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Next to gasses, liquids are probably the most things we take for granted in this world. We waste water as if it's an infinite source, but we put no though at all on how much energy is required to bring them into our rivers and taps. Liquids require a lot of energy to manage and manipulate, and this is due to their chemistry and physics.
May they be water, petrol, or cooking oil, the nature of liquids is a cornerstone to understanding how our world works. |
"Don't get set into one form, adapt it and build your own, and let it grow. Empty your mind, be formless, shapeless — like water. Now you put water in a cup, it becomes the cup; You put water into a bottle it becomes the bottle; You put it in a teapot it becomes the teapot. Now water can flow or it can crash. Be water, my friend."
Bruce Lee, 'Bruce Lee: A Warrior's Journey' (2000)
Liquids have some intermolecular bonding, but it isn't as strong as bonding in a solid. As a result, the molecules are close together, but they are not tightly packed. Instead, they are free to slide past each other. Liquids occur at temperatures above the melting point of a substance, but below its boiling point. At the melting point, the molecules are moving slow enough to form rigid bonds and become a solid. At the boiling point, the molecules are moving so fast that they cannot form any bonds and become a gas.
Liquids have definite volume, but indefinite shape. They are free to form droplets and puddles when they are not inside a container. When a liquid is inside a container, it will take its shape. Unlike gases, a liquid will not change its volume to spread out and completely fill a container. There is enough intermolecular bonding to give liquids a definite volume.
Liquids are fluid, able to flow and take any shape. This occurs due to the weak intermolecular bonding that allows the molecules to slide past each other freely. As a result of being fluid, liquids exhibit many interesting properties that solids do not, including capillary action and diffusion.
Liquids, like gases, undergo diffusion when mixed. This can be seen by adding food coloring to water. Different liquids, when added, will chaotically spread out and mix together. Diffusion will occur faster when the liquid is warmer because the increased kinetic energy allows the molecules to move faster and collide more frequently.
Liquids are usually considered incompressible. The molecules are already close together, so it is difficult to compress them any more. Under very high pressures, liquids will actually compress, but not very much.
Liquids, unlike gases, have a distinct surface—they need not take their container's shape. This allows the formation of droplets and puddles.
Liquids have definite volume, but indefinite shape. They are free to form droplets and puddles when they are not inside a container. When a liquid is inside a container, it will take its shape. Unlike gases, a liquid will not change its volume to spread out and completely fill a container. There is enough intermolecular bonding to give liquids a definite volume.
Liquids are fluid, able to flow and take any shape. This occurs due to the weak intermolecular bonding that allows the molecules to slide past each other freely. As a result of being fluid, liquids exhibit many interesting properties that solids do not, including capillary action and diffusion.
Liquids, like gases, undergo diffusion when mixed. This can be seen by adding food coloring to water. Different liquids, when added, will chaotically spread out and mix together. Diffusion will occur faster when the liquid is warmer because the increased kinetic energy allows the molecules to move faster and collide more frequently.
Liquids are usually considered incompressible. The molecules are already close together, so it is difficult to compress them any more. Under very high pressures, liquids will actually compress, but not very much.
Liquids, unlike gases, have a distinct surface—they need not take their container's shape. This allows the formation of droplets and puddles.
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Cohesion and Adhesion
The molecules of a liquid are attracted to each other. This is called cohesion. Molecules like methane are non-polar, so they are held together only by van der Waals forces (the weakest). These molecules will have minimal cohesion. In contrast, water molecules use hydrogen bonding (very strong), so they display strong cohesion. A cohesive liquid will form more spherical droplets and have much higher surface tension (explained below). Adhesion is the attraction of a liquid molecule to its surroundings. Adhesive liquids will demonstrate capillary action (explained below). They are also more "wet". Mercury is very cohesive, but not adhesive. As a result, it doesn't leave behind residue as it rolls across a surface. Water, on the other hand, is much more adhesive. When water rolls across a surface, it wets that surface because some of the molecules adhere to it. Though there are surfaces that resist adhesion, such as the hydrophobic surface of a taro (keladi) leaf. |
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Surface Tension and Capillary Action
Capillary action, or capillarity, is the ability of a liquid to flow in narrow spaces without the assistance of, and in opposition to, external forces like gravity. The effect can be seen in the drawing-up of liquids between the hairs of a paint-brush, in a thin tube, in porous materials such as paper, in some non-porous materials such as liquified carbon fiber, and in a cell. It occurs because of intermolecular attractive forces between the liquid and solid surrounding surfaces. If the diameter of the tube is sufficiently small, then the combination of surface tension (which is caused by cohesion within the liquid) and adhesive forces between the liquid and the container act to lift the liquid. With some pairs of materials, such as mercury and glass (right), the intermolecular forces within the liquid exceed those between the solid and the liquid, so a convex meniscus forms, and capillary action works in reverse. |
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Pressure
Liquids will distribute pressure evenly. This concept, known as Pascal's Law, is crucial for equipment like hydraulic brakes. It is a result of their incompressibility.
Liquids will distribute pressure evenly. This concept, known as Pascal's Law, is crucial for equipment like hydraulic brakes. It is a result of their incompressibility.
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Liquids will evaporate. Although the average kinetic energy of the molecules is too low to overcome bonding and become a gas, individual molecules will occasionally have above-average energy and break free from the surface of the liquid. The molecule then escapes to the gas phase. At the same time, however, a gas molecule may hit the surface of the liquid and slow down enough to join the liquid. A glass of water left outside in the sun will eventually become empty. The sunlight adds energy to the molecules, allowing some to escape as a gas. Eventually, all molecules will escape. The tendency of a liquid to evaporate depends on its intermolecular forces. Volatile liquids tend to evaporate quickly have relatively weak intermolecular forces keeping the molecules together, making it easier for them to escape the liquid phase. Conversely, non-volatile liquids do not evaporate to any visible extent due to the presence very strong intermolecular forces.
The evaporation increases with temperature. It can be measured by vapor pressure, the amount of pressure exerted by the evaporated gas above the liquid's surface. Vapor pressure increases with temperature, and once it reaches the pressure of the surrounding atmosphere, the liquid will boil. Vapor pressure also depends on the intensity of intermolecular forces in the liquid. Heron's fountain (left) is a hydraulic machine invented by the 1st century AD inventor, mathematician, and physicist Heron of Alexandria that utilises hydraulic and atmospheric pressures to produce work. |
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Viscosity
Viscosity refers to the liquid's resistance to flow. For example, maple syrup has a relatively high viscosity when compared to water because maple syrup flows much slower than water, which flows relatively quickly and easily. The difference in viscosity between these two liquids is due to the attractive forces within the specific liquid. In order to flow, molecules must roll and move over each other. A solution with low attractive forces would allow the molecules to move in a more free and easy manner, decreasing the viscosity. In most cases, the viscosity of a liquid decreases as the temperature of a liquid is increased. Increasing the temperature of a liquid causes the molecules to have a higher kinetic energy. This increase in kinetic energy breaks down the intermolecular forces present in the liquid. Since viscosity is dependent on these attractive forces, the viscosity will decrease when the kinetic energy is increased. A notable experiment is the University of Queensland's pitch drop experiment (right), featuring its then-current custodian, Professor John Mainstone. |
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Ponder this
Why does mercury have such a low melting point, while other metals require higher temperature to change their state of matter?
Supercooled liquid helium is said to have "zero viscosity", what does that mean?
Discuss
Discuss the properties of different type of liquids, their compositions, whether they are compounds or mixtures, and how these affect their properties in relation to surface tension, capillary action, behaviors under pressure or in motion. Find ways to manipulate these liquids to change their properties - for instance diluting them, or better yet, without diluting them.
Further readings
Liquids, for a more comprehensive explanation on the nature and properties of liquids.
Surface tension and capillary action, are tied to the forces between individual molecules of the fluid.
Hydrostatics and hydrodynamics, topics related to the properties of liquids at rest and in motion.
Viscosity, related to surface tension, however this is in relation to the liquid itself without any external factors.
