The Nature of Liquids

1
The Nature of Liquids
2
A Model for Liquids

• According to the kinetic theory, both the
  particles that make up gases and liquids have
  motion.
• While particles in gases are not attracted to
  each other, particles in liquids are.

3
A Model for Liquids

• The attractive forces between molecules are
  called intermolecular forces.
• The forces allow the particles in liquids to
  slide past one another.

4
A Model for Liquids

• Most of the particles of liquids do not have
  enough kinetic energy to overcome the
  intermolecular forces and escape into the gaseous
  state.
• Liquids take the shape of its container.

5
A Model for Liquids

• The interplay between the disruptive motions of
  particles of a liquid and the attractive forces
  between them causes liquids to flow and have
  definite volumes.

6
A Model for Liquids

• Intermolecular forces also reduce the amount of
  space between the particles in a liquid, which
  makes them more dense than gases.
• Increasing the pressure on a liquid has hardly
  any effect on its volume. So, liquids and solids
  are known as condensed states of matter.

7
Evaporation

• The conversion of a liquid to a gas or vapor is
  called vaporization.
• When it goes from a liquid to a gas at the
  surface of a liquid that is not boiling, the
  process is called evaporation.

8
Evaporation vs. Boiling
9
Evaporation

• In evaporation some molecules in the liquid break
  away and enter the gas or vapor state. Only those
  molecules of the liquid with a certain minimum
  kinetic energy can break away from the surface.

10
Evaporation

• Added heat increases the average kinetic energy
  of the liquids particles.
• The energy enables more particles to overcome the
  attractive forces keeping them in the liquid
  state.
• As evaporation occurs, the particles with the
  highest kinetic energy tend to escape first.

11
Vapor Pressure

• When a partially filled container of liquid is
  sealed, some of the particles in the liquid
  vaporize.
• These particles collide with the walls of the
  sealed container and produce a vapor pressure, or
  a force due to the gas above the liquid.

12
Vapor Pressure

• As time passes, the number of particles entering
  the vapor increases and eventually some particles
  will return to the liquid, or condense.
• After a time, the number of vapor particles
  condensing will equal the number of liquid
  particles vaporizing and the vapor pressure will
  remain constant.

13
Vapor Pressure

• In a system at constant vapor pressure, a dynamic
  equilibrium exists between the gas and the
  liquid. Within the system, the rate of
  evaporation of liquid equals the rate of
  condensation of vapor.

14
Vapor Pressure and Temperature Change

• An increase in the temperature of a contained
  liquid increases the vapor pressure.
• The vapor pressure of a liquid can be determined
  by means of a device called a manometer.

15
Vapor Pressure Measurements

• In a simple manometer, one end of a U-shaped
  glass tube containing mercury is attached to a
  container. The other end of the tube is open to
  the surrounding atmosphere.

16
Vapor Pressure Measurements

• When a liquid is added to the container, the
  pressure in the container increases due to the
  vapor pressure of the liquid.
• The vapor pressure of the liquid pushes the
  mercury on the container side of the U-tube the
  levels of mercury in the U-tube are no longer the
  same.

17
Boiling Point

• When the liquid is heated to a high enough
  temperature, many of the particles throughout the
  liquid have enough kinetic energy to vaporize. At
  that point, boiling (or vaporization throughout
  the liquid) occurs.

18
Boiling Point

• The boiling point (bp) is the temperature at
  which the vapor pressure of the liquid is just
  equal to the external pressure.
• Bubbles of vapor form throughout the liquid, rise
  to the surface, and escape into the air as the
  liquid boils.

19
Boiling Point and Pressure Changes

• The boiling point of a liquid varies with the
  external pressure.
• At high altitudes, the atmospheric pressure is
  lower than it is at sea level which allows the
  liquid to boil at a lower temperature.

20
Boiling due to Altitude
21
Boiling Point and Pressure Changes

• Boiling point decreases at lower pressure and
  increases at higher pressure.
• At lower pressure, the boiling point decreases
  because the particles need less kinetic energy to
  escape the liquids.
• At higher external pressures, a liquids boiling
  point increases because the particles in the
  liquid need more kinetic energy to escape.

22
Boiling Point and Pressure Changes

• The temperature of the boiling liquid never rises
  above its boiling point.
• If heat is supplied at a greater rate, the liquid
  only boils faster.
• The vapor produced is at the same temperature as
  that of the boiling liquid.

23
Normal Boiling Point

• Because a liquid can have various boiling points
  depending on pressure, the normal boiling point
  is defined as the boiling point of a liquid at a
  pressure of 101.3 kPa.