Burns produced by 100°C steam are much more severe than those produced by the sa

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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