consider the phase change lab. what is required to bring about a phase change

Section Learning Objectives

By the end of this section, you will be able to practice the following:

• Explain changes in heat during changes of country, and draw latent heats of fusion and vaporization
• Solve problems involving thermal energy changes when heating and cooling substances with stage changes

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Teacher Support

The learning objectives in this section will help your students master the following standards:

• (6) Science concepts. The pupil knows that changes occur inside a concrete system and applies the laws of conservation of free energy and momentum. The pupil is expected to:
  • (E) draw how the macroscopic properties of a thermodynamic system such as temperature, specific heat, and force per unit area are related to the molecular level of matter, including kinetic or potential energy of atoms;
  • (F) contrast and give examples of different processes of thermal energy transfer, including conduction, convection, and radiation.

Section Key Terms

condensation	freezing	latent rut	sublimation
latent oestrus of fusion	latent rut of vaporization	melting	vaporization
phase change	stage diagram	plasma

Instructor Support

Instructor Support

Introduce this department by request students to give examples of solids, liquids, and gases.

Stage Changes

So far, nosotros have learned that adding thermal energy by heat increases the temperature of a substance. But surprisingly, there are situations where adding free energy does non change the temperature of a substance at all! Instead, the additional thermal energy acts to loosen bonds between molecules or atoms and causes a phase alter. Because this energy enters or leaves a organisation during a phase modify without causing a temperature change in the organization, it is known as latent estrus (latent ways hidden).

The iii phases of thing that you frequently encounter are solid, liquid and gas (run into Figure 11.8). Solid has the least energetic country; atoms in solids are in close contact, with forces between them that let the particles to vibrate just not modify position with neighboring particles. (These forces tin can be thought of every bit springs that tin be stretched or compressed, just not easily broken.)

Liquid has a more energetic state, in which particles can slide smoothly past one some other and change neighbors, although they are still held together by their mutual attraction.

Gas has a more than energetic country than liquid, in which particles are broken free of their bonds. Particles in gases are separated by distances that are big compared with the size of the particles.

The most energetic country of all is plasma. Although you may not take heard much near plasma, it is actually the virtually common state of matter in the universe—stars are made up of plasma, as is lightning. The plasma land is reached by heating a gas to the point where particles are pulled apart, separating the electrons from the rest of the particle. This produces an ionized gas that is a combination of the negatively charged gratuitous electrons and positively charged ions, known as plasma.

Effigy 11.8 (a) Particles in a solid always take the same neighbors, held close past forces represented here by springs. These particles are essentially in contact with one another. A stone is an instance of a solid. This rock retains its shape considering of the forces belongings its atoms or molecules together. (b) Particles in a liquid are also in close contact simply can slide over one another. Forces between them strongly resist attempts to push them closer together and too concur them in close contact. Water is an example of a liquid. H2o tin can flow, but it also remains in an open container because of the forces between its molecules. (c) Particles in a gas are separated by distances that are considerably larger than the size of the particles themselves, and they move about freely. A gas must be held in a closed container to foreclose information technology from moving out into its surroundings. (d) The atmosphere is ionized in the farthermost rut of a lightning strike.

During a phase change, matter changes from one phase to another, either through the addition of energy by rut and the transition to a more energetic state, or from the removal of energy by estrus and the transition to a less energetic state.

Phase changes to a more energetic land include the following:

• Melting—Solid to liquid
• Vaporization—Liquid to gas (included boiling and evaporation)
• Sublimation—Solid to gas

Phase changes to a less energetic state are as follows:

• Condensation—Gas to liquid
• Freezing—Liquid to solid

Energy is required to cook a solid considering the bonds between the particles in the solid must exist broken. Since the free energy involved in a phase changes is used to interruption bonds, in that location is no increase in the kinetic energies of the particles, and therefore no ascension in temperature. Similarly, energy is needed to vaporize a liquid to overcome the attractive forces betwixt particles in the liquid. In that location is no temperature change until a stage change is completed. The temperature of a loving cup of soda and ice that is initially at 0 $\text{°C}$ stays at 0 $\text{°C}$ until all of the ice has melted. In the opposite of these processes—freezing and condensation—energy is released from the latent estrus (see Figure 11.9).

Teacher Support

Instructor Support

[BL] [OL] Ask students if the same amount of free energy is absorbed or released in melting or freezing a detail quantity of a substance.

[AL] Ask student how water is able to evaporate even when it is at room temperature and not at 100 $\text{°C}$ .

Figure eleven.9 (a) Energy is required to partially overcome the attractive forces between particles in a solid to form a liquid. That aforementioned energy must be removed for freezing to take place. (b) Particles are separated by large distances when irresolute from liquid to vapor, requiring significant free energy to overcome molecular attraction. The same energy must be removed for condensation to accept identify. In that location is no temperature change until a stage modify is completed. (c) Plenty free energy is added that the liquid country is skipped over completely every bit a substance undergoes sublimation.

The heat, Q, required to change the phase of a sample of mass m is

$Q=m{\mathrm{Fifty}}_f$ (for melting/freezing),

$Q=\mathrm{thousand}{L}_5$ (for vaporization/condensation),

where $L_f$ is the latent heat of fusion, and $L_v$ is the latent heat of vaporization. The latent heat of fusion is the amount of oestrus needed to cause a phase change between solid and liquid. The latent rut of vaporization is the amount of oestrus needed to cause a phase alter betwixt liquid and gas. ${\mathrm{50}}_f$ and ${\mathrm{Fifty}}_{\mathrm{five}}$ are coefficients that vary from substance to substance, depending on the strength of intermolecular forces, and both have standard units of J/kg. Come across Tabular array 11.3 for values of $L_f$ and ${\mathrm{50}}_v$ of different substances.

Substance	Melting Betoken ( $\mathrm{°C}$ )	Lf (kJ/kg)	Boiling Point ( $\mathrm{°C}$ )	Lv (kJ/kg)
Helium	‒269.7	5.23	‒268.nine	twenty.nine
Hydrogen	‒259.three	58.six	‒252.9	452
Nitrogen	‒210.0	25.5	‒195.8	201
Oxygen	‒218.8	13.8	‒183.0	213
Ethanol	‒114	104	78.iii	854
Ammonia	‒78	332	‒33.4	1370
Mercury	‒38.nine	11.eight	357	272
Water	0.00	334	100.0	2256
Sulfur	119	38.1	444.six	326
Lead	327	24.five	1750	871
Antimony	631	165	1440	561
Aluminum	660	380	2520	11400
Silverish	961	88.3	2193	2336
Aureate	1063	64.5	2660	1578
Copper	1083	134	2595	5069
Uranium	1133	84	3900	1900
Tungsten	3410	184	5900	4810

Table xi.3 Latent Heats of Fusion and Vaporization, along with Melting and Humid Points

Let'southward consider the example of adding heat to ice to examine its transitions through all three phases—solid to liquid to gas. A stage diagram indicating the temperature changes of water as energy is added is shown in Figure xi.10. The ice starts out at −twenty $\text{°C}$ , and its temperature rises linearly, absorbing heat at a abiding rate until it reaches 0 $\text{°}\text{.}$ Once at this temperature, the water ice gradually melts, absorbing 334 kJ/kg. The temperature remains constant at 0 $\text{°C}$ during this phase change. Once all the ice has melted, the temperature of the liquid h2o rises, absorbing estrus at a new constant charge per unit. At 100 $\text{°C}$ , the water begins to boil and the temperature once more remains constant while the water absorbs 2256 kJ/kg during this stage change. When all the liquid has become steam, the temperature rises once more at a constant rate.

Figure 11.ten A graph of temperature versus added energy. The system is constructed so that no vapor forms while water ice warms to become liquid water, and so when vaporization occurs, the vapor remains in the organization. The long stretches of constant temperature values at 0 $\text{°C}$ and 100 $\text{°C}$ reflect the large latent heats of melting and vaporization, respectively.

We have seen that vaporization requires rut transfer to a substance from its environs. Condensation is the reverse procedure, where estrus in transferred abroad from a substance to its surroundings. This release of latent oestrus increases the temperature of the surroundings. Free energy must exist removed from the condensing particles to brand a vapor condense. This is why condensation occurs on cold surfaces: the heat transfers energy away from the warm vapor to the common cold surface. The free energy is exactly the same every bit that required to cause the phase change in the other management, from liquid to vapor, and so information technology can exist calculated from $Q=m{L}_v$ . Latent oestrus is likewise released into the surroundings when a liquid freezes, and tin can be calculated from $Q=m{\mathrm{Fifty}}_f$ .

Fun In Physics

Making Water ice Foam

Figure 11.11 With the proper ingredients, some ice and a couple of plastic bags, you lot could make your ain ice foam in five minutes. (ElinorD, Wikimedia Commons)

Water ice cream is certainly piece of cake plenty to buy at the supermarket, only for the hardcore ice cream enthusiast, that may non exist satisfying enough. Going through the process of making your own ice cream lets yous invent your own flavors and marvel at the physics firsthand (Figure 11.xi).

The first step to making homemade ice cream is to mix heavy cream, whole milk, sugar, and your flavor of choice; information technology could exist as simple as cocoa pulverization or vanilla extract, or as fancy equally pomegranates or pistachios.

The next step is to pour the mixture into a container that is deep enough that yous volition be able to churn the mixture without information technology spilling over, and that is too freezer-rubber. After placing information technology in the freezer, the water ice cream has to be stirred vigorously every 45 minutes for 4 to five hours. This slows the freezing process and prevents the ice cream from turning into a solid block of ice. Most people prefer a soft creamy texture instead of one giant popsicle.

As it freezes, the cream undergoes a phase change from liquid to solid. By now, we're experienced enough to know that this ways that the cream must experience a loss of heat. Where does that heat go? Due to the temperature difference between the freezer and the water ice cream mixture, oestrus transfers thermal energy from the ice foam to the air in the freezer. One time the temperature in the freezer rises plenty, the freezer is cooled by pumping excess heat exterior into the kitchen.

A faster way to make ice cream is to chill it by placing the mixture in a plastic handbag, surrounded by some other plastic pocketbook half total of water ice. (You tin also add a teaspoon of table salt to the outer bag to lower the temperature of the water ice/salt mixture.) Shaking the handbag for five minutes churns the ice cream while cooling it evenly. In this case, the heat transfers energy out of the ice cream mixture and into the ice during the phase alter.

This video gives a demonstration of how to make home-made ice cream using ice and plastic bags.

Why does the ice bag method piece of work so much faster than the freezer method for making water ice cream?

1. Ice has a smaller specific heat than the surrounding air in a freezer. Hence, it absorbs more than energy from the water ice-foam mixture.

2. Ice has a smaller specific rut than the surrounding air in a freezer. Hence, it absorbs less energy from the water ice-cream mixture.

3. Water ice has a greater specific heat than the surrounding air in a freezer. Hence, it absorbs more energy from the ice-foam mixture.

4. Water ice has a greater specific heat than the surrounding air in a freezer. Hence, it absorbs less energy from the water ice-cream mixture.

Solving Thermal Energy Problems with Phase Changes

Worked Instance

Calculating Oestrus Required for a Phase Alter

Calculate a) how much energy is needed to melt 1.000 kg of ice at 0 $\text{°C}$ (freezing point), and b) how much free energy is required to vaporize 1.000 kg of water at 100 $\text{°C}$ (boiling betoken).

Strategy FOR (A)

Using the equation for the heat required for melting, and the value of the latent rut of fusion of h2o from the previous table, nosotros tin solve for part (a).

Strategy FOR (B)

To solve part (b), we use the equation for heat required for vaporization, along with the latent estrus of vaporization of water from the previous table.

Discussion

The amount of free energy demand to cook a kilogram of ice (334 kJ) is the same amount of free energy needed to raise the temperature of 1.000 kg of liquid h2o from 0 $\text{°C}$ to 79.eight $\text{°C}$ . This example shows that the energy for a stage modify is enormous compared to free energy associated with temperature changes. It also demonstrates that the amount of energy needed for vaporization is even greater.

Worked Example

Calculating Final Temperature from Phase Change: Cooling Soda with Ice Cubes

Ice cubes are used to chill a soda at 20 $\text{°C}$ and with a mass of $m_{soda}=0.25\text{ kg}$ . The water ice is at 0 $\text{°C}$ and the total mass of the ice cubes is 0.018 kg. Assume that the soda is kept in a foam container so that estrus loss tin can be ignored, and that the soda has the same specific rut as h2o. Discover the last temperature when all of the ice has melted.

Strategy

The water ice cubes are at the melting temperature of 0 $\text{°C}$ . Estrus is transferred from the soda to the ice for melting. Melting of ice occurs in 2 steps: first, the stage change occurs and solid (water ice) transforms into liquid water at the melting temperature; and then, the temperature of this water rises. Melting yields water at 0 $\text{°C}$ , so more estrus is transferred from the soda to this water until they are the same temperature. Since the amount of heat leaving the soda is the same as the amount of heat transferred to the water ice.

$$Q_{ic\mathrm{due\; east}}=-Q_{soda}$$

xi.20

The heat transferred to the ice goes partly toward the phase modify (melting), and partly toward raising the temperature subsequently melting. Recall from the last section that the relationship between rut and temperature change is $Q=\mathrm{yard}c\Delta T$ . For the ice, the temperature change is $T_f-0\text{°C}$ . The full heat transferred to the ice is therefore

$$Q_{ice}=m_{ice}{L}_f+m_{ice}{c}_{\mathrm{westward}}(T_f-0\text{°C}).$$

11.21

Since the soda doesn't change phase, but only temperature, the oestrus given off by the soda is

$$Q_{soda}=m_{soda}{c}_w(T_f-20\text{°C}).$$

11.22

Since $Q_{ice}=-Q_{\mathrm{south}oda}$ ,

$${\mathrm{chiliad}}_{ice}{L}_f+m_{ic\mathrm{east}}{c}_w(T_f-0\text{°C})=-m_{soda}{c}_w(T_f-\mathrm{twenty}\text{°C}).$$

11.23

Bringing all terms involving $T_f$ to the left-hand-side of the equation, and all other terms to the right-hand-side, nosotros can solve for $T_f$ .

$$T_f=\frac{m_{soda}{c}_w(\mathrm{twenty}\text{°C})-{\mathrm{grand}}_{ice}{\mathrm{Fifty}}_f}{(m_{soda}+m_{ice})c_{\mathrm{due\; west}}}$$

11.24

Substituting the known quantities

$$T_f=\frac{\left(0.25\text{ kg}\right)\left(4186\text{ J/kg}\cdot \text{°C}\right)\left(20\text{°C}\right)-\left(0.018\text{ kg}\right)\left(\text{334,000 J/kg}\right)}{\left(\text{0}\text{.25 kg + 0}\text{.018 kg}\right)\left(4186\text{ G/kg}\cdot \text{°C}\right)}=13\text{°C}$$

11.25

Discussion

This case shows the enormous energies involved during a phase alter. The mass of the ice is about vii pct the mass of the soda, yet it causes a noticeable modify in the soda's temperature.

Tips For Success

If the ice were not already at the freezing point, we would also have to factor in how much free energy would become into raising its temperature up to 0 $\text{°C}$ , before the phase change occurs. This would be a realistic scenario, because the temperature of water ice is frequently below 0 $\text{°C}$ .

Practice Problems

11 .

How much energy is needed to melt 2.00 kg of water ice at 0 °C ?

1. 334 kJ
2. 336 kJ
3. 167 kJ
4. 668 kJ

12 .

If 2500\,\text{kJ} of energy is just enough to cook 3.0\,\text{kg} of a substance, what is the substance's latent heat of fusion?

1. 7500\,\text{kJ} \cdot \text{kg}

2. 7500\,\text{kJ/kg}

3. 830\,\text{kJ} \cdot \text{kg}

4. 830\,\text{kJ/kg}

Check Your Agreement

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Use these questions to assess student achievement of the section'due south learning objectives. If students are struggling with a specific objective, these questions will help identify which and straight students to the relevant content.

13 .

What is latent oestrus?

1. It is the heat that must transfer energy to or from a arrangement in gild to crusade a mass change with a slight alter in the temperature of the system.

2. It is the heat that must transfer free energy to or from a system in order to cause a mass change without a temperature change in the system.

3. It is the heat that must transfer energy to or from a system in lodge to cause a phase modify with a slight alter in the temperature of the system.

4. It is the estrus that must transfer energy to or from a system in order to cause a phase change without a temperature change in the system.

xiv .

In which phases of matter are molecules capable of irresolute their positions?

1. gas, liquid, solid
2. liquid, plasma, solid
3. liquid, gas, plasma
4. plasma, gas, solid

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Source: https://openstax.org/books/physics/pages/11-3-phase-change-and-latent-heat

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