This means that liquid particles are further apart and can move about more easily. Since the particles can move, the liquid can flow and take the shape of its container. Some insects, such as pond-skaters, are able to walk on water without sinking. This is because the forces of attraction between the water particles pull the particles at the surface together.
This creates a tension, called surface tension, that makes the water surface behave as if an invisible, stretchy skin covers it. Mercury is a liquid metal that is poisonous. When mercury is dropped onto a surface, it rolls off in little balls. This is because the forces between the mercury particles are very strong, so the particles clump together. This force between particles of the same type is called cohesion. Water particles do not have such strong cohesion, so they wet surfaces.
A measure of how fast or slowly a liquid can flow is its viscosity. See Figure 1 for an illustration of the various intermolecular forces and interactions. Contrast that with a solid , in which the intermolecular forces are so strong that they allow very little movement. While molecules may vibrate in a solid, they are essentially locked into a rigid structure, as described in the Properties of Solids module.
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. At a molecular level, liquids have some properties of gases and some of solids. First, liquids share the ability to flow with gases. Both liquid and gas phases are fluid , meaning that the intermolecular forces allow the molecules to move around.
Solids are not fluid , but liquids share a different important property with them. Figure 2 shows the differences of gases, liquids, and solids at the atomic level. Most substances can move between the solid , liquid , and gas phases when the temperature is changed. These transitions occur because temperature affects the intermolecular attraction between molecules.
However, the intramolecular forces that hold the H 2 0 molecule together are unchanged; H 2 0 is still H 2 0, regardless of its state of matter. You can read more about phase transitions in the States of Matter module. First, though, we need to briefly introduce the different types of intermolecular forces that dictate how liquids, and other states of matter , behave.
As we described earlier, intermolecular forces are attractive or repulsive forces between molecules , distinct from the intramolecular forces that hold molecules together. Intramolecular forces do, however, play a role in determining the types of intermolecular forces that can form. Intermolecular forces come in a range of varieties, but the overall idea is the same for all of them: A charge within one molecule interacts with a charge in another molecule.
Depending on which intramolecular forces, such as polar covalent bonds or nonpolar covalent bonds , are present, the charges can have varying permanence and strengths, allowing for different types of intermolecular forces.
So, where do these charges come from? In some cases, molecules are held together by polar covalent bonds — which means that the electrons are not evenly distributed between the bonded atoms.
This type of bonding is described in more detail in the Chemical Bonding module. This uneven distribution results in a partial charge: The atom with more electron affinity, that is, the more electronegative atom, has a partial negative charge, and the atom with less electron affinity, the less electronegative atom, has a partial positive charge.
This uneven electron sharing is called a dipole. When two molecules with polar covalent bonds are near each other, they can form favorable interactions if the partial charges align appropriately, as shown in Figure 3, forming a dipole-dipole interaction. Hydrogen bonds are a particularly strong type of dipole-dipole interaction. Hydrogen bonds occur when a hydrogen atom is covalently bonded to one of a few non-metals with high electronegativity , including oxygen, nitrogen, and fluorine, creating a strong dipole.
The hydrogen bond is the interaction of the hydrogen from one of these molecules and the more electronegative atom in another molecule. Hydrogen bonds are present, and very important, in water, and are described in more detail in our Water: Properties and Behavior module. Hydrogen bonds and dipole-dipole interactions require polar bonds, but another type of intermolecular force , called London dispersion forces , can form between any molecules , polar or not.
The basic idea is that the electrons in any molecule are constantly moving around and sometimes, just by chance, the electrons can end up distributed unequally, creating a temporary partial negative charge on the part of the molecule with more electrons. This partial negative charge is balanced by a partial positive charge of equal magnitude on the part of the molecule with fewer electrons, with the positive charge coming from the protons in the nucleus Figure 4.
These temporary partial charges in neighboring molecules can interact in much the same way that permanent dipoles interact. The overall strength of London dispersion forces depends on the size of the molecules: larger molecules can have larger temporary dipoles, leading to stronger London dispersion forces.
Now, you might ask, if molecules can develop temporary partial charges that interact with each other, these temporary charges should also be able to interact with permanent dipoles , right?
And you would be correct. These interactions are called, very creatively, dipole-induced dipole interactions. As you might have guessed, London dispersion forces and dipole-induced dipole interactions are generally weaker than dipole-dipole interactions.
These forces , as well as hydrogen bonds , are all van der Waals forces , which is a general term for attractive forces between uncharged molecules. The primary intermolecular forces present in most oils and many other organic liquids — liquids made predominantly of carbon and hydrogen atoms , also referred to as non-polar liquids — are London dispersion forces , which for small molecules are the weakest types of intermolecular forces.
These weak forces lead to low cohesion. On the other end of the cohesion spectrum , consider a dewdrop on a leaf in the early morning Figure 6. How can such a thing exist if, as explained earlier, liquids flow and take the shape of the container holding them? As described above and in the Water module, water molecules are held together by strong hydrogen bonds.
These strong forces lead to high cohesion: The water molecules interact with each other more strongly than they interact with the air or the leaf itself. This high cohesion also creates surface tension. Surface tension results from the strong cohesive forces of some liquids. These forces are strong enough to be maintained even when they experience external forces like the gravity of an insect walking across its surface. Adhesion is the tendency of a compound to interact with another compound.
Remember that, in contrast, cohesion is the tendency of a compound to interact with itself. Cohesive forces are greatest beneath the surface of the liquid, where the particles are attracted to each other on all sides. Particles at the surface are more strongly attracted to the identical particles within the liquid than they are to the surrounding air.
This accounts for the tendency of liquids to form spheres, the shape with the least amount of surface area. When these liquid spheres are distorted by gravity, they form the classic raindrop shape. Adhesion is when forces of attraction exist between different types of particles. Particles of a liquid will not only be attracted to one another, but they are generally attracted to the particles that make up the container holding the liquid. Particles of the liquid are drawn up above the surface level of the liquid at the edges where they are in contact with the sides of the container.
The combination of cohesive and adhesive forces means that a slight concave curve, known as the meniscus, exists at the surface of most liquids. The most accurate measurement of the volume of a liquid in a graduated cylinder will be observed by looking at the volume marks closest to the bottom of this meniscus.
Adhesion also accounts for capillary action when a liquid is drawn up into a very narrow tube. One example of capillary action is when someone collects a sample of blood by touching a tiny glass tube to the blood droplet on the tip of a pricked finger.
Viscosity is a measure of how much a liquid resists flowing freely. A liquid that flows very slowly is said to be more viscous than a liquid that flows easily and quickly.
A substance with low viscosity is considered to be thinner than a substance with higher viscosity, which is usually thought of as being thicker. For example, honey is more viscous than water. Honey is thicker than water and flows more slowly.
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