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13.2 The Nature of Liquids>

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Chapter 13 States of Matter 13.1 The Nature of Gases 13.2 The Nature of Liquids 13.3 The Nature of Solids 13.4 Changes of State

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How hot should water be when you make coffee?

CHEMISTRY & 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.

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A Model for Liquids

A Model for Liquids Factors that determine the physical properties of a liquid

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A Model for Liquids

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

–  The ability to flow = conform to shape of container

–  Rotational motion

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A Model for Liquids

How Liquids Differ From Gases

–  Intermolecular attractions keep the particles in a liquid close together

–  definite volume

– Strength of interparticle/intermolecular forces determines

– boiling point, melting point, surface tension, vapor pressure, shape of meniscus, volatility, specific heat, etc…

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A Model for Liquids

Liquids are much more dense than gases.

•  Increasing the pressure on a liquid (or a solid) has hardly any effect on its volume.

•  Liquids and solids are known as “condensed states” of matter.

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13.2 The Nature of Liquids>

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In what ways are liquids and gases similar? In what ways are they different?

13.2 The Nature of Liquids>

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Evaporation

Evaporation & Kinetic Energy

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Evaporation

The conversion of a liquid to a gas or vapor is called vaporization (general term)

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

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

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13.2 The Nature of Liquids>

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Evaporation

•  Open system

•  Closed System

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Evaporation

•  increases the average kinetic energy of the particles.

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

Liquids evaporates faster when heated

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Evaporation

•  The particles left have a lower average kinetic energy.

•  Therefore, the liquid’s temperature decreases.

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

–  Evaporation is a cooling process.

–  Perspiration

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13.2 The Nature of Liquids>

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Vapor Pressure

•  sealed, partially filled container of liquid

•  some particles at the surface vaporize

•  producing pressure in container

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

Vapor Pressure

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Vapor Pressure

Over time, gas and liquid phase reach dynamic equilibrium

Liquid Vapor (gas) evaporation

condensation

•  number of particles condensing = number of particles vaporizing.

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Vapor Pressure

Acetone has much weaker interparticle attractions between its molecules than water does. Knowing this, what properties of acetone can you infer?

Vapor pressure? Boiling point? Volatility? Surface tension?

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Vapor Pressure

Vapor Pressure and Temperature Change

What temperature is increased?

INCREASES VAPOR PRESSURE

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Vapor pressure indicates how volatile a given liquid is (how easily it evaporates)

Interpret 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

•  diethyl ether is the most volatile

•  water is the least volatile.

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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

43.9 mm Hg or 5.85 kPa

Air at standard temperature and pressure

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

Air Mercury

Ethanol Mercury

Ethanol Mercury

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13.2 The Nature of Liquids>

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Vapor Pressure

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

12.2 mm Hg or 1.63 kPa

43.9 mm Hg or 5.85 kPa

Air at standard temperature and pressure

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

Air Mercury

Ethanol Mercury

Ethanol Mercury

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Boiling Point

Boiling Point How does boiling differ from vaporization and evaporation?

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Boiling Point

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

Temperature at which vapor pressure within the liquid > atmospheric pressure

Bubbles form and can break free from the surface

Boiling Point

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Boiling Point

Boiling Point and Pressure Changes Because external pressure is a factor in boiling, the same liquid will not always boil (bubble) at the same temperature.

•  atmospheric pressure is lower at higher altitudes

• Therefore, boiling points decrease at higher altitudes.

• Sketch a graph to represent this relationship

• How do pressure cookers work?

• What does this all mean for my Ramen?

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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

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Interpret Graphs

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

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

Normal Atmospheric Pressure

What do the red dots represent?

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Steam Burns

•  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.

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•  boiling point of a liquid at a pressure of 101.3 kPa.

•  Why is the boiling point of water so high, compared to the other compounds listed?

Interpret 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