Chap 12

Liquids and Solids

Properties of Liquids and the Properties of Liquids and the Kinetic-Molecular TheoryKinetic-Molecular Theory

• Liquid- is a form of matter that has a definite volume and takes the shape of its container. The particles are in constant motion.

• The attractive forces are dipole-dipole, London dispersion, and hydrogen bonding.

• The Kinetic-Molecular Theory states that particles of a liquid have no fixed space, and move about constantly.

• Fluid-is a substance that can flow and takes the shape of its container.

Liquid’s molecular structure

Properties

Relatively High Density

The liquids are very dense because the particles of liquids are extremely close together. Also, different liquids have different densities.

Relative Incompressibility

Liquids are much less compressible because they have tightly packed particles, and also transmit pressure equally.

Ability to Diffuse The liquids diffuse with most liquids, but at a slower rate than gases because the particles are more tightly packed, and there are many attractive forces between the particles.

Intermolecular Forces:Van Der Waals Forces

• Significant in molecular substances (gases, most liquids and some solids?)

• Intermolecular forces can effect the B.P. and M.P. of the molecular substances.

• Strong intermolecular forces lead to increased m.p. and b.p.

• What are some properties of molecular substances? (nonconductors, insoluble in water, but soluble in most non polar solvent, low m.p. and b.p.

LDFstemporary dipole interactionsThe strength of LDFs depends upon two things:

1. # of e in atoms that make up the molecule2. The ease with which e are dispersed to form temporary dipoles.

Dipole-Dipole Interactions

• Found in PC molecules.

• Ex: ICl

H Bonds

• When H is bonded to a relatively small, electronegative atom, such as N,O, or F

Properties of Liquids: Surface Tension

Surface Tension and Capillary Action

Surface Tension-a force that tends to pull adjacent parts of a liquid’s surface together, thereby decreasing surface area to the smallest possible size.

~The higher the attraction forces (intermolecular forces), the higher the surface tension. Surface tension causes liquid droplets to take a spherical shape.

Capillary action- the attraction of the surface of a liquid to the surface of a solid.

~Capillary action is the reason water from the roots of a tree goes to the leaves. It is also responsible for the liquid surface called the meniscus.

Liquid- Vapor Equilibrium

• Vapor Pressure: The pressure exerted on the surface of a liquid by the vapor that is in equilibrium with the liquid is called as “vapor pressure”

• Once equilibrium between a liquid and vapor is reached, the number of molecules per unit volume in a vapor does not change with time. Hence, the vapor pressure over the liquid remains constant at a given temperature.

• Vapor Pressure is independent of the volume of the container. Why?

Vapor Pressure Vs. TemperatureVapor pressure increases with the increase in

temperature.

Evaporation and Boiling

Vaporization-the process by which a liquid or solid changes to gas.

Evaporation- is the process where particles escape from the surface of a nonboiling liquid and enters the gas state.

~Evaporation takes place because the particles of liquids have different kinetic energies, and move at speeds, therefore some of the particles with higher kinetic energy overcome the intermolecular forces and evaporate.

Boiling- is the change of a liquid to bubbles or vapor. Boiling occurs when the vapor pressure becomes equal to 1 atmospheric pressure.

Boiling

• A liquid boils at the temp. at which its vapor pressure is equal to the pressure above its surface. (usually atomospheric pressure)

• If the pressure above the liquid’s surface is 1 atm, then this temperature is called as its “Normal Boiling Point”

• B.P. of a liquid is reduced by lowering the pressure above it.

• Why does it take longer to cook at high altitudes?

Boiling: A liquid boils at a temp. when the vapor pressure P1 becomes equal to the external pressure

P2 above the liquid

SolidsSolids• Solids have particles that are much closer

together. Solids are of two types:

• Crystalline solids- consist of crystals which are substances which are organized in symmetric, geometric ways.

• Amorphous solids-noncrystalline solid where the particles are arranged randomly.

Solids

High Density and Incompressibility

~Solids are much more dense than liquids or gases because their particles are so much closer together.

~Solids are also less compressible than liquids, and are mostly thought of as not compressible at all.

Types of Solids

Crystalline SolidsExist either as single crystals or group of crystals fused together.The total three dimesnsional arrangement of particles is called as crystal structure.

Crystalline solids can be of four types:

Ionic, Covalent Network, Metallic and Covalent Crystals

Amorphous Solids

Unlike crystalline solids, amorphous solids do not have a regular shape. Amorphous solids are formed when liquids are cooled gradually, so particles are not arranged in any particular order.

Ex: Plastic, Glasses

Crystalline Solids

• There are four types of crystals:• 1. Ionic crystals: The positive and negative ions crystals have

properties: high melting points, are hard and brittle, and good insulators.

• 2. Covalent network crystals: the sites have single atoms. They are nonconductors or semiconductors and have high melting points. Ex: Graphite, diamond

• 3. Metallic crystals: these are metal atoms with a sea of valence electrons. There is high electric conductivity of metals, and the melting points differ. Ex: Iron, Aluminum etc

• 4. Covalent molecular crystals: they are held together by covalent molecular forces, and have low melting points, are easily vaporized, soft, and good insulators. Ex: Sugar, Dry Ice

Cryatalline Solids

Covalent Network: Diamond

Covalent Network: Graphite

Phase Changes

• Melting(fusion)/Freezing

• Vaporization/condensation

• Sublimation/deposition

Time ( heat energy added )

T

0

100

1 2 3 4

Time Temperature Curve1 & 3 H = m * T * Cp

2 & 4 H = m * Hvap

Melting(Fusion)

Boiling

Condensation

Freezing

CHANGES IN STATE

P 1 atm

O T, in C 100

SolidGas

Liquid

Physical Properties of Water

• Highest density at 0 C. Ice is one of the few solids to have a lower density in solid phase as compared to liquid phase. This property is very useful in ice skating, and fishes in lakes etc.

• Because of H Bonding water has a much higher b.p. and m.p.as compared to other liquids. This property is making water one of the best coolants.

• ( Ex: Perspiration, In car radiators etc)