The latent heat of vaporization accounts for steam at 100 degree celsius transferring more heat to the skin than an equivalent mass of liquid water at 100 degree celsius
We will compare the amount of thermal energy transferred to your skin for the case of steam at 100 deg C versus an equivalent mass of boiling water at 100 degC.
When boiling water at intial temperature Ti = 100 degC hits your skin and cools to a final temperature of Tf = 25 degC the amount of energy, Q, transferred from the water to your skin is given by
where M is the mass of the water, c = 4190 J kg-1K-1 is the specific heat of water 1, and . Taking M = 1 g the total heat transfer to your skin would be 314 J.
When 1 g of steam hits your skin at Ti = 100 degC it must first condense to liquid water undergoing a phase change before it will drop in temperature. The thermal energy released from the steam to your skin when condensing is given by M Lv, where Lv is the heat of vaporization and is 22.6 * 105 J kg-1 for water 1 and M is the mass of the steam in contact with your skin. For 1g of steam this gives M Lv = 2260 J. Once the steam has turned to liquid water additional thermal energy is transferred to the skin when the condensed water goes from Ti = 100 degC to Tf = 25 degC as calculated above. This means the steam lost an additional 2260 J of energy. This heat released due to condensation partially accounts for the severity of steam burns. Other reasons for the severity of steam burns is that steam can be heated many times more than 100 degC and can be extremely pressurized. When pipes containing steam burst or leak in industrial settings nearby personnel are subjected to superheated pressurized steam. Severe burns over a large surface area of the body (and if inhaled in the lungs as well) can result as well as sometimes throwing personnel a very large distance by the force of the steam. Steam burns in industrial settings are therefore extremely dangerous.
Note: You do not normally burn yourself on steam when you are cooking if you stay far enough away from the pots because the steam spreads out rapidly so that the mass of condensed water on your skin per unit area is small. However, in cases where steam is dense such as directly out of the exit hole of a kettle or from a pressure cooker, injury is far more likely.
- 1. a. b. Knight, R.D , Physics for scientists and engineers: a strategic approach (1st ed.), p.527-530, Pearson Addison-Wesley, 2004.
© Physics and Astronomy Outreach Program at the University of British Columbia (Janelle Van Dongen 2012-01-30)