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States of Matter 1.15: Some Properties of Liquids Wiva k12 Chemistry

Terms in this set (19)

A liquid takes the shape of its container but may not fill it.
Liquids are a state of matter where the molecules are held together more tightly than in gases, but less tightly than in solids. Like gases, liquids can flow and take the shape of their containers, but they have a definite volume and might not fill the container. Their properties are determined by the interplay between their molecules' intermolecular forces and kinetic energies. In this lesson, you will learn the properties of liquids and how they can be explained by kinetic molecular theory.
A liquid takes the shape of its container but might not fill the container.
When you think of liquids, probably the first one that comes to mind is water. But other substances can be liquids at room temperature as well. For example, mercury is a liquid metal. Many oils are liquid at room temperature. Under the right conditions of temperature and pressure, gases can also condense into liquids. The water that condenses on the outside of a glass of ice water on a hot summer day comes from water vapor in the atmosphere.

Liquids are characterized by having a definite volume but no definite shape. They take the shape of their containers. However, unlike gases, liquids do not expand to fill the entire container.
Molecules are closer together in liquids than in gases but are not as mobile.
Gas molecules are far apart and move rapidly and randomly throughout their container, colliding with one another and with the walls of the container. By contrast, liquid molecules are much closer together. They contact and slide around one another. They both vibrate and move from place to place, but they move more slowly than gas molecules. Compared to solid molecules, liquid molecules are somewhat farther apart. While solid molecules are rigid and can only vibrate, liquid molecules can slide past one another and move along.

What holds the molecules of a liquid, or a solid, together? The molecules of any substance have kinetic energy. Molecules also have intermolecular forces. The phase of the substance is determined by how the strength of the intermolecular forces compares to the kinetic energy of the molecules.
The phase depends on the interplay between molecular forces and kinetic energy.
Consider what happens to the molecules of a substance as it is heated from a solid state.

-Solid molecules are close together, and the intermolecular forces (IF) are strong. The kinetic energy (KE) may be low, depending on the temperature. Therefore, IF >> KE, and the molecules only vibrate.
-As the solid is heated, molecules absorb energy and move faster (IF > KE). Molecules break free from one another, but IF still hold them close by. As a result, they slide past one another.
-As the liquid is heated, the molecules absorb more energy and move faster. Molecules break free because IF cannot hold them close (IF << KE).
What determines whether a substance will be a liquid at certain conditions?
Three factors determine whether a substance will be a liquid at any given temperature and pressure:

-Strength of intermolecular forces: Strong IF will draw the molecules close together and hold them there.
-Molecular mass: KE depends on mass and velocity. For the same KE, higher-mass molecules travel more slowly than lower-mass ones. Slow-moving molecules interact more than fast-moving ones do.
-Molecular shapes: Compact molecular shapes, such as tetrahedrons or linear shapes, will help the molecules pack closer together, thereby allowing stronger IF to hold them together.
The sparse spaces between liquid molecules allow them to flow.
Imagine a cup of liquid as a pickup truck with marbles filling the bed. The marbles are close together, but they can roll along one another. If you put down the back gate, the marbles will roll out. Liquid molecules act in much the same way. This molecular behavior allows liquids to flow.

Even as liquid molecules flow past one another, they exert IF that resist that flow. This resistance is called viscosity, and it is proportional to the strength of the IF. Examples of viscous liquids include syrup, honey, and motor oil. You can change the viscosity of a liquid by heating it. The added energy can break bonds involved in IF; hydrogen bonds are one example. You can see this change when you warm honey or syrup. Similarly, high heat from a car's engine makes the motor oil less viscous.
viscosity
a measure of a liquid´s tendency (or resistance) to flow
The sparse spaces between liquid molecules make the molecules only slightly compressible.
Because of the sparse spaces between molecules, liquids are only slightly compressible—and much less compressible than gases. Liquids transmit pressure; this property is used in hydraulic braking systems like those in a car.

Charles's law says that gases expand a lot when heated. By contrast, the IF between liquid molecules tend to hold them together and keep liquids from expanding very much when heated.
Intermolecular forces give liquids surface tension.
Have you ever noticed a water strider walking across the surface of the water? Maybe you have floated a paper clip on the surface of a glass of water. What holds the water strider and the paper clip up? Surface tension holds them up. It is a property of liquids, and in water it is especially high.

Here's how surface tension works. Any particle deep within a liquid has an equal number of molecules around it, and the IF of the molecules equally attract the particle. However, on the surface, the number of molecules surrounding a particle is not equal. The imbalance draws the liquid molecules closer together, forming a sort of film on the water.
Surface tension
the more rigid arrangements of bonds on the surface of a liquid that give the surface a more integrated arrangement than the rest of the liquid
High-energy liquid molecules escape into vapor phase.
If you pour a liquid, like water, into an empty container and seal it, you will find that some water molecules will enter the gas phase above the liquid. The gas molecules will actually exert a pressure called vapor pressure. This vapor pressure happens even when the liquid is not boiling.

Each molecule in a liquid has a KE. At any given time, some of those molecules have more KE than others. Those few liquid molecules with the highest KE can break the IF and move into the gas phase. As more molecules go into the gas phase, they exert a pressure back down on the surface of the liquid. Some of those vapor molecules slow down and condense back into liquid. Because the container is sealed, a steady state—or equilibrium—occurs, where the rate of vaporization equals the rate of condensation, and a steady vapor pressure is achieved. At higher temperatures, more liquid molecules have sufficient KE to go into the gas phase, thus increasing the vapor pressure.
Vapor pressure
the pressure exerted by a gas on a liquid in a closed system
Vapor pressure and boiling are closely related.
Vapor pressure increases with temperature until at some point the liquid boils.

You can increase the total pressure above a liquid artificially by putting a lid on the pot. In this case, the trapped water vapor exerts more pressure back on the liquid. Therefore, the temperature of the liquid must go higher for more molecules to escape. As a result, the liquid will boil at a higher temperature. This is the principle behind a pressure cooker. By contrast, if you reduce the total pressure above a liquid, as happens when you cook at high altitudes, then the liquid does not need to reach as high a temperature to boil.
Intermolecular forces explain many liquid properties.
Liquids have volume but no definite shape. The strength of intermolecular forces (IF) compared to kinetic energy (KE) determines the phase of a substance. For liquids, IF > KE. IF hold liquid molecules close together, but the molecules can slide past one another and move about.

The sparse spaces between liquid molecules allow the molecules to flow and compress only slightly. IF bind liquid molecules at a surface to form a film and support particles (surface tension). Liquid molecules with high KE can escape into the vapor phase and exert vapor pressure. When vapor pressure equals total pressure above it, the liquid boils.
Which of the following conditions occurs as a liquid freezes?
Strengths of intermolecular forces of the liquid molecules exceed the kinetic energy of the molecules.
Two pots are at the same atmospheric pressure, but one has a lid on it. The pressure above the liquid in pot A is 115 kPa, while the pressure above pot B is 102 kPa. Which pot will boil at a higher temperature?
Pot A will boil at a higher temperature.
The molecular masses of four substances are as follows: A = 16, B = 58, C = 86, D = 116. Which substance will have the lowest boiling point?
A
In terms of intermolecular forces, explain why atoms and molecules in a liquid move in random patterns.
vibration and some linear motion like sliding around
Which factor does not influence whether a substance will be a liquid at room temperature and normal atmospheric pressure?
surface tension
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