measured in several ways. For example, it can be measured as the time it takes for a certain quantity of liquid to flow through a narrow vertical tube. It can also be measured as the time it takes for a solid to fall through a given volume of the liquid. This is measured in poise units. The higher the number, the higher the viscosity. Strong intermolecular forces will increase the viscosity. Long flexible molecules will have higher viscosities. This is why long-chain hydrocarbons like certain motor oils have high viscosity. Hot liquids will have lower viscosity because of increased kinetic energy. The vapor pressure of a liquid is the pressure of a vapor when it is in equilibrium with its more condensed phases at a given temperature and in a closed system. The vapor pressure of a liquid will increase with temperature until the liquid boils. As mentioned, the normal boiling point of a liquid is that at 1 atmosphere of pressure. This is exactly 100 degrees Celsius with water. Of course, the actual boiling point is pressuredependent. Molecules of a liquid need a greater kinetic energy (higher temperature) to escape under higher pressure. This is how cooking in pressure cookers can be so successful.
LIQUID FORCES Liquid forces are more similar to that of solids than they are to gases. It’s the intermolecular force between the molecules that affects the properties of the liquid. These forces will be less than that seen in covalent bonding. Using water as an example, it takes 927 kJ of energy to break the covalent bond between a hydrogen ion and the hydroxyl ion but only about 41 kJ of energy to overcome intermolecular forces in order to convert liquid water to gaseous water. The melting/freezing points of solids and the boiling points of liquids are determined by intermolecular forces. These forces are primarily electrostatic in nature—between positively-charged and negatively-charged molecules. These forces fall off rapidly as intermolecular distance falls so they become more important between molecules in liquid and solid form versus those in gaseous form (except, of course, at high pressures). Ionic forces are referred to as Coulombic forces because they involve the phenomena of “like repels like” and “opposites attract”. These forces are very strong—even stronger
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