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Unformatted text preview: Chem 400 Properties of Liquids and Phase Changes Properties of Liquids • You already know some of the properties of liquids: fixed volume, but no fixed shape. • But there are several important properties of liquids which you need to know (and probably have already heard of). • Viscosity : this is the resistance to flow which a liquid has. The higher the viscosity, or the more viscous a liquid is, the slower and more sluggishly it flows. (think of your ketchup or honey or molasses) • Why are some liquids more viscous than others? Intermolecular forces! • The stronger the intermolecular forces, the more the molecules are “glued” together, so they resist flowing more. • After all, to flow freely implies that the molecules “slide” past each other. • They can’t “slide” if they are “glued” together. • Surface Tension : this is a liquid’s resistance to increasing its surface area. • We can think of it as the tendency of a liquid to “bead” up or not spread. • Mercury and water are 2 liquids which bead up or have a high surface tension. • Again, high intermolecular forces “glue” molecules (or atoms) together, so they resist spreading. • The atoms or molecules on the surface of the liquid are strongly attracted to the inner portion of the liquid, which helps “hold” a bead together, or keep the liquid from spreading. • Capillary Action : Now we have 2 forces: the atoms or molecules of a liquid are holding them together, but the atoms or molecules are also attracted to the surface of the container. So the molecules of a liquid tend to “creep” up the sides of the container. • This is what causes the meniscus that you know and love when you pour water into a glass container. • In glass containers, water molecules are strongly attracted to the glass sides and so “creep” up the sides. But water molecules not along the container walls don’t feel this attraction, and so undergo the normal surface tension force. • So we see a curved meniscus, where capillary action is stronger than the surface tension. Phase Changes • We’ve already talked about the 6 phase changes and their signs for Δ S and Δ H. • Because of the very high entropy and energy of the gas phase, the enthalpy and entropy changes are very large for any transition which involves the gas phase....
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- Fall '06
- pH, Vapor pressure, Phase transition, Hvap