Properties of Liquids

Learning Goals

We cannot set up a model for liquids, but we can study some of their properties such as vaporization, condensation, vapor pressure, boiling point, and the critical temperature and pressure. We will also examine the concepts of surface tension, capillary action, and viscosity. After completing this section you should have a basic understanding of these fundamental properties of liquids.

Synopsis

Liquids are the most difficult states of matter to be studied because they do not possess the complete randomness of gases, nor do they possess the order of solids. The liquid state has some local order - areas where the molecules do exist in some order for a very short period of time, but that just makes the study even more difficult. We will not attempt to construct a model for this state, but we will study some of its physical properties.

Vaporization is the process of molecules in the liquid state going into the gaseous state. To make this transformation the molecules must possess enough kinetic energy to break the intermolecular forces holding them in the liquid state. For molecules to possess this kinetic energy, energy must be put into the liquid system and so this process is endothermic. This heat energy is often given as the molar enthalpy of vaporization, H_vap, and values for some compounds are given in Table 13.4. Note the correlation between the molar enthalpy of vaporization and the boiling point. Also note the increase in H_vap in going from the C2 alkane to the C4 alkane. As molecules become larger, the primary intermolecular forces involved are dispersion forces that increase as the size increases. Water has an exceptionally large H_vap, which means that the evaporation of water from the surface of your body will have a large cooling effect on your body.

If the system is closed so that the gas molecules cannot escape, some of the molecules will collide with each other, lose kinetic energy in the transfer, and return to the liquid state. This process is called condensation. Condensation is an exothermic process and the H_condensation is equal in magnitude but opposite in sign to H_vap.

If we put a liquid in a closed container, the liquid molecules will evaporate and the gas molecules will condense until we reach a state of dynamic equilibrium. In a dynamic equilibrium, the same number of molecules is constantly in the two states, but not the same molecules. We indicate such a dynamic equilibrium by the use of double arrows between the species. For our example of a liquid and a gas being in dynamic equilibrium, the number of molecules in each state obviously depends upon the temperature of the system. The pressure exerted by the gas molecules is called the equilibrium vapor pressure, or just the vapor pressure. Because the number and kinetic energy of molecules in the gas state depend upon the temperature, obviously the vapor pressure is temperature dependent.

The volatility of a molecule is the tendency of a liquid molecule to join the gas state. The more volatile the compound, the higher will be the vapor pressure. The mathematical relationship between the temperature and the vapor pressure was discovered by Clausius: ln P = -(H_vap /RT) + C. The lnP term is the natural logarithm (Appendix A.3) of the vapor pressure at the temperature T. C is a constant for each molecule. Note that this equation is of the type Y = ax + b, the equation of a straight line. So if we plot lnP vs. 1/T, we can determine H_vap. We usually do not use the equation in that form since we don't know C for most molecules. If we integrate the main equation using proper limits we get the more useful equation called the Clausius-Clapeyron equation:

ln(P₂/P₁) = H_vap/R(1/T₁ - 1/T₂)

If we have our liquid in an open container, the gas molecules are under the external pressure of the atmosphere. As we increase the temperature, more and more molecules will leave the liquid state to enter the gaseous state and the vapor pressure will increase. The temperature at which the vapor pressure equals the external pressure is called the boiling point. If the external pressure is 1 atm, the temperature is called the normal boiling point. Table 13.4 lists normal boiling points of some liquids. As we increase in altitude, the atmospheric pressure decreases, so the boiling point decreases. If we are cooking an egg by boiling it, the length of time to transfer sufficient energy to the egg to make it "done" will then vary with altitude.

Figure 13.19 shows the vapor pressure curves for three different molecules and their normal boiling points.

You might think that these curves just continue increasing, but they don't. The curves have a point at which they stop and that point, called the critical point, is different for every substance as shown in Figure 13.21.

The temperature at which the curve stops is called the critical temperature (T_C), and the corresponding pressure is called the critical pressure (P_C). The critical temperature and pressure for water is 374 ^oC and 217.7 atm, respectively. However, CO₂ has a critical temperature and pressure of 30.99 ^oC and 72.8 atm, which are not extreme conditions. Molecules at their T_C and P_C are called supercritical fluids, and they have very special properties that make them ideal for some applications. Because some supercritical fluids, such as carbon dioxide, are exceptionally good solvents, they are used to extract caffeine from coffee, for example. The density of a supercritical fluid is like a liquid, but its viscosity (ability to flow) is like a gas.

The molecules at a surface have a very unique arrangement in that the forces acting on them are not balanced. The molecules in the bulk (below the surface) of the liquid have the same number of molecules on all sides of them to balance all the forces. However, the molecules on the surface have no molecules above them as illustrated rather crudely below:

surface

This unique arrangement of the surface molecules leads to a force imbalance with a resulting force parallel to the surface of the liquid. This parallel force is called the surface tension and makes the surface of the liquid seem to have a protecting "force field". The presence of this surface tension allows you to float a needle on the surface of water, and allows some bugs to literally walk on water. As you slowly push some water out of a dropper, you will observe that the water maintains a curved shape as it comes out of the dropper. This curvature is also caused by the surface tension.

The intermolecular forces between the molecules of the liquid are called the cohesive forces. If the liquid comes in contact with another substance, such as a glass tube inserted in the liquid, the forces of attraction between the liquid and the substance are called adhesive forces. If the adhesive forces are stronger than the cohesive forces the liquid will rise in the tube. The movement of the liquid in the tube is called capillary action.

Review Question

1.) Discuss the variation in deltaH_vap and boiling points of the molecules NH₃, H₂O, and CH₄ based upon the intermolecular forces involved..

2.) Use the Clausius-Clapeyron equation to calculate the boiling point of water at vapor pressures of 1 atm and 0.8 atm.

3.) Use a medicine dropper to watch the different behaviors of water and cooking oil as you slowly force the liquids out of the dropper. Use a light behind the dropper to help you see the details. Discuss your observations and explanations for any differences observed on the WebCT bulletin board.