# Ch. 10 States of Matter Ch The Nature of Gases

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Ch. 10 States of Matter Ch. 10.1 The Nature of Gases
Ch The Nature of Liquids Ch The Nature of Solids Ch Changes of State

Ch. 10.1 The Nature of Gases Kinetic theory
States that the tiny particles in all forms of matter are in constant motion Makes 3 basic assumptions: (see pg. 267) Gases are composed of tiny particles with very little volume and a great deal of empty space between them; no attractive forces The particles move rapidly in random motion; they travel independently in straight paths and collisions occur All collisions are perfectly elastic – total kinetic energy is conserved

Ch. 10.1 The Nature of Gases Gas pressure
Force exerted by a gas per unit surface area Atmospheric pressure Results from the collisions of air molecules with objects Decreases as you move higher in the atmosphere Barometers are used to measure atmospheric pressure (dependent on weather) SI unit is the pascal (Pa); two older units are mm HG and atm One standard atm is the pressure necessary to support 760mm Hg in a mercury barometer at 25oC

Ch. 10.1 The Nature of Gases Gas pressure
Modern barometers are aneroid Measure the # of collisions of air molecules It is important to measure gases under a standard condition STP is defined as OoC and 101.3kPa or 1 atm Kinetic energy and Kelvin temperature When heated, particles absorb some energy as potential, the rest is kinetic and speeds up the particles Particles have varying amounts of energy, so average kinetic energy is used At absolute zero, 0 K, the motion of particles theoretically ceases This has never been achieved in a lab

Ch. 10.2 The Nature of Liquids
A model for liquids Particles that make up a liquid vibrate and spin as they move from place to place this motion contributes to the kinetic energy Most particles do not have enough energy to escape the intermolecular forces and enter the gaseous state Liquids and solids are known as the condensed states of matter

Ch. 10.2 The Nature of Liquids
Evaporation Conversion of a liquid to a gas or vapor is called vaporization When the process occurs in a liquid that is not boiling, it is called evaporation Liquid particles with enough kinetic energy break away from the surface and enter the gas phase Some particles collide with air molecules and return to the liquid phase Liquids evaporate faster when heated Evaporation is a cooling process When particles with high kinetic energy escape, the overall energy of the liquid is lowered

Ch. 10.2 The Nature of Liquids
Evaporation in a closed container has different results Particles that escape collide with the walls if the container and produce a vapor pressure the pressure of the gas above the liquid Particles will continue to leave the liquid, but some will re-enter A dynamic equilibrium exists (see pg. 276) Rate of evaporation = rate of condensation Increasing temperature increases the vapor pressure

Ch. 10.2 The Nature of Liquids
Vapor pressure can be measured with a manometer (see pg. 277) Boiling point The temperature at which the vapor pressure of a liquid is equal to the external pressure Normal boiling point occurs under standard pressure Boiling occurs more easily at high altitudes, and more slowly at low altitudes Boiling is also a cooling process The temperature never rises above boiling point Liquids can only boil faster if heat increases Vapor pressure is at the same temperature, but it has a much higher potential energy

Ch. 10.3 The Nature of Solids A model for solids
Particles in solids tend to vibrate about a fixed point They are dense and incompressible They do not flow or take the shape of their containers Melting point When solids are heated, the particles gain kinetic energy Organization of the particles is disrupted The melting point is the temperature at which a solid becomes a liquid Disruptive vibrations overcome the strong forces holding the particles together Melting and freezing are reverse processes (see pg. 280)

Ch. 10.3 The Nature of Solids Crystal structure and unit cells
Crystals have sides or faces There are 7 types of crystal systems (see pg. 281) The shape of a crystal depends on the arrangement of the particles The smallest group of particles within the crystal system is called a unit cell There are 3 types of unit cells Some substances can take more that one crystal form – these are called allotropes

Ch. 10.3 The Nature of Solids Amorphous solids
Not all solids are crystalline, some are amorphous their atoms are randomly arranged Rubber, plastic and asphalt are amorphous solids Glass is another type Sometimes called supercooled liquids Structure is intermediate between a crystalline solid and a free-flowing liquid Does not melt, but gradually softens Breaks in irregular shapes

Ch. 10.4 Changes of State Phase diagrams Sublimation
Give the conditions of temperature and pressure at which a substance exists as a solid, liquid and gas (see pg. 284) The point at which all three curves meet is known as the triple point Sublimation Change of state from a solid to a gas without passing through the liquid phase Solids have a vapor pressure If the vapor pressure is high enough, a solid can change directly to a gas Dry ice (solid CO2), and mothballs (naphthalene) are examples

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