These transitions occur because temperature affects the intermolecular attraction between molecules. Consider the molecule H 20: It takes the form of ice, a crystalline solid, below 0° C water, a liquid, between 0° and 100° C and water vapor, or steam, a gas, above 100° C. Most substances can move between the solid, liquid, and gas phases when the temperature is changed. (Figure 2 shows the differences of gases, liquids, and solids at the atomic level.)įigure 2: The three states of matter at the atomic level: solid, liquid, and gas. Liquids and solids are both held together by strong intermolecular forces and are much more dense than gases, leading to their description as “condensed matter” phases because they are both relatively incompressible. Solids are not fluid, but liquids share a different important property with them. In both of these phases, the materials don’t have fixed shapes and instead are shaped by the containers holding them. Both liquid and gas phases are fluid, meaning that the intermolecular forces allow the molecules to move around. First, liquids share the ability to flow with gases. At the other end of the spectrum are gases, in which the molecules are so far apart that the intermolecular forces are effectively nonexistent and the molecules are completely free to move and flow independently.Īt a molecular level, liquids have some properties of gases and some of solids. While molecules may vibrate in a solid, they are essentially locked into a rigid structure, as described in the Properties of Solids module. image ©UC Davis ChemWikiĬontrast that with a solid, in which the intermolecular forces are so strong that they allow very little movement. Panel B shows a sample liquid with several molecules both attracting and repulsing with their dipole-dipole interactions. In (c) and (d), repulsive interactions are shown with orientations that juxtapose the positive or negative ends of the dipoles on adjacent molecules. Attractive interactions are show in (a) and (b) with orientations where the positive end is near the negative end of another molecule. (See Figure 1 for an illustration of the various intermolecular forces and interactions.)įigure 1: Panel A shows the variety of attractive and repulsive dipole–dipole interactions. In liquids, the intermolecular forces can shift between molecules and allow them to move past one another and flow. (These are not to be confused with intramolecular forces, such as covalent and ionic bonds, which are the forces exerted within individual molecules to keep the atoms together.) The forces are attractive when a negative charge interacts with a nearby positive charge and repulsive when the neighboring charges are the same, either both positive or both negative. Intermolecular forces are the forces between neighboring molecules. Liquids flow because the intermolecular forces between molecules are weak enough to allow the molecules to move around relative to one another. Then we will go through a brief overview of intermolecular forces, and finally we’ll explore how intermolecular forces govern the way that liquids behave. In this module we’ll first discuss liquids in the context of the other two main states of matter, solids and gases. These varied behaviors arise primarily from the different types of intermolecular forces that are present in liquids. You may also notice a similar phenomenon in some salad dressings that separate into an oil layer that rests atop a layer of vinegar, which is primarily water. Consider oil spills, where the oil floats in a sticky, iridescent layer on top of the water. Others, though, don’t seem to mix at all. When it comes to interactions between different liquids, some mix well: Think of a Shirley Temple, made of ginger ale and grenadine. Where do these different behaviors come from? Some liquids flow relatively easily, like water or oil, while others, like honey or molasses, flow quite slowly. Beyond this feature, though, the behaviors of different liquids span a broad range. One of the key defining properties of liquids is their ability to flow. These are just three examples of a highly diverse state of matter: liquids. Intermolecular occurring between different molecules intramolecular occurring within a molecule polar having an electrical charge viscous thick not flowing easily
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