Adding Heat To A Substance

seven.3: Phase Changes

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Learning Objectives

Determine the oestrus associated with a phase alter.

Matter tin can exist in one of several different states, including a gas, liquid, or solid country. The amount of energy in molecules of affair determines the state of affair.

A gas is a country of matter in which atoms or molecules take enough free energy to move freely. The molecules come into contact with one another only when they randomly collide.
A liquid is a state of matter in which atoms or molecules are constantly in contact simply have enough energy to go on changing positions relative to ane some other.
A solid is a state of matter in which atoms or molecules do not have plenty energy to move. They are constantly in contact and in stock-still positions relative to one some other.

Figure \(\PageIndex{1}\): States of Matter. All three containers comprise a substance with the aforementioned mass, merely the substances are in different states. In the left-hand container, the substance is a gas, which has spread to fill up its container. Information technology takes both the shape and volume of the container. In the middle container, the substance is a liquid, which has spread to take the shape of its container but non the book. In the right-hand container, the substance is a solid, which takes neither the shape nor the book of its container.

The following are the changes of land:

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 instance, heat is increasing the speed of the molecules causing them movement faster (examples: solid to liquid; liquid to gas; solid to gas).
If heat is removed from a substance, such every bit in freezing and condensation, then the process is exothermic. In this example, heat is decreasing the speed of the molecules causing them move slower (examples: liquid to solid; gas to liquid). These changes release heat to the surroundings.
The amount of oestrus needed to change a sample from solid to liquid would exist the same to reverse from liquid to solid. The only difference is the direction of oestrus transfer.

Case \(\PageIndex{1}\)

Label each of the post-obit processes as endothermic or exothermic.

water boiling
ice forming on a pond

Solution

endothermic - yous must put a pan of water on the stove and requite it heat in order to get water to eddy. Because you are calculation heat/energy, the reaction is endothermic.
exothermic - think of ice forming in your freezer instead. You put water into the freezer, which takes heat out of the water, to go it to freeze. Because heat is existence pulled out of the h2o, it is exothermic. Heat is leaving.

Exercise \(\PageIndex{1}\)

Label each of the following processes as endothermic or exothermic.

h2o vapor condensing
gold melting

Answer

a. exothermic

b. endothermic

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 oestrus at a particular temperature, known as the melting point or the boiling point of the substance. The melting signal 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 management of the heat transfer. Oestrus 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.

2 key points are worth emphasizing. Kickoff, at a substance's melting point or humid indicate, two phases tin exist simultaneously. Take h2o (H₂O) every bit an example. On the Celsius scale, H₂O has a melting point of 0°C and a humid point of 100°C. At 0°C, both the solid and liquid phases of H₂O tin coexist. Notwithstanding, if heat is added, some of the solid H_iiO will melt and turn into liquid H_iiO. If estrus is removed, the opposite happens: some of the liquid H₂O turns into solid H₂O. A similar process tin occur at 100°C: adding heat increases the amount of gaseous H₂O, while removing heat increases the amount of liquid H_twoO (Figure \(\PageIndex{1}\)).

Effigy \(\PageIndex{two}\): Heating curve for water. Every bit heat is added to solid h2o, the temperature increases until it reaches 0 °C, the melting point. At this point, the phase change, added heat goes into changing the state from a solid to liquid. Only when this phase change is consummate, the temperature can increase. (CC BY 3.0 Unported; Community Higher Consortium for Bioscience Credentials).

H2o is a good substance to use as an example considering many people are already familiar with it. Other substances accept melting points and boiling points also.

Second, equally shown in Figure \(\PageIndex{1}\), the temperature of a substance does non change as the substance goes from one stage to another. In other words, phase changes are isothermal (isothermal ways "constant temperature"). Over again, consider H₂O as an instance. Solid water (water ice) can exist at 0°C. If oestrus is added to water ice at 0°C, some of the solid changes phase to make liquid, which is too at 0°C. Remember, the solid and liquid phases of H₂O can coexist at 0°C. Simply after 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 change per gram (or per mole) of material. The rut of fusion (ΔH _fus) is the amount of estrus per gram (or per mole) required for a phase 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 phase change that occurs at the boiling point. If yous know the total number of grams or moles of fabric, you can use the ΔH _fus or the ΔH _vap to determine the total heat being transferred for melting or solidification using these expressions:

\[\text{heat} = north \times ΔH_{fus} \label{Eq1a} \]

wher e \(n\) is thursday due east number of moles and \(ΔH_{fus}\) is expressed in free energy/mole or

\[\text{oestrus} = m \times ΔH_{fus} \label{Eq1b} \]

where \(m\) is the mass in grams and \(ΔH_{fus}\) is expressed in energy/gram.

For the boiling or condensation, use these expressions:

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

wher due east \(northward\) is the number of moles) and \(ΔH_{vap}\) is expressed in energy/mole or

\[\text{heat} = m \times ΔH_{vap} \label{Eq2b} \]

wh ere \(m\) i s the mass in grams and \(ΔH_{vap}\) is expressed in energy/gram.

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

Case \(\PageIndex{2}\)

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

Solution

We can utilise the relationship between heat and the heat of fusion (Equation \(\PageIndex{1}\)) to determine how many cal of estrus are needed to melt this ice:

\[ \begin{align*} \ce{rut} &= \ce{grand \times ΔH_{fus}} \\[4pt] \mathrm{estrus} &= \mathrm{(55.8\: \cancel{g})\left(\dfrac{79.9\: cal}{\cancel{g}}\correct)=4,460\: cal} \end{align*} \nonumber \]

Exercise \(\PageIndex{2}\)

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

Respond: \[ \begin{align*} \ce{heat} &= \ce{m \times ΔH_{vap}} \\[4pt] \mathrm{heat} &= \mathrm{(685\: \abolish{g})\left(\dfrac{540\: cal}{\cancel{chiliad}}\right)=370,000\: cal} \end{align*} \nonumber \]

Tabular array \(\PageIndex{one}\) lists the heats of fusion and vaporization for some common substances. Note the units on these quantities; when y'all employ these values in problem 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/g)	ΔH _vap (cal/m)
aluminum (Al)	94.0	two,602
gilded (Au)	15.iii	409
iron (Atomic number 26)	63.2	1,504
water (H₂O)	79.9	540
sodium chloride (NaCl)	123.5	691
ethanol (C_iiH₅OH)	45.2	200.three
benzene (C₆H₆)	xxx.4	94.1

Sublimation

There is also a stage modify where a solid goes directly to a gas:

\[\text{solid} \rightarrow \text{gas} \characterization{Eq3} \]

This phase alter is called sublimation. Each substance has a feature heat of sublimation associated with this process. For example, the heat of sublimation (ΔH _sub) of H_iiO is 620 cal/g.

We encounter sublimation in several ways. You may already be familiar with dry ice, which is simply solid carbon dioxide (CO_two). At −78.5°C (−109°F), solid carbon dioxide sublimes, irresolute straight from the solid stage to the gas phase:

\[\mathrm{CO_2(south) \xrightarrow{-78.5^\circ C} CO_2(yard)} \label{Eq4} \]

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

Fifty-fifty at temperatures below 0°C, solid H₂O will slowly sublime. For instance, a sparse layer of snow or frost on the ground may slowly disappear as the solid H₂O sublimes, fifty-fifty though the outside temperature may be below the freezing point of water. Similarly, water ice cubes in a freezer may get smaller over time. Although frozen, the solid water slowly sublimes, redepositing on the colder cooling elements of the freezer, which necessitates periodic defrosting (frost-gratuitous freezers minimize this redeposition). Lowering the temperature in a freezer volition reduce the need to defrost equally often.

Under like circumstances, h2o volition besides sublime from frozen foods (e.chiliad., meats or vegetables), giving them an unattractive, mottled appearance chosen freezer fire. It is not really a "burn," and the food has not necessarily gone bad, although information technology looks unappetizing. Freezer burn can exist minimized by lowering a freezer'south temperature and by wrapping foods tightly and so water does not have whatsoever space to sublime into.

Cardinal Takeaway

There is an energy alter associated with any phase change.