The Nature of Liquids

Properties of Liquids Definite volume Indefinite shape Particles are close together, but they can move a little bit… so liquids can flow. Density of liquids is much greater than gases. Ex: D H2O(l) is 1250X greater than D H2O(g) at 25  C. Liquids can be compressed… but change in volume is very slight and requires enormous pressure.

Viscosity Liquids exhibit viscosity.  Viscosity = resistance to flow Viscosity depends on  strength of intermolecular forces  sizes & shapes of molecules  temperature The stronger the intermolecular forces, the higher the viscosity.  As temperature , viscosity .  As temperature , viscosity .

Viscosity and Molecular Shape (a) Carbon tetrachloride, CCl 4, consists of small, symmetrical molecules with fairly weak intermolecular forces. It has a low viscosity. (b) Octadcane, C 18 H 38, consists of long molecules with fairly strong intermolecular forces. It has a relatively high viscosity.

Need a thin film of oil on bearing surfaces to prevent flaking of metal. If the oil is too thick, it won’t circulate at low temperatures. If the oil is too thin, it will lose film strength at high temperatures. Oil in an engine prevents direct metal to metal contact.

Where does the marble drop fastest? Slowest? Slowest? water glycerol

Surface Tension unbalanced Particles at the surface of a liquid exist in an unbalanced environment. No attraction from above to balance attractions from below. Net attractive force pulling down. Surface seeks smallest possible area.

Surface Tension  Energy required to increase the surface area by a given amount  Measure of inward pull Strong intermolecular attractions 

Surfactants Compounds that lower the surface tension of water Disrupt hydrogen bonds between H 2 O molecules. video

Illustration of Surface Tension

Capillary Action Water forms a concave meniscus in a glass tube. Attractive forces between water and glass greater than the attractive forces between the water molecules. capillary action Upward movement of a liquid in a narrow tube is called capillary action.

Cohesive and Adhesive Forces Cohesive forces are forces of attraction between molecules of a liquid. Adhesive forces are forces of attraction between molecules of a liquid and another substance.

Meniscus Force (Hg-glass)  Force (Hg-Hg) Force (H 2 O-glass)  Force (H 2 O- H 2 O)