Chapter 7 Matter

Objectives Define Density Define pressure and calculate the pressure of a fluid Use the kinetic model to describe the structures of solids, liquids, and gases and to explain changes of state and the pressure of a gas

Density Density is a macroscopic property of matter Macro = large scale It tells us about how concentrated the matter is Density is a constant for a given material Density = mass/volume Ρ (Greek letter rho)= m/V

Density continued SI unit for density is kg m-3 can also be quoted in g cm-3 Density of water is approximately 1000 kg m-3

Pressure Pressure tells you about how the force is shared out over the area it acts on Pressure is defined as the normal force acting per unit cross-sectional area. Pressure = normal force/cross sectional area p= F/A Pressure unit are Newtons per square meter, which are called pascals 1 Pa = 1 N m-2

Pressure in a fluid Pressure in fluid increases with depth Decreases with height Pressure is due to the weight of the atmosphere above us, pressing down, when pulled by gravity

Pressure of a fluid Depends on three variables The depth h below the surface The density of the fluid ρ The acceleration due to gravity g p= ρgh Pressure = density x acceleration x depth

Pressure of a fluid Volume of water = A h Mass of water = density x volume = ρ A h weight of water = mass x g = ρ A h g Pressure = Force /area = ρ A h g/A = ρ g h

Kinetic Model Used to describe matter and explain its properties and behavior, based on these assumptions Matter is made up of tiny particles These particles tend to move about For simplicity, matter refers to atoms

Three states of matter Describe the differences between each state based on three criteria The spacing of the atoms – how far apart? The ordering of the atoms – order or random Motion of the atoms – moving quickly or slowly

Order and Disorder Solids are classified to how the molecules are arranged Crystalline solid –particles are arranged in a regular pattern – like metals or crystals (duh) Amorphous – particles are arranged in a random way (glass) Polymer - a solids made up of long molecules, and cross links with other chains (rubber), can deform under force and return to original shape

Atoms of a Gas We picture the atoms of a gas as fast moving They moved about in a disordered way Scottish botanist, Robert Brown provided direct evidence of air molecules Brownian motion describes the jerky and erratic path that a single particle travels in a gas

Atoms of gas Since air molecules are much smaller than smoke grains, it can be determined that the air molecules are traveling much faster Brownian motion can also be explained in liquids as well

Fast Molecules Air molecules average speed is 400 ms-1 at standard temperature and pressure (STP = 0oC and 100kPA) Velocity and direction are constantly changes every time it collides with anything else. Air molecules move faster than the speed of sound (330 ms-1 at STP). Faster molecules can escape Earth’s gravitational pull (more than 11Km s-1), why we have an atmosphere.

Explaining Pressure A gas exerts pressure on any surface with which it comes into contact. Pressure is a macroscopic property Force of air pressure by the atmosphere acting on a person is about 200 000N. Since our internal pressure is equal to our external pressure, we don’t collapse under the force

Pressure continued When air molecules are colliding with in inside of a container, they exert a small amount of force on the container, Two factors affect the force and therefor the pressure, that a gas exerts on a the box The number of molecules that hit each side of the box in one second The force with which one molecule collides with the wall.

Pressure continued still… The higher the pressure of a container will increase if the molecules are moving faster If temperature remains constant, but compression is applied, then more molecules are hitting the surface of the container, increasing pressure Temperature can also increase the rate in which molecules hit the sides faster as well, increasing pressure

Changes of State Many materials change states when heated, either to a liquid, and or a gas When a solid is heated enough, it breaks the bonds with their neighboring molecules, becoming more disordered in their arrangement, and a liquid A Liquid heated up with the molecules will move farther apart, becoming a gas, Liquids boil to become a gas, and boiling is dependent on atmospheric pressures (less pressure; lower boiling point temp, higher pressure; higher boiling point temp)

Gas A gas will increase in volume by a 1000 as compared to a liquid 99.9% of the volume of a gas is empty space The molecules in a gas are about 10 times farther apart as in a liquid state

Evaporation When a liquid changes into a gas without boiling Becomes a vapor Faster moving molecules break away from the rest of the liquid Evaporation causes a cooling affect, as the higher energy molecules break away, reducing the kinetic energy of the molecules, thus dropping the temperature.

If air has a density of 1.225 kg/m3, and a pressure at sea level of 101325 pascals, what is the height of the atmosphere? If seawater has a density of 1030 kg/m3, how many times atmospheric pressure is the pressure at a depth of 30 meters? Draw three simple diagrams to show how particles are arranged in a solid, liquid, and gas