# Gases, Liquids and Solids

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Gases, Liquids and Solids
States of Matter, Chapter 10

Describing a Gas A gas expands to completely fill the container.
A gas can flow. A gas can be compressed. Between the particles of a gas there is empty space.

Gas Pressure Gas pressure is the force exerted by a gas per unit surface area of an object. It is caused by gas particles colliding with an object. An empty space, with no particles and no pressure, is a vacuum. Atmospheric pressure results from air particles colliding with an object.

Atmospheric Pressure Changing weather conditions

Atmospheric Pressure Changing altitude

Measuring Pressure Barometer – device that measures atmospheric pressure Invented by Evangelista Torricelli in 1643 The pressure was measured by finding the height of the Hg column (using the unit mmHg).

Measuring pressure in a sample of gas:
A manometer measures the pressure of a gas in a container.

Units of Pressure: Metric unit of pressure is the pascal (Pa). A more familiar units include the atmosphere (atm) and the millimeter of mercury (mmHg). Know this: 1 atm = mm Hg = torr = kilopascal (kPa)

Objectives Describe the nature of a liquid in terms of the attractive forces between the particles Differentiate between evaporation and boiling of a liquid

A Model for Liquids Liquid molecules are free to slide past each other, so liquids, like gases, can flow. Liquids take the shape of their container. Liquids are condensed, not compressible.

Vaporization Vaporization is the conversion of a liquid to a gas or vapor. If the conversion occurs at the surface of a liquid that is not boiling, the process is called evaporation. Evaporation is a cooling process. The particle with the highest kinetic energy tend to escape first, leaving cooler particles behind.

Vapor Pressure Vapor pressure that results from the vapor particles colliding with the walls of the container. It is a force due to a gas above the surface of a liquid. (Vapor is the gaseous state of a substance that is a liquid or solid at room temperature).

Phase changes with liquids
Evaporation… Liquid  Vapor (gas) Condensation… Vapor (gas)  Liquid

Equilibrium vapor pressure…
When the rate of vaporization is equal to the rate of condensation, no net change in vapor pressure occurs. This is a dynamic equilibrium.

Temperature affects the rate of vaporization
Increasing the temperature of a contained liquid increases the vapor pressure over the surface of a liquid.

Boiling Boiling occurs when the vapor pressure is equal to the external pressure. In Denver, where atmospheric pressure is low, boiling occurs at a lower temperature. In a pressure cooker, boiling occurs at a higher temperature.

Boiling point… the temperature at which boiling occurs.
Normal boiling point is the temperature at which boiling occurs at 1 atm pressure (normal pressure). Boiling is also a cooling process.

The Nature of Solids The particles in a solid tend to vibrate about fixed points. They are not free to flow. Most solids are highly organized, dense and incompressible.

Melting point…. the temperature at which a solid turns into a liquid.
Solid  liquid Freezing… Liquid  solid

Distinguish between a crystal lattice and a unit cell
Objectives Describe how the degree of organization of particles distinguishes solids from liquids and gases Distinguish between a crystal lattice and a unit cell Explain how allotropes of an element differ

Solids Crystal Unit cell Allotrope Amorphous solids (glass, soot)
Several forms of an element (carbon) Amorphous solids (glass, soot) Supercooled solids Network solids (diamonds, graphite)

The Solid State: Types of Solids
Crystalline solids

A. The Solid State: Types of Solids

The Solid State: Types of Solids

Bonding in Solids

Changes of state Solid to liquid… Liquid to solid… Liquid to gas…
melting Liquid to solid… freezing Liquid to gas… boiling, evaporating Gas to liquid… condensation Solid to gas… sublimation Gas to solid… deposition

Phase diagrams A phase diagram is a graph showing the relationship between pressure and temperature for solid, liquid and gas states of a substance. Triple point describes the set of conditions where all three phases, solid, liquid, and gas, can exist in equilibrium with each other.

Phase diagram for CO2

Phase diagram for water

Note: The solid-liquid equilibrium line for CO2 has a negative slope, while the solid-liquid line for water has a positive slope. This shows that increasing the pressure on a solid sample of water can cause the solid to melt, while a solid sample of CO2 would remain a solid. This unique property of water allows a skater to glide over the surface of ice on a layer of water under their thin skate blades.

Heating curve A heating curve shows the relationship between temperature and time as heat is added to a system. Notice that as a phase change occurs, even though heat is being added, no change in temperature occurs.

Heating curve

Heating curve for water