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Chapter 14: Q8Q (page 390)

Explain why water cools (its temperature drops) when it evaporates, using the concepts of latent heat and internal energy.

Short Answer

Expert verified

In evaporation, water molecules leave the intermolecular bonds that keep them together in the liquid phase. The latent heat of vaporization and the internal energy help to break the bonds between the molecules.

Step by step solution

01

Effect of latent heat of vaporization on evaporation

Different molecules have different kinetic energies in a liquid, and move randomly within the liquid. All these molecules are constantly acted upon by the intermolecular forces and the pressure due to the atmosphere, which keeps them within the liquid. It takes energy for the molecules to break these bonds. This energy is the latent heat of vaporization.

02

Effect of internal energy on evaporation

On reaching the exposed surface of the liquid, the most energetic molecules overcome the intermolecular forces and the pressure due to the atmosphere. Hence, they escape into the atmosphere in the form of vapors.

The molecules having lower kinetic energy remain in the system. This lowers the average kinetic energy, along with the internal energy and temperature.

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Most popular questions from this chapter

A leaf of area \({\bf{40}}\;{\bf{c}}{{\bf{m}}^{\bf{2}}}\) and mass \({\bf{4}}{\bf{.5 \times 1}}{{\bf{0}}^{{\bf{ - 4}}}}\;{\bf{kg}}\) directly faces the Sun on a clear day. The leaf has an emissivity of 0.85 and a specific heat of \({\bf{0}}{\bf{.80}}\;{\bf{kcal/kg}} \cdot {\bf{K}}\) (a) Estimate the energy absorbed per second by the leaf from the Sun, and then (b) estimate the rate of rise of the leaf’s temperature. (c) Will the temperature rise continue for hours? Why or why not? (d) Calculate the temperature the leaf would reach if it lost all its heat by radiation to the surroundings at 24°C. (e) In what other ways can the heat be dissipated by the leaf?

Explorers on failed Arctic expeditions have survived by covering themselves with snow. Why would they do that?

Using the result of part (a) in Problem 65, take into account radiation from the leaf to calculate how much water must be transpired (evaporated) by the leaf per hour to maintain a temperature of 35°C.

(II) How long does it take the Sun to melt a block of ice at 0°C with a flat horizontal area \({\bf{1}}{\bf{.0}}\;{{\bf{m}}^{\bf{2}}}\) and thickness 1.0 cm? Assume that the Sun’s rays make an angle of 35° with the vertical and that the emissivity of ice is 0.050

A 12-g lead bullet traveling at 220 m/s passes through a thin wall and emerges at a speed of 160 m/s. If the bullet absorbs 50% of the heat generated, (a) what will be the temperature rise of the bullet? (b) If the bullet’s initial temperature was 20°C, will any of the bullet melt, and if so, how much?
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