13.2 The Nature of Liquids> 1 Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved. Chapter 13 States of Matter 13.1 The Nature

13.2 The Nature of Liquids>

1 Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved.

Chapter 13States of Matter

13.1 The Nature of Gases

13.2 The Nature of Liquids

13.3 The Nature of Solids13.4 Changes of State



How hot should water be when you make coffee?

CHEMISTRY & YOUCHEMISTRY & YOU

Ground coffee beans contain many different oils, which contribute to the flavor and aroma of the brew. If the water used for brewing the coffee is too hot, some of these oils vaporize, leaving the coffee less rich and flavorful.



A Model for Liquids

A Model for Liquids

What factors determine the physical properties of a liquid?



A Model for Liquids

Substances that can flow are referred to as fluids.• Both liquids and gases can flow.

– The ability of gases and liquids to flow allows them to conform to the shape of their containers.



A Model for Liquids

Gases and liquids have a key difference between them.• According to kinetic theory, there are no

attractions between the particles in a gas.

• The particles in a liquid are attracted to each other.

– These intermolecular attractions keep the particles in a liquid close together, which is why liquids have a definite volume.



A Model for Liquids

The interplay between the disruptive motions of particles in a liquid and the attractions among the particles determines the physical properties of liquids.



A Model for Liquids

Liquids are much more dense than gases.

• Increasing the pressure on a liquid has hardly any effect on its volume.

• The same is true for solids.

– Liquids and solids are known as condensed states of matter.



Describe one way in which liquids and gases are similar, and one way in which they are different.



Describe one way in which liquids and gases are similar, and one way in which they are different.

Both liquids and gases can flow, so they can take the shape of their container. The molecules in a liquid have intermolecular attractions that are not present in gases. Therefore, liquids have a definite volume and will not simply fill their container.



Evaporation

Evaporation

What is the relationship between evaporation and kinetic energy?



Evaporation

The conversion of a liquid to a gas or vapor is called vaporization.

• When this conversion occurs at the surface of a liquid that is not boiling, the process is called evaporation.



Evaporation

The process of evaporation has a different outcome in an open system, such as a lake or an open container, than in a closed system, such as a sealed container.

• In an open system, molecules that evaporate can escape from the system.

• In a closed system, the molecules collect as a vapor above the liquid. Some condense back into a liquid.



Evaporation

During evaporation, only those molecules with a certain minimum kinetic energy can escape from the surface of the liquid.

• Even some of the particles that do escape collide with molecules in the air and rebound back into the liquid.



Evaporation

• Heating the liquid increases the average kinetic energy of its particles.

• The added energy enables more particles to overcome the attractive forces keeping them in the liquid state.

A liquid evaporates faster when heated.



Evaporation

• The particles left in the liquid have a lower average kinetic energy than the particles that have escaped.

• As evaporation takes place, the liquid’s temperature decreases.

As evaporation occurs, the particles with the highest kinetic energy tend to escape first.

– Evaporation is a cooling process.



Evaporation

• When you perspire, water molecules in your perspiration absorb heat from your body and evaporate from the skin’s surface.

• This evaporation leaves the remaining perspiration cooler.

You can observe the effects of evaporative cooling on hot days.

– The perspiration that remains cools you further by absorbing more body heat.



Explain why heating a liquid causes evaporation to occur faster.



Explain why heating a liquid causes evaporation to occur faster.

Evaporation occurs when particles have enough kinetic energy to overcome the forces holding the particles together in a liquid. Heating a liquid increases the average kinetic energy of its particles. Increasing the average kinetic energy increases the number of particles with enough energy to escape the liquid by evaporating.



Vapor Pressure

Vapor Pressure

When can a dynamic equilibrium exist between a liquid and its vapor?



Vapor Pressure

The evaporation of a liquid in a closed system differs from evaporation in an open system.• When a partially filled container of liquid is

sealed, some of the particles at the surface of the liquid vaporize.

• These particles collide with the walls of the sealed container, producing pressure.

– A measure of the force exerted by a gas above a liquid is called vapor pressure.



Vapor Pressure

Over time, the number of particles entering the vapor increases and some of the particles condense and return to the liquid state.

Liquid Vapor (gas)evaporation

condensation



Vapor Pressure

• Eventually, the number of particles condensing will equal the number of particles vaporizing.

Over time, the number of particles entering the vapor increases and some of the particles condense and return to the liquid state.

Liquid Vapor (gas)evaporation

condensation



Vapor Pressure

In a system at constant vapor pressure, a dynamic equilibrium exists between the vapor and the liquid. The system is in equilibrium because the rate of evaporation of liquid equals the rate of condensation of vapor.



Vapor Pressure

In a system at constant vapor pressure, a dynamic equilibrium exists between the vapor and the liquid. The system is in equilibrium because the rate of evaporation of liquid equals the rate of condensation of vapor.

• At equilibrium, the particles in the system continue to evaporate and condense, but no net change occurs in the number of particles in the liquid or vapor.



To make the best-tasting coffee, many people grind the coffee beans just prior to brewing the coffee. Also, they are careful not to grind the coffee beans too much. Explain how both of these methods help prevent the natural oils in coffee beans from vaporizing.




To make the best-tasting coffee, many people grind the coffee beans just prior to brewing the coffee. Also, they are careful not to grind the coffee beans too much. Explain how both of these methods help prevent the natural oils in coffee beans from vaporizing.


The natural oils can vaporize only from the surface of the coffee bean, so waiting to grind the beans keeps the surface area small as long as possible, preventing vaporization. Also, not grinding the beans too much keeps the surface area lower and gives less area for vaporization.



Vapor Pressure

Vapor Pressure and Temperature Change

An increase in the temperature of a contained liquid increases the vapor pressure.



Vapor Pressure

Vapor Pressure and Temperature Change

An increase in the temperature of a contained liquid increases the vapor pressure.

• This happens because the particles in the warmed liquid have increased kinetic energy.

– More of the particles will reach the minimum kinetic energy necessary to escape the surface of the liquid.



The vapor pressure data indicates how volatile a given liquid is, or how easily it evaporates.

Interpret DataInterpret Data

Vapor Pressure (in kPa) of Three Substances at Different Temperatures

Substance 0°C 20°C 40°C 60°C 80°C 100°C

Water 0.61 2.33 7.37 19.92 47.34 101.33

Ethanol 1.63 5.85 18.04 47.02 108.34 225.75

Diethyl ether 24.70 58.96 122.80 230.65 399.11 647.87

• Of the three liquids shown, diethyl ether is the most volatile and water is the least volatile.



Vapor Pressure

Vapor Pressure Measurements

The vapor pressure of a liquid can be determined with a device called a manometer.

12.2 mm Hg or 1.63 kPa


Air at standard temperature and pressure

Ethanol at 0°C Ethanol at room temperature (20°C)

Air

Mercury

Ethanol

Mercury

Ethanol

Mercury



Vapor Pressure

Vapor Pressure MeasurementsThe vapor pressure is equal to the difference in height of the mercury in the two arms of the U-tube.



Air at standard temperature and pressure

Ethanol at 0°C Ethanol at room temperature (20°C)

Air

Mercury

Ethanol

Mercury

Ethanol

Mercury



In a sealed gas-liquid system at a constant temperature, eventually

A. there will be no more evaporation.

B. the rate of condensation decreases to zero.

C. the rate of condensation exceeds the rate of evaporation.

D. the rate of evaporation equals the rate of condensation.



In a sealed gas-liquid system at a constant temperature, eventually

A. there will be no more evaporation.

B. the rate of condensation decreases to zero.

C. the rate of condensation exceeds the rate of evaporation.

D. the rate of evaporation equals the rate of condensation.



Boiling Point

Boiling Point

Under what conditions does boiling occur?



Boiling Point

When a liquid is heated to a temperature at which particles throughout the liquid have enough kinetic energy to vaporize, the liquid begins to boil.



Boiling Point

When a liquid is heated to a temperature at which particles throughout the liquid have enough kinetic energy to vaporize, the liquid begins to boil.• Bubbles of vapor form throughout the liquid,

rise to the surface, and escape into the air.



Boiling Point

When a liquid is heated to a temperature at which particles throughout the liquid have enough kinetic energy to vaporize, the liquid begins to boil.• Bubbles of vapor form throughout the liquid,

rise to the surface, and escape into the air.

• The boiling point (bp) is the temperature at which the vapor pressure of the liquid is just equal to the external pressure on the liquid.



Boiling Point

Boiling Point and Pressure Changes

Because a liquid boils when its vapor pressure is equal to the external pressure, liquids don’t always boil at the same temperature.

• Because atmospheric pressure is lower at higher altitudes, boiling points decrease at higher altitudes.



Boiling Point

Boiling Point and Pressure Changes

Atmospheric pressure at the surface of water at 70°C is greater than its vapor pressure. Bubbles of vapor cannot form in the water, and it does not boil.

At the boiling point, the vapor pressure is equal to the atmospheric pressure. Bubbles of vapor form in the water, and it boils.

At higher altitudes, the atmospheric pressure is lower than it is at sea level. Thus, the water boils at a lower temperature.

101.3 kPa 101.3 kPa 34 kPa 70°C 70°C

100°C

Sea Level Sea Level Atop Mount Everest



You can use this graph to show how the boiling point of a liquid is related to vapor pressure.

Interpret GraphsInterpret Graphs

• At a lower external pressure, the boiling point decreases.

• At a higher external pressure, the boiling point increases.



Boiling Point

• The vapor produced is at the same temperature as that of the boiling liquid.

Boiling is a cooling process, similar to evaporation.

– Although the vapor has the same average kinetic energy as the liquid, its potential (or stored) energy is much higher.

– Thus, a burn from steam is more severe than one from an equal mass of boiling water, even though they are both at the same temperature.



The normal boiling point is defined as the boiling point of a liquid at a pressure of 101.3 kPa.

Interpret DataInterpret Data

Normal Boiling Point

Normal Boiling Points of Several Substances

Substance Boiling Point (°C)

Carbon disulfide (CS2) 46.0

Chloroform (CHCl3) 61.7

Methanol (CH4O) 64.7

Carbon tetrachloride (CCl4) 76.8

Ethanol (C2H6O) 78.5

Water (H2O) 100.0



Is the boiling point of water at the top of Mount McKinley (the highest point in North America) higher or lower than it is in Death Valley (the lowest point in North America)?



Is the boiling point of water at the top of Mount McKinley (the highest point in North America) higher or lower than it is in Death Valley (the lowest point in North America)?

The boiling point of water decreases as altitude increases. Therefore, the boiling point of water is lower atop Mount McKinley than it is in Death Valley.



Key Concepts

The interplay between the disruptive motions of particles in a liquid and the attractions among the particles determines the physical properties of liquids.

During evaporation, only those molecules with a certain minimum kinetic energy can escape from the surface of the liquid.



Key Concepts

In a system at constant vapor pressure, a dynamic equilibrium exists between the vapor and the liquid. The rates of evaporation and condensation are equal.

At a temperature at which particles throughout a liquid have enough kinetic energy to vaporize, the liquid begins to boil.



Glossary Terms

• vaporization: the conversion of a liquid to a gas or a vapor

• evaporation: vaporization that occurs at the surface of a liquid that is not boiling

• vapor pressure: a measure of the force exerted by a gas above a liquid in a sealed container; a dynamic equilibrium exists between the vapor and the liquid



Glossary Terms

• boiling point: the temperature at which the vapor pressure of a liquid is just equal to the external pressure on the liquid

• normal boiling point: the boiling point of a liquid at a pressure of 101.3 kPa or 1 atm



END OF 13.2

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13.2 The Nature of Liquids> 1 Copyright © Pearson Education, Inc., or its affiliates. All Rights Reserved. Chapter 13 States of Matter 13.1 The Nature