|
Sci Tech
Burning question
QUESTION: Steam produces greater burns than boiling water. Why?
H.Naveen , Bangalore
ANSWER: May be not necessarily. This question is making the rounds in our schools and examinations and has entered the popular science literature too. Often, as here, it is posed in such a way that we are not asked to choose whether it is right or wrong but explain why it is so.
The expected answer is that steam causes a more severe burn due to the high latent heat of evaporation (latent heat is the energy required to convert water into steam). This question is vague and ill posed and strictly speaking cannot be answered. But we can rationalize the question and analyse to understand the related issues.
Steam and boiling water can exist at varying temperatures but it is only correct to assume here that both are at the same temperature. But the same temperature at the source does not assure that they will have the same temperature when they contact the skin.
Steam tends to cool down considerably due to its mixing with air. Water has a high latent heat of evaporation of 2.257 MJ/kg at 100degrees C (and this energy is available in steam).
But if we consider only this aspect of the latent heat we are ignoring other important factors coming into picture.
The severity of a burn depends on the high temperature to which the skin and the flesh are raised, how long this temperature is held and the area (or volume) of the affected body.
If a hot fluid (water or steam) comes in contact with skin, several factors decide how much it affects the skin — the temperature, density, velocity, thermal conductivity, specific heat, latent heat, how much fluid is there and whether it is replenished, how long it is in contact, etc. A droplet of boiling water that falls on the skin quickly loses its heat and cannot cause much damage but a larger volume of it can cause a severe scald.
Though water has the same latent heat as steam, it has higher density, thermal conductivity and specific heat compared to steam. These factors counteract and tend to cause heat transfer from water to the skin more effective causing more severe burns.
Hence it is not possible to make a simple model of the problem of burn due to an accident to draw a general conclusion.
One has to consider all the details and quantify. Hence the question as it stands is ill posed and cannot be answered. Hot tea can cause a severe burn but we have mastered drinking it without getting hurt.
The sip we take is a sheet that gets cooled by air before coming in contact with the lips. Try drinking it with a straw; but be careful, you may burn your tongue. It may be interesting to mention here about the damage due to cold where the heat is extracted from the body.
Air at 1 degree C makes us uncomfortable but we can survive it. But a human being in water at 1 degree C cannot survive it for more than 30 minutes.
This is because of higher density, thermal conductivity and specific heat of water as compared to those of air. The question of latent heat does not come into picture here but this illustrates the other three factors which are important too. It has not been verified experimentally or otherwise that steam causes a more severe burn.
But such notions persist because we get conditioned at a weaker moment, say when we study a chapter on latent heat. I have discussed this popular question at length in Current Science, Vol. 72, February 1997.
Mohan D. Deshpande, Deputy Director, NAL, Bangalore
Send this article to Friends by
E-Mail
Sci Tech
|
