1. Properties of Liquids

2. Hold your cursor over the image to see molecular state of the underlying material.

6. Properties of Liquids and KMT
Kinetic-Molecular Theory (moving particle)
Fixed Volume and flexible shape
Smaller velocity  Lower kinetic energy
Due to intermolecular forces
Rate of diffusion  slower
What happen when we increase temperature?
Temp , Velocity , therefore, Diffusion 

7. Properties of Liquids Particles are much closer together
Why: Attractive forces are more effective
How: Dipole-dipole, London dispersion (instant dipole) and hydrogen bonding
So? Relatively high density (more mass in a specific volume) compared to gas
Denser than solid (exception: Water)
And? Relative incompressibility

8. Properties of Liquids
More ordered than gases due to strong intermolecular forces
Particles are not bound in fixed positions
Constantly moving  Fluid (ability to flow)
Most liquid flow downhill due to gravity
Few do flow uphill (unusual)
Example: liquid Helium near absolute zero

9. Intermolecular Forces and the Properties of Liquids
In summary, intermolecular forces play a large role in many of the physical properties of liquids and gases. These include:
vapor pressure
boiling point
surface tension
capillary action
viscosity 2

10. Intermolecular Forces and the vapor pressure
The vapor pressure of a liquid depends on intermolecular forces.
When the intermolecular forces in a liquid are strong, you expect the vapor pressure to be low. 2

11. Intermolecular Forces and the normal boiling point
The normal boiling point is related to vapor pressure and is lowest for liquids with the weakest intermolecular forces.
When intermolecular forces are weak, little energy is required to overcome them.
Consequently, boiling points are low for such compounds. 2

12. Intermolecular Forces and the surface tension
Surface tension increases with increasing intermolecular forces.
Surface tension is the energy needed to increase the surface area of a liquid.
To increase surface area, it is necessary to pull molecules apart against the intermolecular forces of attraction. 2

13. Surface Tension
How are surface particles different from internal particles with respect to attractive forces?

14. Surface Tension
The resistance of a liquid to an increase in its surface area.

15. Surface Tension
Attraction of surface
molecules that cause the liquid
surface to contract and become
more spherical
2.Amount of energy required to
stretch or increase the surface

16. Surface Tension Common to ALL liquids
A force tends to pull adjacent parts of a liquid’s surface together
Directly related to the force of attraction between particles
Decreasing surface area to the smallest possible size
Liquid droplets are spherical
Reason: Least surface area

17. Consequences of Surface Tension
Chemistry; The Science in Context; by Thomas R Gilbert, Rein V. Kirss, and Geoffrey Davies, Norton Publisher, 2004, p 457

18. Consequences of Surface Tension
Chemistry; The Science in Context; by Thomas R Gilbert, Rein V. Kirss, and Geoffrey Davies, Norton Publisher, 2004, p 458

19. 19

22. Capillary action
To understand how capillary action works, we must first differentiate between cohesion and adhesion.
Cohesive forsce: intermolecular forces that bind similar molecules to one another, such as the hydrogen bonding in water, are called cohesive forces.
Adhesive forces: intermolecular forces that bind a substance to a surface are called adhesive forces.

23. Cohesion and Adhesion
Cohesion is the intermolecular attraction between like molecules
Adhesion is an attraction between unlike molecules
Adhesion
Cohesion

24. Cohesion/Adhesion Why is this so?
In mercury the cohesive forces are stronger than the adhesive forces to glass.

25. Cohesion/Adhesion
Water tends to “wet” glass because the adhesive forces between the liquid molecules and the glass molecules are greater than the cohesive forces between the liquid molecules.
Water beads up on a wax surface because the cohesive forces between the water molecules are greater than the adhesive forces between the water molecules and the wax molecules.

26. Concave Meniscus of Water
Chemistry; The Science in Context; by Thomas R Gilbert, Rein V. Kirss, and Geoffrey Davies, Norton Publisher, 2004, p 458

27. Capillary action
Attraction of the surface of a liquid to the surface of a solid
Pull liquid molecules upward along the surface against the pull of gravity
Why so important?
Transportation of water from roots of a plant to its leaves in the opposite direction to gravity

28. Capillary action
Capillary action: the spontaneous rising of a liquid up a very narrow tube.

29. Capillary action
Capillary action ; Rise of Liquid up a Very Small Diameter Glass Tube
Chemistry; The Science in Context;
by Thomas R Gilbert, Rein V. Kirss,
and Geoffrey Davies, Norton
Publisher, 2004, p 459

30. Capillary action
A liquid rises in a narrow tube because of capillary action, a result of surface tension and adhesive forces. When the adhesive forces are greater than the cohesive forces, the liquid will rise up the tube.
Adhesive forces
Weight of the water column

31. Capillary action
When cohesive forces between molecules of the liquid exceed adhesive forces, the liquid level in the capillary tube is below the surface of the surrounding fluid.

32. Capillary action The liquid will rise highest in small tubes.
The height h to which capillary action will lift water depends upon the weight of water which the surface tension will lift.

33. Capillary action
When these forces balance the water will stop rising.

34. Capillary action
Hot wax is pulled up the wick by capillary action. It is the wax that burns, not the wick.

35. Capillary action
Water rises in plants and trees by capillary action.

37. Viscosity The resistance of a liquid to flow is called its viscosity.
The greater a liquid’s viscosity, the more slowly it flows.
Viscosity is related to the ease with which individual molecules of the liquid can move with respect to one another.
It depends on the attractive forces between molecules and on whether structural features exist that cause the molecules to become entangled.

38. Viscosity Viscosity decreases with increasing temperature.
Because at higher temperatures the greater average kinetic energy of the molecules more easily overcomes the attractive forces between molecules.

39. Viscosity Viscosity – Resistance to Flow
Increase Intermolecular Force; Increase Viscosity
Increase Temperature; Decrease Intermolecular Force; Decrease Viscosity

40. Intermolecular Forces and the viscosity
Viscosity increases with increasing intermolecular forces because increasing these forces increases the resistance to flow.
Other factors, such as the possibility of molecules tangling together, affect viscosity.
Liquids with long molecules that tangle together are expected to have high viscosities. 2

41. Size and Viscosity
Which molecule would be easier to pour from a bottle?
Which would experience more friction?
How does that affect viscosity?
When looking at the molecules on the slide it is easy to see that the oil molecule is much more likely to become entangled with other oil molecules when being poured. This is vital to viscosity. Consider the shape of these two molecules. The oil molecule is quite long and somewhat thick. The water molecule in comparison is compact and small. Will they actually pour? Experience tells us yes. Being poured is pretty much like rolling downhill. The molecules bump into one another as they tumble out of the bottle. If we think of the oil as being shaped close to a big thick tree and the water like a almost round boulder, which one would roll downhill easier? The boulder. Why? The big heavy tree would experience more friction. Now imagine billions of big oil molecules trying to roll down at the same time – Log Jam, lots of friction and the process would take a while. In other words, it would resist flow. A bunch of roundish boulders on the other hand would make for a pretty easy roll downhill and in fact landslides and avalanches are a real problem in some parts of the world. The boulders would resist the flow much less than logs with big branches on them.

42. Size and Viscosity
When looking at the molecules on the slide it is easy to see that the oil molecule is much more likely to become entangled with other oil molecules when being poured. This is vital to viscosity.
Consider the shape of these two molecules. The oil molecule is quite long and somewhat thick. The water molecule in comparison is compact and small. Will they actually pour? Experience tells us yes.
Being poured is pretty much like rolling downhill. The molecules bump into one another as they tumble out of the bottle.

43. Size and Viscosity
If we think of the oil as being shaped close to a big thick tree and the water like a almost round boulder, which one would roll downhill easier? The boulder. Why?
The big heavy tree would experience more friction. Now imagine billions of big oil molecules trying to roll down at the same time – Log Jam, lots of friction and the process would take a while. In other words, it would resist flow.
A bunch of roundish boulders on the other hand would make for a pretty easy roll downhill and in fact landslides and avalanches are a real problem in some parts of the world. The boulders would resist the flow much less than logs with big branches on them.

44. Increasing Strength of Intermolecular Force
Higher Boiling Point
Lower Vapor Pressure
Higher Surface Tension
Higher Viscosity

45. REFERENCES