Burns produced by 100°C steam are much more severe than those produced by the sa
ID: 1480665 • Letter: B
Question
Burns produced by 100°C steam are much more severe than those produced by the same amount of 100°C water -- and the
following calculations illustrate why. Use 540 kcal/kg for the latent heat of vaporization of water.
(a) How much heat must your skin absorb to cool 20 g of boiling water from 100°C to 50°C?
Qwater = kcal
(b) How much heat must your skin absorb to transform the same amount of 100°C steam to 50°C water?
Qsteam = kcal
(c) For each case, calculate the mass of flesh raised from the average body temperature of 37°C
to 50°C in each case, given that the specific heat of human flesh is 0.83 kcal/kg°C.
water: kg
steam: kg
Explanation / Answer
we know, specific heat of water = 1 kcal/(kg K)
m = 20 g = 0.02 kg
a) delta_A = m*c*dT
= 0.02*1*(100-50)
= 1 kcal
b) delta_Q = m*Lf + m*c*dT
= 0.02*540 + 0.02*1*(100-50)
= 11.8 kcal
c)
in the forst case
delta_Q = m*c*dT
==> m = delta_Q/(c*dT)
= 1/(0.83*37)
= 0.0326 kg or 32.6 grams
in the seond case
delta_Q = m*c*dT
==> m = delta_Q/(c*dT)
= 11.8/(0.83*37)
= 0.384 kg or 384 grams
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