Behavior of Liquids and Gases: Diffusion and Pressure

Review: How do substances change state?

• How does a liquid change to a solid?– Freezing (freezing point temperature)

• How does a solid change to a liquid?– Melting (melting point temperature)

• How does a liquid change to a gas? (2)– Boiling (boiling point temperature)– Evaporation

• How does a gas change to a liquid?– Condensation

• What is the name for the process in which a solid changes to a gas or gases change to a solid?– Sublimation

How do liquids and gases behave?

• Diffusion• Pascal’s principle• Archimedes’ principle

Diffusion

• Diffusion is the mixing of particles– Movement of particles from an area of high

concentration to an area of low concentration• Why can we smell a fire even though we are

far away?– Particles in the gas state are far apart

and move freely to mix with the air • Many toxic gases are mixed with

chemicals so you can smell them and move to safety!

Pressure and Pascal’s principle

• Pressure is force applied over a certain area– Pressure = force area- Unit of pressure is called a pascal (Pa)

• Pascal’s principle states that:If you apply pressure at any point in a confined substance, the pressure travels unchanged throughout the substance

• Example: if you squeeze toothpaste thatforce will move through the toothpaste and push it out of the hole in the tube

Gas Pressure andPascal’s principle

• You may increase the pressure of a gas in a confined space by forcing more particles into the same amount of space

• Ex: inflating a balloon or a ball– Increasing the pressure by adding more

air particles to the space– What do you think would happen to the pressure

inside the ball if you inflated it inside a warm building then took it outside to a cold temperature?

Behavior of Liquids and Gases: Archimedes’ principle

7SCIENCE

Review: What are some factors that affect the behavior of liquids and gases?

• Diffusion- mixing of particles

• Pressure - Pascal’s principle– When pressure is applied to a point in

a liquid or gas the pressure will travel unchanged through the liquid or gas

Density and Archimedes’ principle• Every substance (solid, liquid or gas) has a density• Density refers to if particles in a substance are closely

packed together Density (g/mL)= mass (g)

volume (mL)

Higher density = particles are packed more closely

Lower density = particles are further apart

Density and Archimedes’ principleWhy do some objects float and others sink?

• If an object has a density lower than that of the surrounding liquid or gas, it will float

• OPPOSITE: If an object has a density higher than that of the liquid or gas, it will sink

Buoyancy and Archimedes’ principle• The idea of buoyancy is that objects tend

to float or sink in a liquid

• Buoyancy is a decrease in weight caused by an upward force in a liquid or gas

- Objects have a specific buoyancy

– Ex: floating in a swimming pool or lake

Archimedes’ principle states:An object will sink until the volume of liquid displaced equals the weight of the object (then it will float)

Application of Archimedes’ principle“Plimsoll line”- Line painted on the side of a ship or boat- If the boat is sitting low and the line is under the

water, the ship is overloaded (too heavy)