# Which Statements Describe Phase Changes Check All That Apply

## 7.3: Phase Changes

##### Learning Objectives
• Decide the estrus associated with a phase modify.

Matter tin can exist in ane of several different states, including a gas, liquid, or solid state. The corporeality of energy in molecules of matter determines the
land of thing.

• A
gas
is a state of matter in which atoms or molecules have enough energy to movement freely. The molecules come into contact with one another only when they randomly collide.
• A
liquid
is a country of matter in which atoms or molecules are constantly in contact but take enough energy to keep changing positions relative to one some other.
• A
solid
is a state of matter in which atoms or molecules do not take enough energy to move. They are constantly in contact and in fixed positions relative to one another.

The following are the changes of state:

 Solid → Liquid Melting or fusion Liquid → Gas Vaporization Liquid → Solid Freezing Gas → Liquid Condensation Solid → Gas Sublimation
• If heat is added to a substance, such every bit in melting, vaporization, and sublimation, the process is
endothermic. In this case, rut is increasing the speed of the molecules causing them move faster (examples: solid to liquid; liquid to gas; solid to gas).
• If rut is removed from a substance, such as in freezing and condensation, then the process is
exothermic. In this instance, heat is decreasing the speed of the molecules causing them motility slower (examples: liquid to solid; gas to liquid). These changes
release heat
to the surroundings.
• The amount of heat needed to modify a sample from solid to liquid would exist the same to opposite from liquid to solid. The but difference is the direction of oestrus transfer.
##### Example $$\PageIndex{1}$$

Label each of the following processes as endothermic or exothermic.

1. h2o boiling
2. ice forming on a swimming
###### Solution
1. endothermic – you must put a pan of water on the stove and give it heat in order to get water to eddy. Because you are adding heat/energy, the reaction is endothermic.
2. exothermic – think of ice forming in your freezer instead. You put water into the freezer, which takes rut out of the h2o, to get it to freeze. Because oestrus is being pulled out of the water, it is exothermic. Estrus is leaving.

##### Exercise $$\PageIndex{one}$$

Label each of the following processes as endothermic or exothermic.

1. water vapor condensing
2. gold melting

a. exothermic

b. endothermic

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A phase change is a physical process in which a substance goes from one phase to another. Usually the change occurs when adding or removing heat at a particular temperature, known as the melting point or the boiling point 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 boiling point 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 phase change depends on the direction of the heat transfer. Heat going
into
a substance changes it from a solid to a liquid or a liquid to a gas. Removing heat
from
a substance changes a gas to a liquid or a liquid to a solid.

Two central points are worth emphasizing. Outset, at a substance’s melting point or boiling point, ii phases can exist simultaneously. Take h2o (HiiO) as an case. On the Celsius scale, H2O has a melting point of 0°C and a boiling point of 100°C. At 0°C, both the solid and liquid phases of HtwoO can coexist. However, if heat is added, some of the solid H2O will cook and plow into liquid HiiO. If heat is removed, the opposite happens: some of the liquid H2O turns into solid H2O. A similar process can occur at 100°C: adding heat increases the amount of gaseous H2O, while removing oestrus increases the amount of liquid H2O (Figure $$\PageIndex{1}$$).

Water is a expert substance to employ as an example considering many people are already familiar with it. Other substances have melting points and boiling points as well.

2d, equally shown in Figure $$\PageIndex{1}$$,
the temperature of a substance does not change
every bit the substance goes
from one phase to another. In other words, stage changes are isothermal (isothermal means “constant temperature”). Over again, consider H2O every bit an case. Solid water (ice) tin exist at 0°C. If estrus is added to ice at 0°C, some of the solid changes stage to brand liquid, which is also at 0°C. Retrieve, the solid and liquid phases of H2O can coexist at 0°C. Only subsequently all of the solid has melted into liquid does the addition of heat change the temperature of the substance.

For each phase change of a substance, there is a characteristic quantity of heat needed to perform the phase modify per gram (or per mole) of textile. The heat of fusion (ΔH
fus) is the amount of rut per gram (or per mole) required for a stage alter that occurs at the melting point. The heat of vaporization (ΔH
vap) is the amount of heat per gram (or per mole) required for a stage change that occurs at the boiling point. If yous know the total number of grams or moles of material, you lot tin can use the ΔH
fus
or the ΔH
vap
to determine the total heat being transferred for melting or solidification using these expressions:

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$\text{heat} = n \times ΔH_{fus} \label{Eq1a}$

wher
e $$n$$ is thursday
due east number of moles

and $$ΔH_{fus}$$ is expressed in energy/mole

or

$\text{rut} = chiliad \times ΔH_{fus} \label{Eq1b}$

where

$$chiliad$$ is

the mass in grams

and $$ΔH_{fus}$$ is expressed in free energy/gram.

For the boiling or condensation, use these expressions:

$\text{oestrus} = n \times ΔH_{vap} \label{Eq2a}$

wher
e $$n$$ is

the number of moles)

and $$ΔH_{vap}$$ is expressed in energy/mole

or

$\text{estrus} = one thousand \times ΔH_{vap} \label{Eq2b}$

wh
ere $$m$$ i
s the mass in grams

and $$ΔH_{vap}$$ is expressed in energy/gram.

Call back that a stage change depends on the management of the heat transfer. If oestrus transfers in, solids get liquids, and liquids get solids at the melting and boiling points, respectively. If heat transfers out, liquids solidify, and gases condense into liquids. At these points, there are no changes in temperature as reflected in the above equations.

##### Example $$\PageIndex{2}$$

How much estrus is necessary to melt 55.8 g of water ice (solid H2O) at 0°C? The rut of fusion of H2O is 79.9 cal/one thousand.

###### Solution

We tin use the relationship between heat and the heat of fusion (Equation $$\PageIndex{1}$$) to decide how many cal of estrus are needed to melt this ice:

\brainstorm{align*} \ce{heat} &= \ce{grand \times ΔH_{fus}} \\[4pt] \mathrm{heat} &= \mathrm{(55.viii\: \cancel{k})\left(\dfrac{79.9\: cal}{\cancel{one thousand}}\right)=4,460\: cal} \end{align*} \nonumber

##### Practise $$\PageIndex{2}$$

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

\brainstorm{align*} \ce{estrus} &= \ce{1000 \times ΔH_{vap}} \\[4pt] \mathrm{heat} &= \mathrm{(685\: \cancel{g})\left(\dfrac{540\: cal}{\cancel{g}}\correct)=370,000\: cal} \end{align*} \nonumber

Tabular array $$\PageIndex{1}$$ lists the heats of fusion and vaporization for some common substances. Note the units on these quantities; when you use these values in trouble solving, brand certain 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.

Table $$\PageIndex{1}$$: Heats of Fusion and Vaporization for Selected Substances
Substance ΔH
fus
(cal/yard)
ΔH
vap
(cal/g)
aluminum (Al) 94.0 2,602
gold (Au) 15.3 409
iron (Fe) 63.2 i,504
h2o (HtwoO) 79.9 540
sodium chloride (NaCl) 123.5 691
ethanol (CtwoH5OH) 45.2 200.3
benzene (Chalf dozenH6) 30.4 94.1
##### Sublimation

In that location is too a phase change where a solid goes direct to a gas:

$\text{solid} \rightarrow \text{gas} \label{Eq3}$

This phase modify is chosen
sublimation. Each substance has a feature estrus of sublimation associated with this process. For instance, the heat of sublimation (ΔH
sub) of H2O is 620 cal/k.

We encounter sublimation in several ways. Y’all may already exist familiar with dry out water ice, which is just solid carbon dioxide (CO2). At −78.v°C (−109°F), solid carbon dioxide sublimes, changing directly from the solid phase to the gas phase:

$\mathrm{CO_2(southward) \xrightarrow{-78.5^\circ C} CO_2(g)} \label{Eq4}$

Solid carbon dioxide is called dry ice because it does not pass through the liquid stage. Instead, information technology does straight to the gas phase. (Carbon dioxide
tin can
exist as liquid simply only under high pressure.) Dry ice has many practical uses, including the long-term preservation of medical samples.

Even at temperatures below 0°C, solid H2O will slowly sublime. For example, a thin layer of snow or frost on the footing may slowly disappear equally the solid HtwoO sublimes, even though the outside temperature may be beneath the freezing betoken of water. Similarly, ice cubes in a freezer may go smaller over time. Although frozen, the solid water slowly sublimes, redepositing on the colder cooling elements of the freezer, which necessitates periodic defrosting (frost-costless freezers minimize this redeposition). Lowering the temperature in a freezer will reduce the need to defrost as frequently.

Nether like circumstances, water will also sublime from frozen foods (e.g., meats or vegetables), giving them an unattractive, mottled appearance called freezer burn. It is not really a “burn,” and the food has non necessarily gone bad, although it looks unappetizing. Freezer fire tin be minimized past lowering a freezer’s temperature and by wrapping foods tightly so h2o does not have whatever space to sublime into.

## Cardinal Takeaway

• At that place is an energy alter associated with any phase change.

## Which Statements Describe Phase Changes Check All That Apply

Source: https://chem.libretexts.org/Bookshelves/Introductory_Chemistry/The_Basics_of_GOB_Chemistry_(Ball_et_al.)/07%3A_Energy_and_Chemical_Processes/7.03%3A_Phase_Changes

Originally posted 2022-08-04 21:00:50.

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