The Nature of Liquids

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.

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.

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.

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.

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.

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.

Evaporation vs. Boiling

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.

Evaporation • Added heat increases the average kinetic

energy of the liquid’s 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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

Boiling due to Altitude

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.

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.

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.