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13.2: Liquids and Solids

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Lesson Objectives

  • Compare and contrast the properties of liquids and solids both at the macroscopic level and at the molecular level.
  • Explain the concept of viscosity and give examples of substances that are more viscous and less viscous than water.
  • Define surface tension and vapor pressure.
  • List the four types of solid crystals based on how the particles are bonded together, and give examples of each.

Lesson Vocabulary

  • viscosity: Measures a fluid’s resistance to flow.
  • surface tension: The amount of energy required to increase the surface area of a liquid.
  • adhesion: A liquid that displays more attraction between the liquid and the glass than attraction between the liquid particles.
  • cohesion: A liquid that that displays more attraction between the liquid particles than attraction between the liquid and the glass
  • vapor pressure: A measure of the pressure exerted by a gas above a liquid in a sealed container.
  • crystalline solids: Solid substances in which the arrangement of particles is highly ordered.

Check Your Understanding

  • Would particles in the solid, liquid, or gas phase have the greatest average kinetic energy?

Introduction

You encounter solids and liquids in many forms in your everyday life. Solids, unlike liquids, hold a definite shape. Both solids and liquids hold a definite volume. However, on a molecular level, these two states of matter are quite different. In this lesson we will introduce some of the properties of liquids and solids that affect your interactions with the substances all around you.

Liquids

In this image, you see small pools of elemental mercury. The Latin name for mercury is hydrargyrum, a compound word meaning “water-silver” (hydr- = water, -argyros = silver), since it is liquid like water but shiny like silver. Mercury is the only pure metal that exists as a liquid at room temperature.

Liquids have a definite volume, but no definite shape. A gallon of milk will keep the same volume whether it is stored in a one-gallon milk jug or a ten-gallon barrel. The volume is the same, but the fluid will either fill the jug or spread out over the bottom of the barrel.

Intermolecular Forces in Liquids

Molecules in the liquid state are held in close proximity to one another by intermolecular forces. Any molecule containing an –OH group (such as water and the various alcohols) will form hydrogen bonds with adjacent molecules, causing a group of molecules to take on the liquid state. A polar molecule like CH2Cl2 cannot form hydrogen bonds, but molecules are held together in the liquid state by dipole-dipole interactions. Br2 is not polar, but interactions with other Br2 molecules through dispersion forces cause Br2 to be a liquid at room temperature as well. Intermolecular forces are also responsible for various properties of liquids, such as viscosity, surface tension, and vapor pressure.

Viscosity

Viscosity measures a fluid’s resistance to flow – the higher the viscosity, the slower the flow of the material. One factor that affects viscosity is the strength of the intermolecular forces in the material. Molecules that exhibit higher intermolecular forces tend to have higher viscosities. Temperature also influences viscosity. A higher temperature results in a decrease in viscosity, since molecules are moving faster and the intermolecular forces are more easily disrupted. Most cooking oils are more viscous than water, but when heated, the viscosity decreases and the oil spreads more easily. Motor oils are ranked by viscosity. Lower viscosity oils provide less drag on the engine, but they are also used up faster than a higher-viscosity material.

Surface Tension

Surface tension is a physical property of liquids that is caused by the attraction of liquid molecules due to various intermolecular forces. This property is allows the surface of a liquid to resist an external force, and is a measure of the amount of energy required to increase the surface area of a liquid. Surface tension is basically a measure of the “pull” of surface molecules on one another.

Intermolecular forces on a molecule in the interior of a liquid are exerted in all directions. The forces on a surface molecule are more directional, causing the molecule to be pulled toward other surface molecules and down toward the interior of the liquid. This property has consequences both for the behavior of the liquid in contact with solids and the influence of temperature on the liquid.

A liquid that can interact with glass displays the property of adhesion. The attraction of the glass to the molecules is stronger than the attraction of the liquid molecules to each other. The liquid molecules are pulled up due to their stronger attraction to the glass. Water is one liquid that displays this property.

Some liquids, such as mercury, display more attraction between the liquid particles than attraction between the liquid and the glass. This is known as cohesion. The pull between the liquid particles is stronger, so the liquid pulls away from the glass.

Vapor Pressure

Vapor pressure is a measure of the pressure exerted by a gas above a liquid in a sealed container. While viscosity and surface tension increase as the strength of intermolecular forces increase, vapor pressure decreases. This is because the stronger the intermolecular forces are, the harder it is for liquid molecules to escape into the gas phase. Vapor pressure a measure of how much of a substance is in the gas phase when that substance is at equilibrium; that is, the number of molecules in the gas and liquid phases are not changing. The more molecules that are able to escape into the gas phase, the higher the vapor pressure. Vapor pressure is also proportional to temperature. As temperature increases, vapor pressure also increases.

Solids

Solids are materials that have both a defined shape and a defined volume. They do not take on the shape of their container, as liquids and gases do. Solids can be either amorphous or crystalline. Amorphous solids (such as glass) do not have a well organized three-dimensional arrangement of molecules or atoms, so they lack a high level of order. On the other hand, crystalline solids display a highly ordered and predictable three-dimensional structure. In this section we will discuss the different types of crystalline solids.

Types of Solids

Solids can be categorized by the forces holding the individual atoms, ions, or molecules together. The Table below summarizes the properties of different solids:

Type Connecting Forces Properties Examples
ionic ionic bonds brittle, high melting points, poor conductors NaCl, LiBr
molecular intermolecular forces soft, low melting points, poor conductors water, glucose
covalent covalent bonds hard, high melting points, poor conductors diamond, quartz
metallic metallic bonds variable hardness and melting points, good conductors Cu, Fe, other metals

Ionic crystals have very strong interactions between individual particles, due to the electrostatic attraction between oppositely charged ions. This is reflected in the very high melting points of most ionic solids. The shape of the solid depends upon the relative sizes of the ions and the ratio of cations to anions. Although ionic substances are poor conductors in the solid state, the ions become mobile upon melting, and the resulting liquid is a very good conductor of electricity.

Molecular solids are held together by weak dispersion forces or other polar interactions. Because these are based on attractions between partial charges, they are not as strong as ionic forces. Due to the weaker forces involved, this class of solids will melt at much lower temperatures than ionic crystals.

Covalent solids are composed of atoms connected by covalent bonds in a three-dimensional network. This extensive series of strong connections produces a very stable structure that is not affected much by changes in temperature. The hardness and stability of diamond is a reflection of the many strong carbon-carbon bonds that form its covalent network.

Metallic solids are composed entirely of metallic atoms. In other solid structures, the electrons involved in bonding tend to be localized, or fixed in place in the covalent bonds. However, the electrons in metallic bonds are delocalized over the entire crystal. The fact that the electrons are free to move between different atoms causes metallic solids to be very good conductors of electricity.

Crystalline solids can also be characterized by their shape. The Table below illustrates some of the common crystal forms. The crystal shapes are determined by the type and arrangement of the individual atoms, ions, or molecules from which they are constructed.

Seven Basic Crystal Systems
Crystal System Diagram

Cubic

a = b = c; α = β = γ = 90°

Tetragonal

a = b ≠ c; α = β = γ = 90°

Orthorhombic

a ≠ b ≠ c; α = β = γ = 90°

Monoclinic

a ≠ b ≠ c; α ≠ 90° = β = γ

Rhombohedral

a = b = c; α = β = γ ≠ 90°

Triclinic

a ≠ b ≠ c; α ≠ β ≠ γ ≠ 90°

Hexagonal

a = b ≠ c; α = β = 90°, γ = 120°

Lesson Summary

  • At the molecular level, the major difference between solids and liquids is based on how freely the particles can move within the substance.
  • Viscosity measures a liquid's resistance to flow.
  • Surface tension results from the directional pull on surface molecules towards the interior of a liquid.
  • Adhesion and cohesion are demonstrated by liquids as they interact with the surface of a container.
  • Vapor pressure is the pressure resulting from the gas molecules above a liquid in a closed container at equilibrium.
  • Ionic, molecular, covalent, and metallic solids are characterized by the types of bonds holding the substance's particles together in the solid form.

Lesson Review Questions

  1. Define viscosity. Give an example of how temperature influences viscosity.
  2. Define surface tension. Would you expect water to have a stronger or weaker surface tension than olive oil?
  3. Draw a picture of a liquid displaying adhesion and a liquid displaying cohesion in a glass jar.
  4. What does vapor pressure measures? Does vapor pressure increase or decrease as intermolecular forces when intermolecular forces grow stronger?
  5. What distinguishes crystalline and amorphous solids?
  6. List and define the four different types of crystalline solids.
  7. Order the four types of crystalline solids from lowest to highest melting point.
  8. Research to find one example for each type of crystalline solid, different from those provided in the text.

Further Reading / Supplemental Links

Points to Consider

  • In this lesson, we studied the differences between liquids and solids at the molecular level. As we will see, these differences can be accounted for by looking at the amount of energy within a given system.
  • How do you suppose pressure might affect particles at the molecular level in terms of the phase that a given substance exhibits?

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Date Created:

Sep 09, 2013

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Oct 20, 2014
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