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 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

Fluid Pressure

• Equation

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.

Brownian Motion

Brownian Motion

Brownian Motion

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

• Molecules are in constant random motion• Molecules hit the walls of the container• Force that the gas molecules exert as they hit

the walls of the container

Energies and Molecules

• Potential Energy• Distance between molecules

• Kinetic Energy• Proportional to temperature of the substance

• Internal Energy• The sum of the microscopic kinetic and potential

energies of a substance due to random motion.

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