Density and Pressure

Density and Pressure

• Define density• Understand relative density• Define and use the term pressure• Learn the instruments used for measuring

pressure• Brownian Motion and the Kinetic Model• Pressure in a Gas• Melting, Boiling and Evaporation

Density

• Mass per unit volume• Units: kgm-3, gcm-3, gml-1

• Density is changes with temperature• Relative Density– Density of a substance divided density of water– Dimensionless– Density of water is 1000 kgm-3

Densities of Some Substances

Comparing Densities• When most substances change from a solid

state to a liquid state their volume does not change much. – Density of a solid is almost the same as a liquid.

Comparing Densities• When a liquid changes into a gas, the spacing

between the particles increases by a factor of about ten.

• As a result, the density of a substance decreases by a factor of approximately 1000 when it changes from a liquid to a gas.

Sample Calculations

• A cup of gold colored metal beads was measured to have a mass 425 grams. By water displacement, the volume of the beads was calculated to be 48.0 cm3. Identify the metal.

• A 60-ml flask is filled with mercury at 0C. When the temperature rises to 80C, 1.47 g of mercury spills out of the flask. Assuming that the volume of the flask is constant, find the density of mercury at 80C if its density at 0C is 13,645 kgm-3.

Pressure

• Force per unit area• Force is normal to the area• Units: Nm-2, kgm-1s-2, Pascal (Pa)• Fluid Pressure• Equation

Fluid Pressure

• Equation• Absolute pressure• Gauge pressure

Measuring Pressure

• Mercury Barometer• U-tube Manometer• Equation

Sample Calculations

• A U-tube contains liquid of unknown density. An oil of density 800 kgm-3 is poured into one arm of the tube until the oil column is 12 cm high. The oil-air interface is 5.0 cm above the liquid level in the other arm of the tube. Find the density of the liquid.

• P400, #22.18 (a)

Brownian Motion

Brownian Motion

Brownian Motion Video 1

Brownian Motion Video 2

Kinetic Theory of Matter

All matter is made up of a large number of tiny atoms or molecules

which are in continuous motion

Pressure of a Gas

• Force that the gas molecules exert as they hit the walls of the container

Energies and Molecules

• Potential Energy• Distance between molecules

• Kinetic Energy• Related to temperature of the substance

• Internal Energy• The sum of the kinetic and potential energies of a

substance.

Melting and Boiling

• Heat is added to break bonds between molecules

• Some part of the energy is used to work against the environment

• Both occur at a specific constant temperature• Let us look at the energies involved

Boiling and Evaporation

• Both involve changes from liquid to gas

• Boiling occurs at a specific temperature• Evaporation occurs at any temperature

• Boiling occurs inside the liquid• Evaporation occurs at the surface of the liquid