Substances can adjust phase — often since of a temperature change. At low temperatures, most substances are solid; together the temperature increases, they end up being liquid; at higher temperatures still, they end up being gaseous.

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The procedure of a solid coming to be a fluid is called melting (an larger term the you might see sometimes is fusion). The opposite process, a liquid coming to be a solid, is called solidification. For any pure substance, the temperature at which melt occurs — recognized as the melting suggest — is a characteristic of the substance. It requires energy for a solid come melt into a liquid. Every pure substance has a details amount of power it needs to readjust from a solid come a liquid. This lot is called the enthalpy of blend (or warm of fusion) of the substance, represented as ΔHfus. Part ΔHfus worths are noted in Table 10.2 “Enthalpies of combination for miscellaneous Substances”; that is assumed that these values room for the melting point of the substance. Note that the unit of ΔHfus is kilojoules every mole, therefore we need to understand the amount of material to know how much energy is involved. The ΔHfus is always tabulated as a positive number. However, it can be supplied for both the melting and also the solidification processes as long as you store in mind the melting is always endothermic (so ΔH will be positive), while solidification is always exothermic (so ΔH will be negative).

Table 10.2 Enthalpies of blend for assorted SubstancesSubstance (Melting Point)ΔHfus (kJ/mol)
Water (0°C)6.01
Aluminum (660°C)10.7
Benzene (5.5°C)9.95
Ethanol (−114.3°C)5.02
Mercury (−38.8°C)2.29

Example 10.4

What is the energy readjust when 45.7 g of H2O melt in ~ 0°C?


The ΔHfus of H2O is 6.01 kJ/mol. However, our quantity is offered in units of grams, not moles, for this reason the very first step is to convert grams come moles making use of the molar mass of H2O, i m sorry is 18.0 g/mol. Climate we have the right to use ΔHfus together a conversion factor. Because the problem is melting, the procedure is endothermic, for this reason the energy change will have actually a positive sign.


Without a sign, the number is suspect to it is in positive.

Test Yourself

What is the energy adjust when 108 g that C6H6 freeze at 5.5°C?


−13.8 kJ

During melting, power goes solely to an altering the phase of a substance; it does no go into changing the temperature that a substance. For this reason melting is one isothermal process since a substance continues to be at the very same temperature. Only when every one of a problem is melted walk any extr energy walk to changing its temperature.

What happens when a solid i do not care a liquid? In a solid, separation, personal, instance particles space stuck in place since the intermolecular forces cannot be conquer by the power of the particles. When much more energy is provided (e.g., by increasing the temperature), over there comes a suggest at i m sorry the particles have actually enough power to move about but not enough energy to separate. This is the fluid phase: particles room still in contact yet are able come move approximately each other. This describes why liquids deserve to assume the shape of their containers: the particles move about and, under the affect of gravity, to fill the lowest volume possible (unless the fluid is in a zero-gravity environment — see number 10.16 “Liquids and Gravity”).

Figure 10.16 “Liquids and Gravity.” (a) A liquid fills the bottom the its container together it is drawn downward by gravity and the corpuscle slide over each other. (b) A fluid floats in a zero- heaviness environment. The corpuscle still slide over each other because they space in the fluid phase, but now over there is no gravity to traction them down.

The phase change between a liquid and also a gas has some similarities to the phase adjust between a solid and a liquid. In ~ a specific temperature, the corpuscle in a liquid have actually enough energy to become a gas. The procedure of a liquid coming to be a gas is dubbed boiling (or vapourization), while the procedure of a gas ending up being a liquid is called condensation. However, uneven the solid/liquid counter process, the liquid/gas conversion process is noticeably influenced by the surrounding pressure ~ above the liquid because gases are strongly affected by pressure. This method that the temperature at which a fluid becomes a gas, the boiling point, can readjust with bordering pressure. Therefore, we define the normal boil point as the temperature at which a liquid alters to a gas once the bordering pressure is exactly 1 atm, or 760 torr. Uneven otherwise specified, it is assumed that a boiling suggest is because that 1 atm of pressure.

Like the solid/liquid phase change, the liquid/gas phase adjust involves energy. The quantity of power required to convert a fluid to a gas is called the enthalpy of vaporization (or warm of vaporization), stood for as ΔHvap. Some ΔHvap values are provided in Table 10.3 “Enthalpies the Vaporization for assorted Substances”; it is assumed that these values space for the typical boiling allude temperature that the substance, which is additionally given in the table. The unit because that ΔHvap is additionally kilojoules every mole, so we need to recognize the amount of material to know how much power is involved. The ΔHvap is also always tabulated as a optimistic number. It deserve to be offered for both the boiling and also the condensation processes as long as you store in mind the boiling is always endothermic (so ΔH will certainly be positive), if condensation is always exothermic (so ΔH will certainly be negative).

Table 10.3 Enthalpies that Vaporization for miscellaneous SubstancesSubstance (Normal boil Point)ΔHvap (kJ/mol)
Water (100°C)40.68
Bromine (59.5°C)15.4
Benzene (80.1°C)30.8
Ethanol (78.3°C)38.6
Mercury (357°C)59.23

What is the energy adjust when 66.7 g that Br2(g) condensation to a fluid at 59.5°C?


The ΔHvap of Br2 is 15.4 kJ/mol. Also though this is a condensation process, we can still use the numerical value of ΔHvap as long as we realize that we have to take power out, therefore the ΔH value will it is in negative. To identify the size of the energy change, we must very first convert the quantity of Br2 to moles. Then we can use ΔHvap together a conversion factor.


Because the procedure is exothermic, the actual worth will it is in negative: ΔH = −6.43 kJ.

Test Yourself

What is the energy readjust when 822 g of C2H5OH(ℓ) boil at its typical boiling point of 78.3°C?


689 kJ

As v melting, the power in boiling goes specifically to an altering the step of a substance; that does not go into changing the temperature of a substance. For this reason boiling is likewise an isothermal process. Only when all of a substance has actually boiled walk any additional energy go to an altering its temperature.

What happens once a fluid becomes a gas? us have already established the a liquid is written of particles in call with every other. Once a fluid becomes a gas, the particles separate from every other, with each particle going that is own way in space. This is just how gases have tendency to fill their containers. Indeed, in the gas phase many of the volume is north space; only about one one-thousandth of the volume is in reality taken up by issue (see figure 10.17 “Liquids and also Gases”). That is this home of gases that defines why they deserve to be compressed, a truth that is taken into consideration in chapter 6 “Gases”.

Figure 10.17 Liquids and Gases. In (a), the particles room a liquid; the particles space in contact however are also able come move approximately each other. In (b), the particles space a gas, and most the the volume is in reality empty space. The particles are not come scale; in reality, the dots representing the particles would certainly be around one one-thousandth the dimension as depicted.

Under some circumstances, the solid step can shift directly come the gas phase without going with a fluid phase, and also a gas deserve to directly end up being a solid. The solid-to-gas change is dubbed sublimation, when the reverse procedure is dubbed deposition. Sublimation is isothermal, choose the other phase changes. Over there is a measurable energy adjust during sublimation; this energy change is dubbed the enthalpy that sublimation, represented as ΔHsub. The relationship between the ΔHsub and also the various other enthalpy changes is as follows:

ΔHsub = ΔHfus + ΔHvap

As such, ΔHsub is not constantly tabulated since it have the right to be just calculated indigenous ΔHfus and also ΔHvap.

There are several common examples the sublimation. A renowned product — dry ice — is in reality solid CO2. Dry ice is dry because it sublimes, v the hard bypassing the liquid phase and also going right to the gas phase. The sublimation wake up at temperature the −77°C, for this reason it need to be tackled with caution. If friend have ever before noticed the ice cubes in a freezer have tendency to obtain smaller end time, it is since the hard water is very slowly subliming. “Freezer burn” isn’t in reality a burn; it occurs when particular foods, such together meats, slowly lose hard water content because of sublimation. The food is still great but look at unappetizing. Reducing the temperature that a freezer will slow the sublimation of hard water.

Chemical equations can be provided to represent a phase change. In together cases, the is vital to use phase brand on the substances. For example, the chemistry equation because that the melting of ice to make liquid water is as follows:

H2O(s) → H2O(ℓ)

No chemical adjust is acquisition place; however, a physical readjust is taking place.

Heating Curves

A plot that the temperature versus the amount of heat included is well-known as a heating curve (see figure 10.18). These are frequently used come visually present the relationship between phase changes and also enthalpy because that a provided substance.

Figure 10.18 “Generic heater curve diagram.”

In number 10.18<1>, the solid gains kinetic energy and consequently rises in temperature as warm is added. At the melt point, the heat added is offered to rest the attractive intermolecular forces of the solid instead of raising kinetic energy, and also therefore the temperature continues to be constant. After all the solid has actually melted, once again, the heat included goes to boosting the kinetic power (and temperature) that the fluid molecules till the boil point. In ~ the cook point, when again, the heat included is supplied to rest the attractive intermolecular forces instead of offering kinetic energy, and also the temperature remains constant until every liquid has been turned to gas.

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Phase alters can take place between any kind of two phases of matter.All phase changes occur with a simultaneous adjust in energy.All phase transforms are isothermal.
QuestionsWhat is the difference in between melting and also solidification?What is the difference in between boiling and condensation?Describe the molecular changes when a solid becomes a liquid.Describe the molecular transforms when a liquid becomes a gas.What is the energy readjust when 78.0 g the Hg melt in ~ −38.8°C?What is the energy readjust when 30.8 g the Al solidify at 660°C?What is the energy readjust when 111 g that Br2 boil at 59.5°C?What is the energy readjust when 98.6 g that H2O condensation at 100°C?Each of the adhering to statements is incorrect. Rewrite them therefore they space correct.Temperature changes during a step change.The process of a liquid becoming a gas is dubbed sublimation.Each of the following statements is incorrect. Rewrite them so they space correct.The volume of a gas has only around 10% matter, v the rest being empty space.ΔHsub is same to ΔHvap.Write the chemistry equation for the melt of elemental sodium.Write the chemistry equation because that the solidification that benzene (C6H6).Write the chemistry equation because that the sublimation that CO2.Write the chemical equation because that the cook of propanol (C3H7OH).What is the ΔHsub the H2O? (Hint: see Table 10.2 “Enthalpies of blend for various Substances” and also Table 10.3 “Enthalpies that Vaporization for assorted Substances”.)The ΔHsub of I2 is 60.46 kJ/mol, when its ΔHvap is 41.71 kJ/mol. What is the ΔHfus the I2?AnswersMelting is the phase adjust from a solid come a liquid, whereas solidification is the phase adjust from a liquid to a solid.The molecules have enough power to move around each other yet not sufficient to fully separate from every other.890 J10.7 kJTemperature does not adjust during a step change.The process of a liquid becoming a gas is called boiling; the procedure of a solid ending up being a gas is dubbed sublimation.Na(s) → Na(ℓ)CO2(s) → CO2(g)46.69 kJ/mol