Open Course Info


Introduction to Astronomy


Lecture 8: The Phases of Matter


For a charm of powerful trouble,
Like a hell-broth boil and bubble.
Double, double toil and trouble;
Fire burn and cauldron bubble.

-- William Shakespeare, Macbeth

8.1 Solids, Liquids, Gases, and Plasmas

(Discovering the Universe, 5th ed., not!)
  • Large quantities of molecules form the phases of matter you are familiar with: solids, liquids, and gases.

    A glass of water, for example, contains about 1025 molecules.

  • Solids consist of closely packed molecules which form a rigid structure.

    Solids are held together by a residual electrostatic attraction which is generally weaker than a molecular bond.

    Solids usually consist of a simple geometric pattern of atoms or molecules which is repeated over and over again in all directions, which is known as a crystal.

  • Some crystals allow their electrons to freely move from atom to atom; they are called metals.

    Metals therefore conduct electricity very easily.

    Most metals consist of relatively massive atoms, for example iron, and these heavy elements are sometimes referred to as metals even when they aren't in solid form.

  • Liquids also contain closely packed molecules, but they do not have a rigid orientation.

    They still feel an electrostatic attraction, and they regularly bump into each other, but they can move around with relative ease.

  • Some liquids are extremely viscous, to the point where they appear solid; they are called glasses.

    Glasses can change their shape, but you might not notice it for years!

  • Gases or vapors are the simplest phase: the molecules are widely spaced, and rarely bump into each other.

  • A gas whose atoms have been ionized into a mixture of positive ions and electrons is called a plasma.

    Stars consist of plasmas.

    Question: where else have you seen the term plasma commonly used? How is it related to this definition?

  • Liquids, gases, and plasmas are all referred to as fluids, because they can flow and take the shape of whatever container they are placed in.

    Gases and plasmas, in addition, will expand to fill their container.
  • Generally, a solid is denser than a liquid, because it packs its molecules into a smaller volume.

    Similarly, liquids are denser than gases.
  • Extra: in recent years, two more unusual phases of matter have been discovered, both a type of quantum-mechanical condensate.

8.2 Heat Energy

(Discovering the Universe, 5th ed., not!)
  • The molecules which make up solids, liquids, and gases all move around to a lesser or greater extent.

    In rigid solids, the molecules can vibrate back and forth, and also rotate slightly.

    In liquids and gases the molecules are free not only to vibrate and rotate but also translate through space.

  • Therefore, molecules in general have some amount of kinetic energy.

    As the molecules bump into each other, they may gain or lose energy, but the total amount is conserved.

    The total of all of the random kinetic energy of the molecules which make up an object is called its heat energy, or simply heat.

  • A solid has the least amount of heat energy.

    As a solid is heated up, the electrostatic bonds which hold the molecules in their rigid positions begin to break, and the solid undergoes a phase change to become a liquid.

    This is what happens when ice melts to form liquid water.

    If energy is removed from a liquid, it will reverse the process, and solidify or freeze.

  • If a liquid is heated, the remaining electrostatic forces holding the molecules close together will eventually be overcome, and the liquid will undergo another phase change to become a gas.

    This is what happens when liquid water boils to become water vapor.

    We also may say that the liquid vaporizes or evaporates.

    If energy is removed from a gas, it will condense back to a liquid.

  • Some solids will change phase into a gas, without becoming a liquid first.

    For example, frozen carbon dioxide sublimates directly into a gas, hence its name dry ice.

    The reverse process, gas into solid, is also called condensation.

  • If the gas is extremely hot, its molecules will be broken down into atoms, which will also become ionized as electrons are knocked loose by collisions.

    Such a gas has then become a plasma.

8.3 Temperature

(Discovering the Universe, 5th ed., Toolbox A-1)
  • The amount of heat energy in a material is described by its temperature.

    For example, at room temperature, the oxygen molecules in air have a total kinetic energy of 10-23 J, which corresponds to an average translational speed of 480 m/s!

    Several different temperature scales are used for different purposes:





    Water boils 212 °F 100 °C 373 K
    Human body  99 °F 37 °C  310 K
    Room Temperature 72 °F 22 °C 295 K
    Water freezes 32 °F 0 °C 273 K
    Salt water freezes 0 °F -18 °C 255 K
    Absolute zero  -460 °F -273 °C 0 K

  • The Fahrenheit temperature scale is commonly used in the United States, but nowhere else.

    The Fahrenheit scale ensures that most of the temperatures we experience are positive values, and it also places body temperature near 100 °F.

  • The rest of the world uses the Celsius temperature scale, which is based on water, a convenient standard.

    Originally the Celsius scale was called the centigrade scale because there are 100 degrees between freezing and boiling.

    The Celsius scale is simply related to the Fahrenheit scale by the equation

    °F = °C (9/5) + 32 .

    Note that the degree Fahrenheit is roughly half the size of the degree Celsius.

  • Scientists worldwide use the Celsius scale, as well as the Kelvin temperature scale, which is related to the Celsius scale by the equation

    K = °C + 273 .

    Note that the Kelvin is the same size as the degree Celsius.

    Note also that we say simply "Kelvin" rather than "degree Kelvin".

  • The Kelvin scale sets the zero point at absolute zero, the temperature at which all heat energy has been removed from an object.

    Absolute zero is therefore the lowest possible temperature.

    As a result, the Kelvin scale is sometimes also called the absolute scale.

8.4 Pressure

(Discovering the Universe, 5th ed., not!)
  • As the molecules in a gas speed around, they will eventually bump into the walls of their container, applying a force to it.

    Because there are so many molecules bumping into the walls surrounding a gas, it is easiest to characterize their effect by the force per unit area, or pressure.

  • A common metric unit for pressure is the bar, which is useful for describing planetary atmospheres:

    1 bar = 105 N/m2 .

  • At sea level, the average atmospheric pressure on Earth is called one atmosphere:

    1 atmosphere = 1 atm = 1.013 bar .

  • Another unit of pressure that is still in common use is based on the original barometers, devices designed to measure pressure.

    Here, a column of liquid mercury (Hg) is supported by the pressure, and its height defines the value:

    1 atm = 76 cm Hg = 29.9 in Hg .

    Question: where have you seen the latter unit comonly used?

  • A unit of pressure that you may be more familiar with is pounds per square inch:

    1 bar = 14.5 pounds per square inch = 14.5 psi .

    Question: where have you seen the latter unit comonly used?

  • In a gas, the more molecules there are in a container, the greater the force they apply per unit area.

    The force will also be greater if the molecules have more energy.

    So, a simple relation between pressure, density, and temperature is given by:

    P ~ dT

    This relation only roughly describes the behavior of liquids.

    It is not applicable at all to solids, which don't change their density easily because of the electrostatic bonds between molecules.

  • Just as temperature affects the phase of a material, so does pressure.

    For example, if a liquid has only a low pressure applied to it, its molecules have little external force holding them together, and they will vaporize more easily.

    Similarly, it is easier for a solid to liquify (or sublimate) if the pressure is low.

    These characteristics can be described by a phase diagram such as the one at the right.

    The phase diagrams of different materials will usually be similar to the above, but with different temperatures and pressures.

    For example, at atmospheric pressure water melts at 0 °C and boils at 100 °C (by definition), but methane melts at -182 °C and boils at -162 °C.

    Question: what would happen to the Earth's oceans if its atmosphere suddenly disappeared?

  • Note that most solid materials will sublimate at a low enough pressure.

    For water that only occurs below 6.1 mbar, far below atmospheric pressure.

    Carbon dioxide, on the other hand, can only become a liquid above 5.2 bar.

8.5 Sound

(Discovering the Universe, 5th ed., not!)
  • Solids, liquids, and gases can all carry directed, energetic disturbances through them, which are called sound.

    For example, when you speak, your vocal cords push against air molecules, which bump into other air molecules, etc.

    The sound moves through the air until it reaches a listener's ear and pushes against their ear drum, which is then interpreted by the brain.

  • Sound is a type of wave, a natural phenomenon in which something oscillates back and forth, passing energy along as it does so.

    For sound waves, it is the molecules which move back and forth as they bump into their neighbors; the molecules themselves don't travel very far.

    Because the molecules alternately get closer together and farther apart, the density oscillates as well.

    In the picture at the right, the higher density regions are darker and the lower density regions are lighter.

    For this reason sound is also called a density wave.

  • Sound waves travel at a particular speed, the speed of sound, that depends on the material, its density, and the temperature.

    In air, it is about 340 m/s; it is generally faster in a liquid and even faster in a solid, since the molecules don't have to travel as far to bump into each other.

  • If an object travels through a fluid faster than its speed of sound, it pushes the fluid faster than it prefers to move.

    As a result, the fluid is compressed into a narrow, dense layer called a shock wave.

    When produced by high-speed jet planes in air, this is also known as a sonic boom.

    An example of a (non-sound) shock wave are the bow waves produced by a boat in water as it travels faster than the speed of water waves.

    A shock wave travels at the speed of sound, and at an angle to the motion of the causal object.

Picture Information
Runaway Star Photo

In the picture at the right, the "runaway" star HD 77581 can be seen plowing through the interstellar medium at 80 Km/s, producing a highly visible bow shock.


The star chart background was produced on a Macintosh with the Voyager II program, and are ©1988-93 Carina Software, 830 Williams St., San Leandro, CA 94577, (510) 352-7328. Used under license.
©1996-2001 Scott R. Anderson
Last update: 2001 July 15
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