Just about everyone at one time or another has been burned by hot water or steam. This problem compares the heat input to your skin from steam as opposed to hot water at the same temperature. Assume that water and steam, initially at 100? C, are cooled down to skin temperature, 34? C, when they come in contact with your skin. Assume that the steam condenses extremely fast. We will further assume a constant specific heat capacity c=4190J / (kg? K) for both liquid water and steam. How much heat H1 is transferred to the skin by 25.0 g of steam onto the skin? The latent heat of vaporization for steam is L=2.256

H1 = 63.3 kJ

Explanation:

When the steam at 100C contacts your skin, it is first condensed to water at 100C. The heat involved with this change in state is calculated using the latent heat of vaporization:

Q = ML = (25.0 g) (2.256 kJ/g) = 56.4 kJ

The water at 100C is then cooled to 34C. The heat involved with this temperature change is calculated using the specific heat capacity. (Because a temperature change is used here, it is not necessary to convert °C to K in order for the units to properly cancel)

Q = mcΔt = (25.0 g) (1kg/1000g) (4190Jkg⁻¹K⁻¹) (100C - 34C) = 6913.5 J = 6.9135 kJ

The total heat H1 that is transferred to the skin is the sum of the two heat values above:

H1 = (56.4 kJ) + (6.9135 kJ) = 63.3 kJ