Chapter 16Chapter 16Chapter 16Chapter 16

Section 3Section 3

Pressure• What did we learn about gas

particles from the kinetic theory?– They are constantly moving and

colliding with anything in their path– The collisions result in pressure– Pressure is the amount of force per

unit of area

Pressure• Gases are often confined within a

container

• Ex: – Balloon– Bicycle tire

Pressure• Why does the balloon and bicycle tire

remain inflated?

– Due to the collisions of the air particles with the walls of the container

– The collection of forces pushes the walls outward

– The more air that is pumped in the more collisions that occur and the more the container expands.

Pressure• What about a bicycle tire? Can

that continue to expand?

– Since the tire can’t expand much the pressure increases inside the tire

Pressure• Pascal (Pa)

– The unit we use to measure pressure

– SI unit

– Pressure = Force / Area

– At sea level atmospheric pressure is 101.3 kPa (kilopascals)

• This means at the Earth’s surface the atmosphere exerts 101,300 N on every square meter

Boyle’s Law• What happens to gas pressure if we

decrease the size of the container?– The particles are now squeezed into a

smaller space– Now they hit the wall of the container

more often– The pressure increases!

Boyle’s Law• What happens if we increase the

size of the container?– The particles have more space

– They hit the walls less often

– The gas pressure decreases!

Boyle’s Law• Boyle’s Law:

– If you decrease the volume of a container of gas and hold the temperature constant the pressure will increase.

– An increase in the volume of a container can cause the pressure to drop if the temperature remains constant.

Boyle’s Law• Figure 21

– As a balloon rises the pressure drops and the volume of the balloon increases

• As the pressure decreases the volume increases

• As the pressure is increased the volume will decrease

Boyle’s Law in Action• Pressure x Volume = a constant

– If the temperature is held constant

– As the pressure and volume change indirectly the constant remains the same

– P1V1 = P2V2

Boyle’s Law in Action• Using the equation P1V1 = P2V2

– We know that the initial pressure and volume is equal to the final pressure and volume

– Volume-Pressure Equation

Pressure-Temperature Relationship

• What happens if you heat an enclosed gas?

– The particles will strike the wall more often– Because the canister is a solid the volume

cannot increase– If the pressure is greater than the canister can

hold it will explode– At a constant volume an increase in

temperature results in an increase in pressure

Charles’s Law• What did we learn about the

thermal expansion of gas particles from section one?– The particles in the hot air balloon

are further apart then particles in cool air

– The hot air is less dense than the cool air and the balloon rises

Charles’s Law• Charles’s Law:

– Volume of a gas increases with increasing temperature as long as pressure does not change

– Volume of a gas decreases with decreasing temperature as long as the pressure does not change

Charles’s Law• As the gas is heated the particles

move faster and further apart

• They strike the walls of the container more often and with more force

• In a balloon the walls have room to expand so instead of the pressure increasing the volume increases

Using Charles’s Law• Formula for Charles’s Law:

– V1 / T1 = V2 / T2

– Temperature is in Kelvin– The pressure must be held constant– Example problem:

• 2.0 L balloon at 25.0 degrees C is placed in a container of ice water at 3 degrees C