determine the heat associated with a phase change.

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Matter deserve to exist in one of several various states, including a gas, liquid, or heavy state. The quantity of energy in molecules of issue determines the state the matter.

A gas is a state of issue in which atoms or molecules have enough energy to move freely. The molecule come into contact with one one more only when they randomly collide. A liquid is a state of matter in which atom or molecules are constantly in contact yet have enough energy to keep transforming positions relative to one another. A solid is a state of matter in which atom or molecules carry out not have actually enough power to move. They are constantly in contact and in addressed positions family member to one another.
Figure \(\PageIndex1\): states of Matter. All three containers contain a substance through the very same mass, however the substances are in different states. In the left-hand container, the problem is a gas, which has spread to to fill its container. That takes both the shape and also volume of the container. In the middle container, the substance is a liquid, which has spread to take it the form of its container but not the volume. In the right-hand container, the substance is a solid, which takes no the shape nor the volume of its container.

The adhering to are the alters of state:

Solid → Liquid Melting or fusion
Liquid → Gas Vaporization
Liquid → Solid Freezing
Gas → Liquid Condensation
Solid → Gas Sublimation
If heat is included to a substance, such together in melting, vaporization, and also sublimation, the process is endothermic. In this instance, warm is boosting the rate of the molecules bring about them move faster (examples: solid come liquid; fluid to gas; solid to gas). If warm is eliminated from a substance, such as in freezing and also condensation, climate the procedure is exothermic. In this instance, warm is to decrease the speed of the molecules resulting in them move slower (examples: liquid to solid; gas come liquid). These transforms release heat to the surroundings. The lot of heat needed to readjust a sample from solid come liquid would be the same to reverse from fluid to solid. The only difference is the direction of warm transfer.

Example \(\PageIndex1\)

Label each of the complying with processes together endothermic or exothermic.

water boiling ice creating on a pond


endothermic - you must put a pan of water on the stove and also give it warm in stimulate to acquire water to boil. Due to the fact that you are including heat/energy, the reaction is endothermic. Exothermic - think of ice developing in your freezer instead. You placed water into the freezer, i m sorry takes heat out the the water, to gain it to freeze. Due to the fact that heat is being pulled the end of the water, it is exothermic. Heat is leaving.

Exercise \(\PageIndex1\)

Label every of the adhering to processes as endothermic or exothermic.

water vapor condensing gold melt Answer

a. Exothermic

b. Endothermic

A phase change is a physical procedure in i beg your pardon a substance goes from one phase to another. Commonly the change occurs when including or removing heat at a specific temperature, recognized as the melting point or the boiling point of the substance. The melting suggest is the temperature in ~ which the problem goes native a solid to a fluid (or indigenous a liquid to a solid). The boiling point is the temperature in ~ which a problem goes from a fluid to a gas (or from a gas to a liquid). The nature that the phase readjust depends on the direction the the warm transfer. Warm going into a substance transforms it indigenous a solid come a liquid or a liquid to a gas. Removing heat from a substance transforms a gas come a liquid or a fluid to a solid.

Two an essential points space worth emphasizing. First, in ~ a substance’s melting point or boiling point, 2 phases deserve to exist simultaneously. Take it water (H2O) as an example. ~ above the Celsius scale, H2O has a melting point of 0°C and a boiling suggest of 100°C. At 0°C, both the solid and also liquid phases of H2O deserve to coexist. However, if warmth is added, several of the hard H2O will melt and also turn right into liquid H2O. If warm is removed, the opposite happens: several of the fluid H2O turns into solid H2O. A similar process can take place at 100°C: adding heat increases the lot of gaseous H2O, while removing warm increases the amount of fluid H2O (Figure \(\PageIndex1\)).

Figure \(\PageIndex2\): heater curve for water. As warm is included to hard water, the temperature increases until the reaches 0 °C, the melt point. At this point, the step change, included heat go into transforming the state native a solid come liquid. Only when this phase readjust is complete, the temperature can increase. (CC by 3.0 Unported; community College Consortium for Bioscience Credentials).

Second, as presented in figure \(\PageIndex1\), the temperature of a substance does not adjust as the substance goes from one phase to another. In other words, phase transforms are isothermal (isothermal method “constant temperature”). Again, think about H2O as an example. Solid water (ice) can exist at 0°C. If warm is included to ice at 0°C, some of the solid changes phase to do liquid, i beg your pardon is also at 0°C. Remember, the solid and also liquid phases that H2O can coexist in ~ 0°C. Just after every one of the solid has actually melted into liquid walk the addition of heat change the temperature of the substance.

For each phase change of a substance, over there is a characteristic amount of warm needed to perform the phase adjust per gram (or every mole) that material. The warm of fusion (ΔHfus) is the lot of warmth per gram (or per mole) compelled for a phase adjust that wake up at the melting point. The warmth of vaporization (ΔHvap) is the lot of warm per gram (or per mole) compelled for a phase change that occurs at the boiling point. If you know the total number of grams or moles of material, you have the right to use the ΔHfus or the ΔHvap to recognize the full heat being transferred for melting or solidification utilizing these expressions:

\<\textheat = n \times ΔH_fus \labelEq1a\>

where \(n\) is the number of moles and \(ΔH_fus\) is to express in energy/mole or

\<\textheat = m \times ΔH_fus \labelEq1b\>

where \(m\) is the fixed in grams and also \(ΔH_fus\) is express in energy/gram.

For the cook or condensation, usage these expressions:

\<\textheat = n \times ΔH_vap \labelEq2a\>

where \(n\) is the number of moles) and \(ΔH_vap\) is expressed in energy/mole or

\<\textheat = m \times ΔH_vap \labelEq2b\>

where \(m\) is the massive in grams and \(ΔH_vap\) is express in energy/gram.

Remember that a phase readjust depends on the direction that the warm transfer. If warm transfers in, solids become liquids, and also liquids become solids in ~ the melting and also boiling points, respectively. If warm transfers out, liquids solidify, and gases condense into liquids. At this points, there are no changes in temperature together reflected in the above equations.

Example \(\PageIndex2\)

How much heat is crucial to melt 55.8 g of ice cream (solid H2O) at 0°C? The warm of fusion of H2O is 79.9 cal/g.


We deserve to use the relationship between heat and the warmth of combination (Equation \(\PageIndex1\)) to determine how countless cal of warm are required to melt this ice:

\< \beginalign* \ceheat &= \cem \times ΔH_fus \\<4pt> \mathrmheat &= \mathrm(55.8\: \cancelg)\left(\dfrac79.9\: cal\cancelg\right)=4,460\: cal \endalign*\>

Exercise \(\PageIndex2\)

How much heat is necessary to vaporize 685 g that H2O at 100°C? The heat of vaporization the H2O is 540 cal/g.


\< \beginalign* \ceheat &= \cem \times ΔH_vap \\<4pt> \mathrmheat &= \mathrm(685\: \cancelg)\left(\dfrac540\: cal\cancelg\right)=370,000\: cal \endalign*\>


There is likewise a phase change where a solid goes directly to a gas:

\<\textsolid \rightarrow \textgas \labelEq3\>

This phase readjust is referred to as sublimation. Every substance has a characteristic heat of sublimation associated with this process. For example, the heat of sublimation (ΔHsub) that H2O is 620 cal/g.

We conference sublimation in number of ways. Friend may already be familiar with dry ice, which is just solid carbon dioxide (CO2). In ~ −78.5°C (−109°F), solid carbon dioxide sublimes, an altering directly indigenous the solid phase to the gas phase:

\<\mathrmCO_2(s) \xrightarrow-78.5^\circ C CO_2(g) \labelEq4\>

Solid carbon dioxide is referred to as dry ice since it does no pass with the fluid phase. Instead, that does straight to the gas phase. (Carbon dioxide can exist as liquid yet only under high pressure.) Dry ice cream has many practical uses, consisting of the permanent preservation of clinical samples.

Even in ~ temperatures below 0°C, heavy H2O will progressively sublime. For example, a thin layer of eye or frost on the ground may slowly disappear together the heavy H2O sublimes, even though the outside temperature may be listed below the freezing point of water. Similarly, ice cubes in a freezer may gain smaller over time. Back frozen, the hard water slowly sublimes, redepositing ~ above the cooler cooling aspects of the freezer, i beg your pardon necessitates periodic defrosting (frost-free freezers minimization this redeposition). Lowering the temperature in a freezer will reduce the should defrost as often.

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Under comparable circumstances, water will additionally sublime indigenous frozen foods items (e.g., meat or vegetables), giving them one unattractive, mottled appearance called freezer burn. The is no really a “burn,” and the food has not necessarily gone bad, although that looks unappetizing. Freezer burn can be lessened by lowering a freezer’s temperature and also by wrapping foodstuffs tightly so water go not have any room to sublime into.

Concept review Exercises

describe what happens when warmth flows into or out of a problem at the melting allude or boil point. Exactly how does the quantity of heat forced for a phase adjust relate to the massive of the substance? What is the direction of warmth transfer in boiling water? What is the direction of warmth transfer in freezing water? What is the direction of warm transfer in sweating?