Unit 5: States of Matter
In this unit, we explore how matter behaves in terms of the three main phases of matter: solids, liquids, and gases. We investigate gases first because their properties are described by well-defined equations. Next, we study phase changes, which we describe in terms of a graph known as a phase diagram. We finish this unit with an exploration of the properties of solids.
Completing this unit should take you approximately 7 hours.
Upon successful completion of this unit, you will be able to:
- use the ideal gas law to calculate properties of a gas;
- compare the motion of particles in a solid, liquid, and gas;
- define the characteristics of bonding in ionic compounds;
- interpret phase diagrams; and
- describe the processes of boiling, evaporation, freezing, melting, and sublimation.
5.1: Gases and Gas Laws
Gases are unique because we can use simple relations or equations to describe many of their properties. This works because gas particles take up the entire space of the container they inhabit, move fast, and do not interact with each other. We learn later in the unit that this quality is different from the particles in the liquid or solid phase.
Read this text, which introduces the important gas properties of pressure, temperature, and volume.
Read this text which discusses the basic gas laws and how pressure, volume, and time are related.
We use the Ideal Gas Law PV = nRT – where P is pressure, V is volume, n is the number of moles, T is temperature, and R is the gas constant – to describe the relationship among pressure, volume, temperature, and the number of moles of a gas.
Watch these videos, which show how to solve for the different variables in an Ideal Gas Law problem. Pay attention to the units used for the variables in the Ideal Gas Law. It is necessary to use the proper units to ensure your units cancel when you complete an Ideal Gas Law problem.
The scientists who formulated the Ideal Gas Law based their theory on Charles' Law, Boyle's Law, and Avogadro's Law, which showed the relationship between just two variables in a gas sample.
Watch these videos to see simulations of what happens when you manipulate one gas variable in a container – pressure, volume, temperature, number of moles – and how the modification affects the other variables.
In many cases different types of gases are mixed. This is true of the air around us. Air consists of nitrogen, oxygen, and a variety of other gases. We can use Dalton's Law of Partial Pressures to determine the pressure exerted by each gas in a given mixture, and the total pressure exerted by a mixture of gases. Dalton's Law of Partial Pressure states that a mixture has a total pressure that is the sum of its component, or partial, pressures.
Watch this video, which introduces Dalton's Law of Partial Pressures, and shows how to use it and the Ideal Gas Law to calculate the partial pressures of gases in a mixture.
Kinetic molecular theory (KMT) explains the properties of gas molecules and their motion. KMT helps explain how radio waves are transmitted and why the sky is blue.
Read this text. The first section defines the main points of KMT. The second section details how KMT explains the gas laws. The third section, "Some Important Practical Applications of KMT," describes many interesting natural phenomena, such as the color of the sky and how lightbulbs work, that come from KMT.
5.2: Phase Changes
In this section, we discuss phase changes, where one substance changes from one phase of matter to another. We are all familiar with this from everyday life. When an ice cube melts, it changes from solid water to liquid water. When we boil water on the stove and see water vapor form, the water is changing from liquid to gas.
Watch this video, which uses the example of water to remind us of the properties of solids, liquids, and gases. It also introduces a graph called a heating curve. Heating curves show us how the temperature changes as we add heat (q or △H) to a substance. Notice that while a phase change occurs, the temperature stays constant. This is why we can define melting points and boiling points for substances.
We will investigate thermodynamics, the study of heat transfer, in detail in Unit 6. However, we need to introduce some thermodynamics terminology here to understand how phase changes occur. We can define the specific heat of a substance as the energy it takes to raise 1 g of a substance by 1 degree Celsius. The unit used for heat is the joule (J), or kilojoule (kJ). We define heat of fusion as the energy or heat necessary to change a given amount of a substance (often one mole) from solid to liquid. Heat of fusion uses the unit of J or kJ. Heat of vaporization is the amount of heat needed to change a given amount of a substance (often one mole) from liquid to gas, with the unit of J or kJ.
Watch the next three videos in order. Our first video uses the heating curve of water to define specific heat, heat of fusion, and heat of vaporization.
Our second video is a worked example showing how to calculate the heat needed to change an ice cube at a given temperature to liquid water at a warmer temperature.
This video is a similar example. You can pause the third video after it presents the problem and try it yourself using the same technique as seen in the "chilling water problem" video.
As we saw above, heating curves show us how the temperature of a substance changes as we apply heat to it. We can use heating curves to determine the phase of a substance at a given temperature.
Another graph used to determine the phase of a substance is a phase diagram. Phase diagrams show the effect of pressure and temperature on the phase of a given substance. In a phase diagram, the temperature is shown on the x-axis and the pressure is shown on the y-axis. Curved lines show where phase changes occur. Areas between the curved lines show the conditions where the substance is a solid, liquid, and gas. All phase diagrams have a similar form, making them relatively straightforward to interpret.
Watch this video to see examples of phase diagrams, and how we can determine the phase of a substance at a given temperature and pressure. Also note the triple point, melting point, boiling point, and supercritical point on the phase diagram.
Now, let's explore the properties of liquids, and what happens at the liquid-gas interface. Have you ever wondered why all soap bubbles are round? This is because of the interfacial properties of water!
Read this text, which highlights many important properties of liquids including viscosity and surface tension. The text explains how surface tension leads to the formation of soap bubbles.
Directly above the surface of a liquid, there is a small amount of vapor that comes from liquid particles that gain enough energy to escape into the gas phase. The pressure exerted by this vapor is called vapor pressure.
Watch this video to learn how this phenomenon occurs and how the strength of intermolecular forces affects vapor pressure.
- Next, watch this video to learn how to perform calculations involving vapor pressure that use the ideal gas law.
Ionic compounds are composed of positively and negatively charged ions held together by Coulombic charges. In other words, opposite charges are attracted to each other and the ionic solids form crystal lattices with a regular, repeating structure. This is different from covalently bonded compounds which are held together by covalent bonds.
Ionic compounds are hard. Hardness measures how resistant a substance is to being deformed. Ionic compounds are also brittle. Brittleness means that one layer of the crystal lattice can "slip" over another when it is hit with a physical stress, which can cause the substance to break.
Read this text about ionic and ion-derived solids.
Our last topic in this unit is the solid phase of matter. There are two types of solids: crystals and amorphous solids. Crystalline solids have a regular, repeating pattern. The repeating unit is called the unit cell. Ionic compounds are examples of crystalline solids. Amorphous solids are things like glass that lack a repeating structure. Here we will focus on crystal structures.
Read this text. Pay attention to the three types of unit cells (cubic, body-centric cubic, and face-centered cubic) and how many atoms are contained in each type of unit cell.