We have already established that a liquid is composed of particles in contact with each other. When a liquid becomes a gas, the particles separate from each other, with each particle going its own way in space. This is how gases tend to fill their containers. Indeed, in the gas phase most of the volume is empty space; only about one one-thousandth of the volume is actually taken up by matter see Figure Under some circumstances, the solid phase can transition directly to the gas phase without going through a liquid phase, and a gas can directly become a solid.
The solid-to-gas change is called sublimation , while the reverse process is called deposition. Sublimation is isothermal, like the other phase changes. There are several common examples of sublimation. A well-known product — dry ice — is actually solid CO 2.
Dry ice is dry because it sublimes, with the solid bypassing the liquid phase and going straight to the gas phase. If you have ever noticed that ice cubes in a freezer tend to get smaller over time, it is because the solid water is very slowly subliming. The food is still good but looks unappetizing. Reducing the temperature of a freezer will slow the sublimation of solid water. Chemical equations can be used to represent a phase change. In such cases, it is crucial to use phase labels on the substances.
For example, the chemical equation for the melting of ice to make liquid water is as follows:. A plot of the temperature versus the amount of heat added is known as a heating curve see Figure These are commonly used to visually show the relationship between phase changes and enthalpy for a given substance.
In Figure At the melting point, the heat added is used to break the attractive intermolecular forces of the solid instead of increasing kinetic energy, and therefore the temperature remains constant. Factors Affecting Freezing Point:.
For the same substance, the freezing point of the liquid is equal to the melting point of the solid. Therefore the factors those affect melting point of solid obviously affect the freezing point of the liquid. Freezing Mixtures:. In a freezing mixture, a soluble salt is added. The heat required to dissolve one mole of soluble solute in a solvent is called heat of solvation.
This heat required for dissolution of solid is taken from the mixture itself and thus the freezing point decreases in steps. Freezing mixtures ate used to preserve perishable foodstuff like meat and fishes. They are used for producing sub-zero temperatures in laboratories and industrial units. The process of conversion of a substance from the liquid state to its vapour state at any temperature below boiling point is called evaporation or vaporization.
Some particles from liquid surface possess kinetic energy sufficient to overcome the attractive forces from remaining particles of the liquid and become completely free and escape out as a gas particle in the surroundings. This phenomenon is called evaporation or vaporization.
The rate of evaporation is directly proportional to the surface area and the temperature of the liquid. During evaporation, the temperature of liquid falls. To maintain temperature balance the liquid particles absorb heat from the surroundings making the surrounding cooler. We have already seen that the molecules with higher kinetic energy leave the surface of the liquid, thus there is an overall decrease in the kinetic energy of liquid. This is one of the reasons for the decrease in the temperature of the liquid.
To increase the rate of evaporation we should increase the surface area, the temperature and the wind speed and should decrease the humidity. Characteristics of Evaporation:. Applications of Evaporation:. Boiling process of change of a liquid into a vapour at a particular temperature and pressure from all part of the liquid. Boiling is a bulk process and takes place throughout the liquid.
For each phase change of a substance, there is a characteristic quantity of heat needed to perform the phase change per gram or per mole of material. Remember that a phase change depends on the direction of the heat transfer. If heat transfers in, solids become liquids, and liquids become solids at the melting and boiling points, respectively.
If heat transfers out, liquids solidify, and gases condense into liquids. How much heat is necessary to melt The heat of fusion of H 2 O is We can use the relationship between heat and the heat of fusion Eq. Note the units on these quantities; when you use these values in problem solving, make sure that the other variables in your calculation are expressed in units consistent with the units in the specific heats, or the heats of fusion and vaporization.
This phase change is called sublimation. Each substance has a characteristic heat of sublimation associated with this process. We encounter sublimation in several ways. You may already be familiar with dry ice, which is simply solid carbon dioxide CO 2. Solid carbon dioxide is called dry ice because it does not pass through the liquid phase.
Instead, it goes directly to the gas phase. Carbon dioxide can exist as liquid but only under high pressure. Dry ice has many practical uses, including the long-term preservation of medical samples. Encyclopedic Entry Vocabulary. Media Credits The audio, illustrations, photos, and videos are credited beneath the media asset, except for promotional images, which generally link to another page that contains the media credit.
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Related Resources. The Water Cycle. View Collection. The Process of Evaporation. View Article. Hydrologic Cycle. View leveled Article. Water Cycle. Educational Resources in Your Inbox. Educational Resources in Your Inbox Join our community of educators and receive the latest information on National Geographic's resources for you and your students.
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