Properties of Liquids

Chapter 11

Viscosity

• Resistance of a liquid to flow• Greater a liquid’s viscosity, the more slowly it flows• Viscosity increases with molecular weight

– Larger molecules get tangled with each other and impede flow

• Viscosity decreases with increases in temperature– Molecules have higher average KE making them move more

easily and overcome the attractive forces between molecules

• The stronger the intermolecular forces, the greater the viscosity of the liquid

Surface Tension

• Surface acts as if it had an elastic skin due to an imbalance of intermolecular forces at the surface of the liquid

• Molecules in the center are equally pulled in all directions while at the surface only net inward force are experienced by the molecules– This force decreases the surface

area and making the molecules at the surface more closely packed together

– Hydrogen bonds increase the attractive forces between molecules increasing the surface tension

Capillary Action• The rising of liquids up very

narrow tubes– Adhesive forces between liquid

and tubing is greater than the cohesive forces between the molecules in the liquid

– Adhesive forces between the liquid and the walls of the tube tend to increase the surface area of the liquid pulling it up the tube

• Reason water rises up against gravity in trees

Vapor Pressure

• At any temperature, some molecules in a liquid have enough energy to escape.

• As the temperature rises, the fraction of molecules that have enough energy to escape increases.

Vapor Pressure

As more molecules escape the liquid, the pressure they exert increases above the surface of the liquid

The liquid and vapor reach a state of dynamic equilibrium: liquid molecules evaporate and vapor molecules condense at the same rate.

Vapor Pressure

• The weaker the attractive forces, the larger the number of molecules that are able to escape and therefore the higher the vapor pressure– Liquids with only dispersion forces will have

the higher vapor pressures

• Vapor pressure increases with increasing temperature

Vapor Pressure

• The boiling point of a liquid is the temperature at which its vapor pressure equals atmospheric pressure.

• The normal boiling point is the temperature at which its vapor pressure is 760 torr.

Boiling Point• The stronger the

intermolecular forces, the higher the boiling points of the liquid– Hydrogen bonding is

the strongest intermolecular force

– Dipole-dipole– Dispersion (weakest

intermolecular force)

Volumes of mixing polar liquids

• When two polar liquids are mixed, the molecules of the liquids can go into the other liquid's available spaces.  They can do this because they are both polar and can mix freely.  Because the molecules of the two liquids are different sizes, their resulting intermolecular lattice can be better arranged to conserve more space.  The mixture of the two liquids takes up less volume than the two liquids did separately.

Volumes of mixing polar and nonpolar liquids

• Since the two molecules have different intermolecular forces, they do not readily mix so the nonpolar liquid will not mix with the polar compound and two distinct bands of the liquids will form and their original volumes will be additive– Immiscible (does not mix)