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Chapter 10: Problem 15

(a) Briefly describe the phenomena of superheating and supercooling. (b) Why do these phenomena occur?

Short Answer

Expert verified
Answer: Superheating is a phenomenon where the temperature of a substance is increased above its boiling point, without the substance changing its phase from liquid to gas. Supercooling is a phenomenon where the temperature of a substance is decreased below its freezing point, without the substance changing its phase from liquid to solid. Superheating occurs due to the presence of impurities, surface tension, and the need for energy to initiate the phase change, while supercooling occurs when a liquid is cooled below its freezing point without transitioning into a solid state, due to the absence of nucleation sites required for the formation of ice crystals.

Step by step solution

01

Define superheating

Superheating is a phenomenon where the temperature of a substance is increased above its boiling point, without the substance changing its phase from liquid to gas (boiling). For example, water can be heated above its boiling point (100掳C or 212掳F) without boiling, as long as it's done under controlled conditions and without any disturbances.
02

Define supercooling

Supercooling, on the other hand, is a phenomenon where the temperature of a substance is decreased below its freezing point, without the substance changing its phase from liquid to solid (freezing). For example, water can be cooled down below its freezing point (0掳C or 32掳F) without turning into ice, under certain conditions.
03

Reasons for superheating

Superheating occurs due to the presence of impurities, surface tension, and the need for energy to initiate the phase change. When a liquid is heated, its molecules require energy to overcome the attractive forces among them. The heat provided to the liquid is used to increase its internal energy and kinetic energy of the molecules rather than causing the phase change. Moreover, impurities and surface tension prevent the formation of bubbles in the liquid, which would allow it to boil. Hence, the liquid remains in its superheated state until the required energy is provided for the phase change.
04

Reasons for supercooling

Supercooling occurs when a liquid is cooled below its freezing point without transitioning into a solid state. This is due to the absence of nucleation sites, which are required for the formation of ice crystals. Nucleation sites can be impurities or irregularities in the container holding the liquid. When there are no nucleation sites, or their density is low, the liquid remains in a supercooled state until it encounters a nucleation site or is disturbed, which then triggers the freezing process.

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Most popular questions from this chapter

Briefly explain why the hardness of tempered martensite diminishes with tempering time (at constant temperature) and with increasing tem- perature (at constant tempering time).

(a) For the solidification of nickel, calculate the critical radius \(r^{*}\) and the activation free energy \(\Delta G^{*}\) if nucleation is homogeneous. Values for the latent heat of fusion and surface free energy are \(-2.53 \times 10^{9} \mathrm{~J} / \mathrm{m}^{3}\) and \(0.255 \mathrm{~J} / \mathrm{m}^{2}\), respectively. Use the supercooling value found in Table 10.1. (b) Now, calculate the number of atoms found in a nucleus of critical size. Assume a lattice parameter of \(0.360 \mathrm{~nm}\) for solid nickel at its melting temperature.

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