13.2 The Nature of Liquids

Describe the particles in a liquid

Liquid particles are attracted to each other The particles in a gas are not

Intermolecular Forces Dipole-Dipole Dispersion Hydrogen Bonds

Particles of a liquid spin and vibrate while they move, adding to their average kinetic energy Most of the particles do not have enough energy to escape into the gaseous state; they would have to overcome their intermolecular attractions with other particles

The intermolecular attractions reduce the amount of space between particles of a liquid Liquids are more dense than gases Increasing pressure on liquid has hardly any effect on it’s volume

Vaporization -the conversion of a liquid to a gas or vapor Evaporation -when this occurs at the surface of a liquid that is not boiling Some of the particles break away and enter the gas or vapor state; but only those with certain minimum kinetic energy escape from the surface of the liquid

Why does a liquid evaporate faster when heated? Evaporation is a cooling process Cooling occurs because those with the highest energy escape first Particles left behind have lower average kinetic energies and the temperature decreases

Evaporation of a liquid in a closed container Fig. 13.6b on page 391 shows that no particles can escape into the outside air When some particles do vaporize, these collide with the walls of the container producing vapor pressure

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

Dynamic equilibrium Rate of evaporation  rate of condensation

An increase in temperature of a contained liquid increases the vapor pressure the particles have an increased kinetic energy, and more minimum energy to escape

Boiling Point (bp) The temperature at which the vapor pressure of the liquid is just equal to the external pressure on the liquid

Since the boiling point is where the vapor pressure equals external pressure, the bp changes if the external pressure changes Normal boiling point- the bp of a liquid at a pressure of kPa (or standard pressure)

Water boils at 100 ° C in Denver = 95 ° C Denver is 1600 m above sea level and average atmospheric pressure is about 85.3 kPa

The Nature of Solids What do you know about the particles in a solid?

Most solids have particles packed tightly in an organized pattern Tend to be dense and incompressible Do not flow Do not take the shape of their container Are still able to move, unless they would reach absolute zero

Heated  particles vibrate faster as the KE increases The organization of particles breaks down  solid melts The melting point (mp) Temp at which a solid turns to liquid

Most ionic solids have high melting points, because of the strong forces holding them together Molecular compounds have relatively low melting points

Crystals Particles are arranged in orderly, repeating, 3-D pattern Called a Crystal Lattice

Type of bonding between atoms determines the mp of crystals Most solid substances are crystalline The angles of the faces are a characteristic of that substance, and are always the same for a given sample of that substance See pages 398, 399

Some solid substances can exist in more than one form Allotropes - two or more different molecular forms of the same element in the same physical state Elemental Carbon

Amorphous Solids No ordered internal structure Rubber, plastic, and asphalt - atoms are randomly arranged Glass substances are cooled to a rigid state without crystallizing sometimes called supercooled liquids