States of Matter

Transcription

1 States of Matter By Ron Kurtus (revised 21 March 2009) Matter exists in three different states or phases: solid, liquid, gas. Some scientists designate plasma as a fourth state of matter. Major physical characteristics define each state and primarily concern volume and shape. Kinetic energy of the molecules and attractive forces determine the physical characteristics of each state. Many common materials can be seen in the various states of matter. Questions you may have include: What are the physical characteristics of the states of matter? Why do the characteristics change? What are some examples of materials in different states? Physical characteristics The primary physical characteristics of the various states of matter are the volume and shape of the material. They are what really define the state. Other characteristics such as color may be different in the various states of matter, but they do not define the state as do volume and shape. Gravity has an effect on the shapes of liquids and gases. Everything on Earth can be explained in terms of 4 states (phases) of matter-- solid, liquid, gas, and plasma. Solid The solid state of matter is when the material has a definite volume or size and distinct shape at a given temperature. At room temperature, a piece of iron at has a shape and size that

2 does not change. Ice is another solid, but its temperature must be below 0 o C (32 o F). Most solid materials expand with higher temperatures, but they retain their shape. What are the properties of a solid? A substance in a solid phase is relatively rigid, has a definite volume and shape. The atoms or molecules that comprise a solid are packed close together and are not compressible. Because all solids have some thermal energy, its atoms do vibrate. However, this movement is very small and very rapid, and cannot be observed under ordinary conditions. Liquid A liquid has a definite volume, but it takes the shape of its container with the help of gravity. For example, if you pour water into a cup or container, it will take the shape of that container. If you put water in a balloon, the water will take the shape of the balloon, no matter how you change the shape of the balloon. On the other hand, in outer space, where there is no gravity such as in the Space Shuttle, water might float out of its container. Its shape will vary, but its volume will remain constant if the air pressure and temperature are constant. Most liquids expand with an increase of temperature and constant air pressure. What are some properties of a Liquid? Liquids have a definite volume, but are able to change their shape by flowing. Liquids are similar to solids in that the particles touch. However the particles are able to move around. Since particles are able to touch the densities of liquid will be close to that of a solid.

3 Since the liquid molecules can move they will take the shape of their container. What are the specific properties of liquids? Viscosity --The resistance of a liquid to flow is called its viscosity. Surface Tension -- The result of attraction between molecules of a liquid which causes the surface of the liquid to act as a thin elastic film under tension. Surface tension causes water to form spherical drops. Vapor Pressure -- The pressure that a solid or liquid exerts when it is in equilibrium with its vapor at a given temperature (when heated water produces steam). Boiling Point -- when vapor pressure = atmospheric pressure. Gas The volume of a quantity of gas is dependent on its temperature and the surrounding pressure. If affected by gravity, it will take the shape of its container, and will also spread out into the surrounding area. If you put a gas in a cylinder and apply pressure with a piston, such as you might do with a tire pump, the volume of the gas can change considerably. This is not the case with water or a solid. Their volumes may change only slightly with an increase of pressure. What are some properties of a Gas? Gases have no definite shape. If unconstrained gases will spread out indefinitely. If confined they will take the shape of their container. Gas Laws -- There are several excellent interactive java applets which we recommend. Molecular Model for an Ideal Gas 2 dimensional gas of hard spheres Gas Simulation Brownian Motion for Gas Plasma At extreme temperatures such as on the Sun matter can often

4 lose all its electrons and become ionized. This matter, along with turbulence caused by the heat, results in a phase called plasma. There is dispute among some scientists whether plasma is truly a state of matter or just an extension of a gas. Since this state is not commonly experienced, we will not say much about it. What is the fourth state of matter? The fourth state of matter is plasma. Plasma is an ionized gas, a gas into which sufficient energy is provided to free electrons from atoms or molecules and to allow both species, ions and electrons, to coexist. In effect a plasma is a cloud of protons, neutrons and electrons where all the electrons have come loose from their respective molecules and atoms, giving the plasma the ability to act as a whole rather than as a bunch of atoms. Plasmas are the most common state of matter in the universe comprising more than 99% of our visible universe and most of that not visible. Plasma occurs naturally and makes up the stuff of our sun, the core of stars and occurs in quasars, x-ray beam emitting pulsars, and supernovas. On earth, plasma is naturally occurring in flames, lightning and the auroras. Most space plasmas have a very low density, for example the Solar Wind which averages only 10 particles per cubic-cm. Inter-particle collisions are unlikely - hence these plasmas are termed collisionless. And now a fifth state -- Bose Einstein? The collapse of the atoms into a single quantum state is known as Bose condensation or Bose-Einstein condensation is now considered a 5th state of matter. Recently, scientists have discovered the Bose-Einstein condensate, which can be thought of as the opposite of a plasma. It occurs at ultra-low temperature, close to the point that the atoms are not moving at all. A Bose-Einstein condensate is a gaseous superfluid phase formed by atoms cooled to temperatures very near to absolute zero. The first such condensate was produced by Eric Cornell and Carl Wieman in 1995 at the University of Colorado at Boulder, using a gas of rubidium atoms cooled to 170 nanokelvins (nk). -- Under such conditions, a large fraction of the atoms collapse into the lowest quantum state, producing a superfluid. This phenomenon was predicted in the 1920s by Satyendra Nath Bose and Albert Einstein, based on Bose's work on the statistical mechanics of photons, which was then formalized and generalized by Einstein.

5 Reasons states have characteristics The reason the various states of matter have their particular physical characteristics and behave in the way they do is a result of the motion of the molecules and the attraction between them. Theories and forces The Molecular Theory of Matter, Theory of Heat and the Molecular Attraction Force affect the states of matter. Molecules in motion The Molecular Theory of Matter and the Theory of Heat state that molecules are in constant motion. The greater the temperature, the greater the kinetic energy and thus the faster the molecules are moving. This also applies to substances made up of only atoms, such as iron or pure aluminum. Molecular attraction But also, Molecules are often attracted to each other, due to the Molecular Attraction Force. This force is strongest among similar molecules or atoms. It is a form of electrostatic attraction that also is the factor in static electricity. Fight between the two The kinetic energy of the molecules or atoms tends to spread them apart, while the molecular attraction tends to pull them together. The temperature or energy will then determine which force wins out. Molecules in solids vibrate If the kinetic energy (and thus the velocity) of the molecules of a material is so low that the molecular attraction between the molecules is much stronger than the forces pulling the molecules apart, the material will be a in solid state of matter. That means the molecule become fixed in place and often line up in a

6 crystalline arrangement. The molecules or atoms still have some energy, but their movement is confined to just vibrating in place or rotation. At the very lowest temperature possible Absolute Zero (0 degrees Kelvin or 0 o K) all motion stops and the atoms and molecules do not vibrate or even spin. Molecules in liquids loosen structure If the molecules of the material have enough kinetic energy and are moving fast enough, they can break out of the constraints of a defined structure. The energy overcomes the much of the molecular force. This force is now only strong enough to hold the material together in the form of a liquid. Molecules in gases run free With a gas, the molecules (or atoms) are more energetic and are moving rapidly. Their kinetic energy is greater than the attractive force between them. Thus, a gas will easily spread and not stay in an open container. Examples with different materials Look at various materials to see their different states. Water A good example of how matter can exist in different states is water. It is normally a liquid at room temperature, but if you cool it below 32 o F (0 o C), it will freeze and become the solid we call ice. If you heat water above 212 o F (100 o C), it will boil and turn into a gas when it evaporates. Iron You've seen pictures of molten iron in a foundry. They heat the iron to a very high temperature, such that it turns into a glowing, yellow liquid. If iron was placed in an extremely hot environment

7 like on the Sun's surface, it would boil and turn into a gas. Helium Helium is a gas at room temperature, but if it is cooled to a few degrees above Absolute Zero (degrees Kelvin), it turns into a liquid. It is the only liquid that cannot be solidified at atmospheric pressure by simply lowering its temperature. Scientists have been only able to solidify liquid Helium at the extreme pressures of 25 atmospheres. (1 atmosphere = 14.7 pounds per square inch). Liquid Helium has some very strange properties. For example, if it is poured in a flask, it will first settle at the bottom of the flask and then start to climb up the sides and soon flow out of the container. Summary There are three accepted states of matter: solid, liquid, gas. There may be a fourth state, called the plasma state. As the energy of the moving molecules overcomes the molecular attraction forces, the material will change its state from solid to liquid to gas. The solid iron, liquid water and helium gas are examples of different states of matter at room temperature.