Chapter 10States of Matter

10.2 The nature of liquids

Things you will learn

• You will understand the nature of liquids in terms of the attractive forces of particles

• You will be able to differentiate between evaporation of a liquid and boiling of a liquid

Things we might know about liquids

• Heavier than gases• Non-compressible• Transparent• Fills a container completely, like gases do• Dissolves certain things• Both gases and liquids can flow

Oil and water CO2 and air

Density of gases and liquids

Differences between gases and liquids

• We learned from the kinetic theory that gases are particles in motion with no attractive forces

• Liquids are particles in motion with attractive forces called intermolecular forces

• Most liquid particles don’t have the kinetic energy to escape the intermolecular forces because of the proximity and attraction of the other particles

Evaporation

• Evaporation happens only at the surface of a liquid that is not boiling

• Some of the molecules break away from the attractive forces of the liquid and enter the atmosphere

• Others only escape for a second before colliding with another particle, losing kinetic energy and rebounding into the liquid

Cooling effect of evaporation

• Occurs because only the particles with the greatest kinetic energy (motion energy) are able to escape, leaving the slower particles behind

• Heating adds energy to the system increasing the average speeds of the particles and allowing more of them to escape

Evaporation in a closed container

• When liquid and gas are in equilibrium, the rate of evaporation equals the rate of condensation.

• An increase in temperature will push the equilibrium towards the side of the vapor

• The pressure that a gas exerts in its vapor phase is called vapor pressure

In a closed container, the evaporation equals the condensation and recombination of the vapor with the liquid. This makes sense.

http://upload.wikimedia.org/wikipedia/commons/9/96/Vapor_Pressure_Chart.png

Boiling point

• Boiling point is when the temperature throughout the liquid is high enough for all the liquid to vaporize

• It’s when the vapor pressure of the liquid equals the external pressure of the system

• The liquid never goes above the boiling point in an open system; the energy being added is simply allowing the liquid to become vapor

The boiling point is reached when all the molecules have enough kinetic energy to escape the liquid phase.

Evel Knievel and his steam powered ‘motorcycle”

Sim #2

Describe the nature of liquids

• Liquid particles are free to slide past one another, but are in much closer proximity than gases so they don’t go as far before colliding with one another. Unlike gases, they are attracted to each other by intermolecular forces. This increases their densities to more than gases, and lowers their average kinetic energy to less than gases.

Use kinetic theory to explain the differences between gases and liquids

• The differences between gases and liquids is in the amount of average kinetic energy each state has. Gases are free to travel in straight lines and when they collide, thay have elastic collisions.

• Liquids have lower average kinetic energy because of the intermolecular forces which tend to make liquid particles stick and have non-elastic collisions.

Difference between evaporation and boiling

• Evaporation occurs at the surface of a liquid where some of the particles have sufficient kinetic energy to escape the liquid phase.

• Boiling is where every particle has enough kinetic energy to escape the liquid phase. This occurs when the vapor pressure of the liquid is equal to the atmospheric pressure.

Explain why the boiling point varies with atmospheric pressure

• At lower atmospheric pressures, the kinetic energy of the liquid can be less and still escape to the gas phase. Boiling is defined as when the kinetic energy of all particles can overcome the atmospheric pressure. If you lower the atmospheric pressure, you can boil a liquid at a lower temperature.

Why does evaporation lower the temperature of a liquid

• The kinetic energies of all the particles of a liquid resemble a bell curve. Only the particles with the highest amount of kinetic energy can escape to the gas phase. When these do escape, the remainder of the particles have, on average, less kinetic energy.

Average kinetic energy