The Nature of Gases Kinetic Kinetic – means motion Kinetic Energy Kinetic Energy – the energy an object has because of its motion. Kinetic Theory Kinetic Theory – all matter consists of tiny particles that are in constant motion.

Kinetic Theory uKinetic theory says that molecules are in constant motion. uPerfume molecules moving across the room are evidence of this.

1. A gas is composed of particles u molecules or atoms u Considered to be hard spheres far enough apart that we can ignore their volume. u Between the molecules is empty space. The Kinetic Theory of Gases Makes three descriptions of gas particles

2. The particles are in constant random motion. u Move in straight lines until they bounce off each other or the walls. 3. All collisions are perfectly elastic

u The Average speed of an oxygen molecule is 1656 km/hr at 20ºC u The molecules don’t travel very far without hitting each other so they move in random directions.

Pressure u Pressure is the result of collisions of the molecules with the sides of a container. u A vacuum is completely empty space - it has no pressure. u Pressure is measured in units of atmospheres (atm). u It is measured with a device called a barometer.

Barometer uAt one atmosphere pressure a column of mercury 760 mm high. Dish of Mercury Column of Mercury 1 atm Pressure

Barometer uAt one atmosphere pressure a column of mercury 760 mm high. uA second unit of pressure is mm Hg u1 atm = 760 mm Hg uThird unit is the Pascal u1 atm = kPa 760 mm 1 atm Pressure

Pressure units u kilopascals – kPa u 1 atm = 760 mm Hg = kPa Convert u 743 mm Hg to atm u 895 kPa to mm Hg

Gas Pressure Gas pressure is the result of simultaneous collisions of billions of rapidly moving particles in a gas with an object. e.g. – a helium-filled balloon maintains its shape because of the pressure of the gas within it. Vacuum – an empty space with no particles and no pressure. (no particles, no collisions)

Atmospheric Pressure Atmospheric pressure Atmospheric pressure results from the collisions of atoms and molecules in air with objects. Atmospheric pressure decreases as you climb a mountain because the density of Earth’s atmosphere decreases as elevation increases. less particles, less pressure

The Nature of Liquids u Particles are in motion. Tends to pull them apart u Attractive forces between molecules keep them close together. u liquids are much more dense than gases. u These are called intermolecular forces. Inter = between

u Vaporization - the change from a liquid to a gas below its boiling point. u Evaporation - vaporization of an uncontained liquid ( no lid on the bottle ).

Evaporation u Molecules at the surface break away and become gas. u Only those with enough KE escape u It requires energy.

Solids u Intermolecular forces are strong u Molecules still move u Can only vibrate and revolve in place. u Particles are locked in place - don’t flow. u Melting point is the temperature where a solid turns into a liquid. u The melting point is the same as the freezing point.

Phase Changes Solid Liquid Gas Melting Vaporization CondensationFreezing

Sublimation – the change of a substance from a solid to a vapor without passing through the liquid state. Sublimation can occur because solids, like liquids, have vapor pressure. Sublimation occurs in solids with vapor pressures that exceed atmospheric pressure at or near room temperature. Sublimation

For example: Solid carbon dioxide (dry ice) sublimes at atmospheric pressure. Used as a coolant. It does not produce a liquid as ordinary ice does when it melts.

Liquid Sublimation Melting Vaporization Deposition Condensation Solid Freezing Gas Require energy Release energy

End of Chapter 13