Aim: How does pressure affect matter?

Do Now:1.Take out your reference tables and a calculator.2.Look at Table H on your reference tables. If the

pressure on ethanoic acid is 10kPa, what is its boiling point?

3.What is the boiling point of a sample of water at 25kPa?

What is pressure?• Pressure is the force exerted on an area.• The metric unit for pressure is the Newton (N).• To get an idea of the size of a Newton, consider that

a small apple weighs about 1Newton.• All molecules exert pressure regardless of their

phase.

Pressure of a solid

• The pressure exerted by a solid is down. Think of it this way: When you put a book on a desk, the pressure exerted by the book is on the desk.

Pressure of a liquid• The pressure of the liquid in a container increases

as the depth increases (This is something divers know very well). Pressure of a liquid is exerted in all directions except up.

A Demonstration• Look at the object(s) placed in the vacuum and

bell jar. What happens to them? Why do you think this occurs?

• Demo #1 – Solid(s)• Demo #2 – Liquid(s)

The solids

• Why didn’t the shape or volume of the solid change?

• Remember – The molecules of a solid are tightly packed. There isn’t a lot of space between them.

The liquids

• The liquids also didn’t change volume. Even though they have weaker bonds than solids, they still don’t have a lot of space between them.

• But they do something the solids don’t. The liquids start to boil at a lower temperature. We can see the relationship between pressure and temperature on boiling point in Table H of the reference tables.

Let’s go back a step…

• How is pressure measured? • Pressure exerted by an object is expressed in

terms of Newtons. • Don’t forget, all molecules exert pressure. • We’ve already talked about pressure exerted

by a solid and a liquid. What about gases?• Gas exerts pressure on all sides of its

container.

We live with the pressure exerted by a gas all the time…

• The atmosphere exerts pressure as a result of its weight and the kinetic energy of the air particles.

• This pressure is called atmospheric pressure or barometric pressure.

• How is air pressure measured?• Air pressure is measured with a tool called a

barometer.

• Why doesn’t the mercury fall out of the tube?• It is held up by the pressure exerted by the atmosphere on the surfaceof the mercury in the container.• What do you think happens when the atmospheric pressure changes?

What can change air pressure?

• Altitude – Higher altitudes mean lower pressure. This is because the weight of the atmosphere is lower.

• Weather – Shortly before rain or snow, the air pressure decreases.

How is air pressure expressed?• Air pressure in a barometer is measured by how

many mm the mercury rises in the tube. • At sea level, the mercury is usually measured at 760

mm. This can be expressed as 760 mm Hg or 760 torr (named after Torricelli the inventor of the barometer. One torr =1mm of mercury).

• Because a barometer measures 760 mm Hg at sea level, that is considered to be standard pressure.

Converting Pressure Units

760 mm Hg = 760 torr = 1 atm = 101.3kPaThis information can also be found on Table A.

What happens if we change the pressure on a gas? Another Demonstration

Activity/Homework #