The amount of energy required to change the state of a substance depends upon the mass and characteristics of that substance. The energy required to change the state of a material is known as the latent heat. It can be:. Specific latent heat The three states of matter are solid, liquid and gas. It can be: specific latent heat of fusion solid to liquid or specific latent heat of vaporisation liquid to gas.
Once at this temperature, the ice begins to melt until all the ice has melted, absorbing Once all the ice has melted, the temperature of the liquid water rises, absorbing heat at a new constant rate of 1. When all the liquid has become steam vapor, the temperature rises again, absorbing heat at a rate of 0. Figure 3. A graph of temperature versus energy added.
The system is constructed so that no vapor evaporates while ice warms to become liquid water, and so that, when vaporization occurs, the vapor remains in of the system. Water can evaporate at temperatures below the boiling point. This heat comes from the skin, and thus provides an effective cooling mechanism in hot weather. High humidity inhibits evaporation, so that body temperature might rise, leaving unevaporated sweat on your brow. Assume that the soda is kept in a foam container so that heat loss can be ignored.
Assume the soda has the same heat capacity as water. Find the final temperature when all ice has melted. Heat is transferred from the soda to the ice for melting.
Melting of ice occurs in two steps: first the phase change occurs and solid ice transforms into liquid water at the melting temperature, then the temperature of this water rises.
Bring all terms involving T f on the left-hand-side and all other terms on the right-hand-side. Solve for the unknown quantity T f :. This example illustrates the enormous energies involved during a phase change. The mass of ice is about 7 percent the mass of water but leads to a noticeable change in the temperature of soda. However, this correction gives a final temperature that is essentially identical to the result we found.
Can you explain why? We have seen that vaporization requires heat transfer to a liquid from the surroundings, so that energy is released by the surroundings. Condensation is the reverse process, increasing the temperature of the surroundings.
This increase may seem surprising, since we associate condensation with cold objects—the glass in the figure, for example. However, energy must be removed from the condensing molecules to make a vapor condense. Condensation forms in Figure 4 because the temperature of the nearby air is reduced to below the dew point.
The air cannot hold as much water as it did at room temperature, and so water condenses. Energy is released when the water condenses, speeding the melting of the ice in the glass. Energy is also released when a liquid freezes. Growers spray water on the plants in orchards so that the water freezes and heat is released to the growing oranges on the trees.
This prevents the temperature inside the orange from dropping below freezing, which would damage the fruit. Figure Water is intentionally sprayed on orchards to help prevent hard frosts.
Sublimation is the transition from solid to vapor phase. You may have noticed that snow can disappear into thin air without a trace of liquid water, or the disappearance of ice cubes in a freezer. The reverse is also true: Frost can form on very cold windows without going through the liquid stage. Sublimation occurs because the equilibrium vapor pressure of solids is not zero.
Certain air fresheners use the sublimation of a solid to inject a perfume into the room. Moth balls are a slightly toxic example of a phenol an organic compound that sublimates, while some solids, such as osmium tetroxide, are so toxic that they must be kept in sealed containers to prevent human exposure to their sublimation-produced vapors.
Figure 5. Direct transitions between solid and vapor are common, sometimes useful, and even beautiful. The visible vapor is made of water droplets. All phase transitions involve heat. L s is analogous to L f and L v , and its value depends on the substance. Sublimation requires energy input, so that dry ice is an effective coolant, whereas the reverse process i. The amount of energy required for sublimation is of the same order of magnitude as that for other phase transitions.
The material presented in this section and the preceding section allows us to calculate any number of effects related to temperature and phase change. Once all the ice has melted, the temperature of the liquid water rises, absorbing heat at a new constant rate of 1. When all the liquid has become steam, the temperature rises again, absorbing heat at a rate of 0.
Heating and Phase Changes of Water : A graph of temperature versus energy added. The system is constructed so that no vapor evaporates while ice warms to become liquid water, and so that, when vaporization occurs, the vapor remains in of the system. A phase change we have neglected to mention thus far is sublimation, the transition of solid directly into vapor.
The opposite case, where vapor transitions directly into a solid, is called deposition. Sublimation has its own latent heat L s and can be used in the same way as L v and L f.
Privacy Policy. Skip to main content. Heat and Heat Transfer. Search for:. Phase Change and Latent Heat. Latent Heat The latent heat is the energy associated with a phase change of a substance. Learning Objectives Describe the latent heat as a form of energy. Key Takeaways Key Points Energy is required to change the phase of a substance, such as the energy to break the bonds between molecules in a block of ice so it may melt.
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