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11.4: Phase Changes

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    218547
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     can exist in one of several different states, including a gas, liquid, or solid state. The amount of in molecules of determines the . is a physical process in which a substance goes from one to another. Usually the change occurs when adding or removing at a particular temperature, known as the melting point or the of the substance. The melting point is the temperature at which the substance goes from a solid to a liquid (or from a liquid to a solid). The is the temperature at which a substance goes from a liquid to a gas (or from a gas to a liquid). The nature of the depends on the direction of the transfer. going a substance changes it from a solid to a liquid or a liquid to a gas. Removing a substance changes a gas to a liquid or a liquid to a solid. , two can exist simultaneously. Take water (HO) as an example. On the Celsius scale, HO has a melting point of 0°C and a of 100°C. At 0°C, both the solid and liquid of HO can coexist. However, if is added, some of the solid HO will melt and turn into liquid HO. If is removed, the opposite happens: some of the liquid HO turns into solid HO. A similar process can occur at 100°C: adding increases the amount of gaseous HO, while removing increases the amount of liquid HO (Figure \(\PageIndex{1}\)). . In other words, changes are ( means “constant temperature”). Again, consider HO as an example. Solid water (ice) can exist at 0°C. If is added to ice at 0°C, some of the solid changes to make liquid, which is also at 0°C. Remember, the solid and liquid of HO can coexist at 0°C. Only after all of the solid has melted into liquid does the addition of change the temperature of the substance. of a substance, there is a characteristic quantity of needed to perform the per gram (or per mole) of material. The of (Δ) is the amount of per gram (or per mole) required for a that occurs at the melting point. The (Δ) is the amount of per gram (or per mole) required for a that occurs at the . If you know the total number of grams or moles of material, you can use the Δ or the Δ to determine the total being transferred for melting or solidification using these expressions: } = n \times ΔH_{fus} \label{Eq1a} \] /mole } = m \times ΔH_{fus} \label{Eq1b} \] /gram. } = n \times ΔH_{vap} \label{Eq2a} \] /mole } = m \times ΔH_{vap} \label{Eq2b} \] /gram. depends on the direction of the transfer. If transfers in, solids become liquids, and liquids become solids at the melting and boiling points, respectively. If transfers out, liquids solidify, and gases condense into liquids. At these points, there are no changes in temperature as reflected in the above equations. and vaporization for some common substances. 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 and vaporization.


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