States of Matter: Liquids and Gases

Physical ScienceChapter 5.1

Liquids and Gases

Atoms and molecules in liquids and gases

Definition: a fluid is any matter that flows when any force is applied, no matter how small

Your Turn

• Write down the names of three liquids and three gases.

• You will use this later.

Forces in Fluids

Definition: pressure is a distribute force per unit area that acts within a fluid

When you apply force to a fluid the force is distributed all over the fluid (pressure). This is what keeps ball round and allows air filled tires to support your bike.

Pressure is caused by the collision of atoms

Your Turn

When you apply force to one of your liquids, what happens?

When you apply force to one of your gases, what happens?

Do they react differently? Why or why not?

Intermolecular Force

Force 1: Holds the molecule together

Force 2: Intermolecular force exists between molecules.

It is why water stays together when you spill it on the floor

Thermal energy pushes things apart (movement) and intermolecular forces pull them together.

In gases -- thermal energy wins

In solids -- intermolecular forces win

Rethink

Now that you know about intermolecular forces and heat energy. Write a better answer to the previous questions about how gases and liquids react to forces.

Melting and Boiling

Melting point is the temperature at which a substance changes from solid to liquid.

What substance?

Boiling point is the temperature at which a substance changes from liquid to gas

It takes energy to change from a solid to a liquid, or from a liquid to a gas. [cold and hot labs]

Your Turn

Write labels on this diagram about what state the matter is in at each part of the graph.

Still Your Turn

What is the difference between temperature and heat energy?

Write it down.

Tell your neighbor.

Tell the class.

Melting and Boiling Points

Water as an exception -- solid less dense than liquid

All elements have their own freezing and boiling points

Evaporation and Condensation

Evaporation happens when molecules go from liquid to gas at temperature below the boiling point.

Water evaporates …

Evaporation takes energy away from the liquid. Your skin cools when sweat evaporates.

Condensation occurs when molecules go from gas to liquid at temperature below boiling point.

Water condenses…

Condensation raises the temperature because it adds energy.

Air is saturated when evaporation and condensation happen at the same rate.

Your Turn

Fill in the chart with where condensation and evaporation happen. Also include freezing and melting.

Convection

Convection is the transfer of heat through the motion of fluids (which are…)

You can see this in boiling water -- hot water rises, cool water flows down

Forced convection is used to heat houses.

Looking Back

Think back to 6th grade science, where have you heard about convection before?

Atmosphere

Convection currents provide weather

Air is nitrogen + oxygen (97%) + other stuff (but mostly nitrogen – an inert gas)

Air pressure is due to all that air above you.

14.7 lbs per square inch at sea level.

Your Turn

Have you ever driven to Tahoe with a sealed bag of potato chips in the car? What happened?

What other effects to lower air pressure have?

Your Turn

Draw a pressure vs. altitude graph with pressure on the x-axis and altitude on the y-axis. Use Redwood City (30.03 in Hg, 20 feet) and Tahoe City (28.17 in Hg, 6400 feet) as your two points.