Why does liquids flow

This is an example of a chaotic or turbulent water flow, which is much harder to control. Cleanup Dry off your workspace, clean the glass and recycle your box if you can. Already a subscriber? Sign in. Thanks for reading Scientific American. Create your free account or Sign in to continue. See Subscription Options. Go Paperless with Digital. Materials Empty rectangular soup or milk box such as shelf-stable soy milk with a spout on top Water Marker Glass or cup to pour into Paper towels or rag to dry up spills A workspace that can tolerate water spills Paper and pen optional Preparation The box has six sides: one on the top with the spout, one on the bottom and four side panels that connect the top and the bottom.

Write the letter A on the side panel farthest from the spout. Go around the box, lettering the next side panel B, then C and, finally, D.

Procedure Fill the box with water. The water level should be about one half inch from the top. Pay attention. Which side panel do you grip while you pour? Is the flow fast or slow?

Does the water flow fluently? Do you think there are other ways to pour? If so, would those work better, as well or worse? If you poured quite slowly, repeat the previous steps but pour faster and see how this changes how the water flows. Empty the glass and add more water to the box so it is filled to the same level as before. Pour some water rather quickly into the glass, but this time hold the box by a different side panel.

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? The power of flowing water can be used to turn wheels to drive machinery and even create electricity.

Fast-moving liquids, such as tidal waves, can also cause a lot of damage. The forces between liquid particles are weaker than the forces between solid particles.

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. These are just three examples of a highly diverse state of matter: liquids.

One of the key defining properties of liquids is their ability to flow. Beyond this feature, though, the behaviors of different liquids span a broad range. Some liquids flow relatively easily, like water or oil, while others, like honey or molasses, flow quite slowly. 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.

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?