what do you call the change from solid to liquid

Affiliate 10. Solids and Liquids

Phase Transitions: Melting, Boiling, and Subliming

1. Describe what happens during a phase alter.
2. Calculate the free energy change needed for a phase change.

Substances can modify stage — often because of a temperature alter. At low temperatures, most substances are solid; as the temperature increases, they become liquid; at college temperatures nevertheless, they become gaseous.

The process of a solid becoming a liquid is called melting (an older term that y'all may meet sometimes is fusion). The reverse process, a liquid becoming a solid, is called solidification. For any pure substance, the temperature at which melting occurs — known every bit the melting point — is a characteristic of that substance. It requires energy for a solid to melt into a liquid. Every pure substance has a certain amount of energy it needs to change from a solid to a liquid. This amount is called the enthalpy of fusion (or heat of fusion) of the substance, represented as ΔH _fus. Some ΔH _fus values are listed in Table 10.2 "Enthalpies of Fusion for Various Substances"; it is causeless that these values are for the melting point of the substance. Annotation that the unit of measurement of ΔH _fus is kilojoules per mole, so we demand to know the quantity of material to know how much energy is involved. The ΔH _fus is ever tabulated every bit a positive number. Nonetheless, it can exist used for both the melting and the solidification processes as long equally you keep in mind that melting is always endothermic (so ΔH will be positive), while solidification is ever exothermic (so ΔH will exist negative).

Tabular array x.2 Enthalpies of Fusion for Diverse Substances

Substance (Melting Indicate)	ΔH fus (kJ/mol)
Water (0°C)	six.01
Aluminum (660°C)	10.7
Benzene (v.5°C)	ix.95
Ethanol (−114.3°C)	five.02
Mercury (−38.viii°C)	two.29

What is the free energy alter when 45.seven grand of H₂O melt at 0°C?

Solution

The ΔH _fus of H₂O is 6.01 kJ/mol. Even so, our quantity is given in units of grams, not moles, and so the kickoff step is to convert grams to moles using the tooth mass of H_iiO, which is eighteen.0 g/mol. And so we can use ΔH _fus as a conversion factor. Because the substance is melting, the process is endothermic, and so the energy change will have a positive sign.

Without a sign, the number is assumed to be positive.

Test Yourself

What is the energy change when 108 g of C_{half dozen}H_{half dozen} freeze at five.5°C?

Answer

−xiii.eight kJ

During melting, energy goes exclusively to changing the phase of a substance; it does not become into changing the temperature of a substance. Hence melting is an isothermal process because a substance stays at the same temperature. Only when all of a substance is melted does any boosted free energy go to changing its temperature.

What happens when a solid becomes a liquid? In a solid, individual particles are stuck in place because the intermolecular forces cannot exist overcome past the energy of the particles. When more than energy is supplied (east.yard., by raising the temperature), in that location comes a bespeak at which the particles take enough free energy to movement around but not enough energy to separate. This is the liquid phase: particles are yet in contact but are able to move around each other. This explains why liquids can presume the shape of their containers: the particles move around and, under the influence of gravity, fill the lowest volume possible (unless the liquid is in a zilch-gravity environment — meet Effigy ten.16 "Liquids and Gravity").

Figure 10.16 "Liquids and Gravity." (a) A liquid fills the bottom of its container as it is drawn downward past gravity and the particles slide over each other. (b) A liquid floats in a zero- gravity environment. The particles still slide over each other because they are in the liquid phase, but now there is no gravity to pull them downward.

The phase change between a liquid and a gas has some similarities to the stage change between a solid and a liquid. At a certain temperature, the particles in a liquid have plenty energy to go a gas. The process of a liquid becoming a gas is called boiling (or vapourization), while the process of a gas becoming a liquid is chosen condensation. However, dissimilar the solid/liquid conversion procedure, the liquid/gas conversion process is noticeably afflicted past the surrounding force per unit area on the liquid because gases are strongly affected by force per unit area. This means that the temperature at which a liquid becomes a gas, the boiling point, can change with surrounding pressure. Therefore, we ascertain the normal boiling point as the temperature at which a liquid changes to a gas when the surrounding pressure is exactly i atm, or 760 torr. Unless otherwise specified, it is assumed that a boiling point is for 1 atm of pressure.

Like the solid/liquid phase modify, the liquid/gas phase change involves energy. The amount of energy required to convert a liquid to a gas is called the enthalpy of vaporization (or heat of vaporization), represented as ΔH _vap. Some ΔH _vap values are listed in Table 10.3 "Enthalpies of Vaporization for Various Substances"; it is assumed that these values are for the normal boiling bespeak temperature of the substance, which is too given in the tabular array. The unit for ΔH _vap is also kilojoules per mole, so we demand to know the quantity of textile to know how much energy is involved. The ΔH _vap is also always tabulated as a positive number. It can be used for both the boiling and the condensation processes every bit long equally you go along in mind that boiling is ever endothermic (and then ΔH will be positive), while condensation is always exothermic (so ΔH will be negative).

Tabular array 10.3 Enthalpies of Vaporization for Various Substances

Substance (Normal Humid Signal)	ΔH vap (kJ/mol)
Water (100°C)	40.68
Bromine (59.5°C)	15.4
Benzene (fourscore.1°C)	30.viii
Ethanol (78.iii°C)	38.vi
Mercury (357°C)	59.23

What is the free energy change when 66.7 g of Br₂(g) condense to a liquid at 59.5°C?

Solution

The ΔH _vap of Br_two is 15.four kJ/mol. Fifty-fifty though this is a condensation procedure, nosotros can withal utilise the numerical value of ΔH _vap as long as we realize that nosotros must take energy out, so the ΔH value will be negative. To make up one's mind the magnitude of the free energy change, we must showtime convert the corporeality of Br₂ to moles. Then we tin utilise ΔH _vap every bit a conversion factor.

Because the process is exothermic, the actual value volition be negative: ΔH = −6.43 kJ.

Exam Yourself

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

Answer

689 kJ

As with melting, the energy in boiling goes exclusively to changing the phase of a substance; information technology does not go into changing the temperature of a substance. So boiling is also an isothermal process. Only when all of a substance has boiled does whatever additional energy get to irresolute its temperature.

What happens when a liquid becomes a gas? Nosotros have already established that a liquid is composed of particles in contact with each other. When a liquid becomes a gas, the particles separate from each other, with each particle going its ain manner in infinite. This is how gases tend to make full their containers. Indeed, in the gas phase about of the volume is empty space; only about one one-thousandth of the volume is actually taken up by affair (see Effigy 10.17 "Liquids and Gases"). Information technology is this property of gases that explains why they can exist compressed, a fact that is considered in Chapter 6 "Gases".

Effigy x.17 Liquids and Gases. In (a), the particles are a liquid; the particles are in contact merely are likewise able to move around each other. In (b), the particles are a gas, and virtually of the volume is actually empty space. The particles are not to scale; in reality, the dots representing the particles would be almost one ane-thousandth the size every bit depicted.

Under some circumstances, the solid phase can transition directly to the gas stage without going through a liquid phase, and a gas can directly get a solid. The solid-to-gas modify is called sublimation, while the reverse process is called deposition. Sublimation is isothermal, similar the other phase changes. There is a measurable energy alter during sublimation; this energy modify is called the enthalpy of sublimation, represented as ΔH _sub. The relationship between the ΔH _sub and the other enthalpy changes is every bit follows:

ΔH _sub = ΔH _fus + ΔH _vap

Every bit such, ΔH _sub is not always tabulated because information technology tin be only calculated from ΔH _fus and ΔH _vap.

At that place are several common examples of sublimation. A well-known product — dry ice — is actually solid CO₂. Dry out ice is dry out because it sublimes, with the solid bypassing the liquid stage and going straight to the gas phase. The sublimation occurs at temperature of −77°C, then it must be handled with caution. If you have e'er noticed that ice cubes in a freezer tend to become smaller over time, information technology is considering the solid water is very slowly subliming. "Freezer burn" isn't actually a burn; it occurs when sure foods, such as meats, slowly lose solid water content considering of sublimation. The nutrient is still good but looks unappetizing. Reducing the temperature of a freezer will slow the sublimation of solid h2o.

Chemical equations can be used to represent a phase modify. In such cases, it is crucial to use phase labels on the substances. For example, the chemical equation for the melting of ice to make liquid water is as follows:

H₂O(s) → H₂O(ℓ)

No chemical alter is taking place; however, a physical change is taking place.

Heating Curves

A plot of the temperature versus the amount of heat added is known equally a heating bend (see Figure ten.xviii). These are usually used to visually show the relationship betwixt phase changes and enthalpy for a given substance.

Figure 10.eighteen "Generic heating bend diagram."

In Figure x.18^[i], the solid gains kinetic energy and consequently rises in temperature as oestrus is added. At the melting point, the heat added is used to break the attractive intermolecular forces of the solid instead of increasing kinetic energy, and therefore the temperature remains constant. After all the solid has melted, once over again, the rut added goes to increasing the kinetic energy (and temperature) of the liquid molecules until the humid bespeak. At the boiling point, one time once again, the heat added is used to break the attractive intermolecular forces instead of supplying kinetic energy, and the temperature remains constant until all liquid has been turned to gas.

• Phase changes tin occur betwixt any two phases of affair.
• All phase changes occur with a simultaneous change in free energy.
• All phase changes are isothermal.

Questions

1. What is the difference between melting and solidification?
2. What is the divergence betwixt humid and condensation?
3. Describe the molecular changes when a solid becomes a liquid.
4. Describe the molecular changes when a liquid becomes a gas.
5. What is the energy change when 78.0 thousand of Hg melt at −38.8°C?
6. What is the energy change when xxx.8 1000 of Al solidify at 660°C?
7. What is the energy change when 111 thousand of Br₂ eddy at 59.five°C?
8. What is the energy modify when 98.half-dozen 1000 of H_iiO condense at 100°C?
9. Each of the following statements is wrong. Rewrite them so they are correct.
   1. Temperature changes during a stage change.
   2. The process of a liquid becoming a gas is called sublimation.
10. Each of the following statements is wrong. Rewrite them and then they are right.
    1. The volume of a gas contains only about x% matter, with the balance being empty infinite.
    2. ΔH _sub is equal to ΔH _vap.
11. Write the chemical equation for the melting of elemental sodium.
12. Write the chemical equation for the solidification of benzene (C_{half dozen}H₆).
13. Write the chemical equation for the sublimation of CO₂.
14. Write the chemical equation for the boiling of propanol (C₃H₇OH).
15. What is the ΔH _sub of H₂O? (Hint: see Table 10.ii "Enthalpies of Fusion for Various Substances" and Table 10.3 "Enthalpies of Vaporization for Various Substances".)
16. The ΔH _sub of I_ii is 60.46 kJ/mol, while its ΔH _vap is 41.71 kJ/mol. What is the ΔH _fus of I₂?

Answers

1. Melting is the phase change from a solid to a liquid, whereas solidification is the phase change from a liquid to a solid.

3. The molecules have enough energy to motility about each other only not enough to completely split up from each other.

5. 890 J

7. ten.7 kJ

9. 1. Temperature does not alter during a phase change.
   2. The process of a liquid becoming a gas is called boiling; the process of a solid condign a gas is called sublimation.

11. Na(due south) → Na(ℓ)

13. CO₂(s) → CO_two(g)

15. 46.69 kJ/mol

Media Attributions

• "Glass of H2o" © 2005 by Derek Jensen is licensed under a Public Domain license
• "Clayton Anderson zero g" © 2010 by NASA is licensed nether a Public Domain license

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Source: https://opentextbc.ca/introductorychemistry/chapter/phase-transitions-melting-boiling-and-subliming/

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