Some are slippery, and some are sticky. Where do these different behaviors come from? When it comes to interactions between different liquids , some mix well: Think of a Shirley Temple, made of ginger ale and grenadine. Consider oil spills, where the oil floats in a sticky, iridescent layer on top of the water.
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. These varied behaviors arise primarily from the different types of intermolecular forces that are present in liquids.
Liquids flow because the intermolecular forces between molecules are weak enough to allow the molecules to move around relative to one another. Intermolecular forces are the forces between neighboring molecules. 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. In liquids , the intermolecular forces can shift between molecules and allow them to move past one another and flow.
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. This cohesive "stickiness" accounts for the surface tension of a liquid.
Surface tension can be thought of as a very thin "skin" of particles that are more strongly attracted to each other than they are to the particles surrounding them. As long as these forces of attraction are undisturbed, they can be surprisingly strong. For example, the surface tension of water is great enough to support the weight of an insect such as a water skipper.
Water is the most cohesive nonmetallic liquid, according to the U. Geological Survey. 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. 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. Crude oil, for example, is a liquid that does not flow very easily. Is this easier or harder compared with the way you usually do it? How does the water flow when you do it this way? Repeat two more times, each time holding the box by a different side panel.
Rank the ways you poured water from most to least preferred and from a laminar or fluent water flow to a turbulent or chaotic one.
If you have paper and a pen, write down your ranking. Repeat the whole activity two more times. Pay attention to how far you tilt the box before water pours out and whether or not the spout is fully covered with water. Do you get the same ranking each time? Can you explain why the water flows differently depending on which side you hold the box to pour? Extra: Do you think your findings are still true if the box is half full or nearly empty?
Try it and see if your prediction was correct! Extra: Test whether your findings are the same if you pour really slowly. Extra: Repeat the activity after making a hole in the top panel of the box. Keep using the spout to pour. How does this change your rankings? Did it make pouring easier? Extra: Test out different water bottles. Do some show turbulent flow when water is poured quickly? Some water bottles have a small hole near the spout in the lid—why would this be?
Why does liquid not flow out from this small hole? Build a Cooler. Holes That Do Not Leak! Lift a Large Load Using Liquids.
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