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The Topic: Re: today's query


Given: a choice of placing your hand into a pot of boiling water or a cloud of steam.

Find: the choice that results in the least damage and/or pain.

Known: Heat of vaporisation of water is 2260 J/gm.

Assumptions: The water and steam are at the same pressure, and "cloud" infers a similar temperature since the steam is condensing into droplets and is visible.

Solution: When water is at its boiling point at standard atmospheric pressure, additional energy is required to change phase from water to steam. The energy is used to break intermolecular bonds and accelerate molecular kinetic energy. Therefore, even though the steam is at the same temperature and pressure as the water, it has 2260 J/gm more energy than the water. The steam would have a greater potential for causing tissue damage, therefore the boiling water would be the best choice.


  Dr. Egon Spengler

You are correct as far as the numbers go, but I'm not sure your line of thinking is valid. Steam burns are considered much worse than burns caused by boiling water. But the conditions under which they occur must also be taken into account.

  Dr. Egon Spengler

Notice that the original question mentioned a pot of water and a cloud of steam. Also note that you mentioned the heat of vaporization at 2260 J per gram... which do you think has more mass, the water in the pot or the cloud of steam?

  Dr. Egon Spengler

The question as posed is too vague. Is the hand to be merely placed quickly in harm's way and then removed? If so, the very motion itself could be skillfully worked to displace most of the steam for that instant, if the steam 'cloud' were small enough. This of course would be impossible with the boiling water. If no new heat is being introduced into the system, and the hand cannot be instantly removed, the greater mass of the water and the fact that it is not dispersing would also come into play.

  Dr. Egon Spengler

As an example given to high school level science students, the question is fine. However, it also stands as a warning not to take theoretical calculations and apply them indiscriminately in real-world situations. For any meaningful results in real-world experimental conditions, you would have to plainly state and monitor any number of variables; the volume of water, and steam, heat inputs, duration of contact, area of contact, and so on.

  Dr. Egon Spengler

Plus, you might have trouble finding volunteers.

Of course, that's what undergraduates are for.

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