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Matter, States of

Matter, states of

Matter is anything that has mass and takes up space. The term refers to all real objects in the natural world, such as marbles, rocks, ice crystals, oxygen gas, water, hair, and cabbage. The term states of matter refers to the four physical forms in which matter can occur: solid, liquid, gaseous, and plasma.

The kinetic theory of matter

Our understanding of the nature of matter is based on certain assumptions about the particles of which matter is composed and the properties of those particles. This understanding is summarized in the kinetic theory of matter.

According to the kinetic theory of matter, all matter is composed of tiny particles. These particles can be atoms, molecules, ions, or some combination of these basic particles. Therefore, if it were possible to look

at the tiniest units of which a piece of aluminum metal is composed, one would be able to observe aluminum atoms. Similarly, the smallest unit of a sugar crystal is thought to be a molecule of sugar.

The fundamental particles of which matter is composed are always in motion. Those particles may rotate on their own axes, vibrate back and forth around a certain definite point, travel through space like bullets, or display all three kinds of motion. The various states of matter differ from each other on the basis of their motion. In general, the particles of which solids are made move very slowly, liquid particles move more rapidly, and gaseous particles move much more rapidly than either solid or liquid particles. The particles of which a plasma are made have special properties that will be described later.

Liquid Crystals

Solid, liquid, and gas: these are the three most common forms of matter. But some materials do not fit neatly into one of these three categories. Liquid crystals are one such form of matter.

Liquid crystals are materials that have properties of both solids and liquids. They exist at a relatively narrow range of temperatures. At temperatures below this range, liquid crystals act like solids. At temperatures above the range, they act like liquids.

The behavior of liquid crystals is due to the shape of the molecules of which they are made. You can think of those molecules as looking like cigars or pencils: they tend to be long and thin. They can be arranged within a material in one of three forms. In nematic crystals, the molecules are all parallel to each other but are free to move back and forth with relation to each other. The molecules in smectic crystals are also parallel to each other but they do not move back and forth; they are, however, further arranged in layers that do pass over each other. And the molecules in cholesteric crystals occur in highly structured layers that are set at slightly different angles than the ones above and below them.

The interesting property about liquid crystals is the way they transmit light. Light can pass through a liquid crystal more easily in one direction than in another. If you look at one of the crystals from one direction, you might see all the light passing through it. But from another direction, no light would be visible. The crystal would be dark.

The arrangement of molecules in a liquid crystal can be changed by adding energy to the crystal. If you warm the crystal, for example, molecules may change their position with relation to each other. This fact is utilized in new kinds of medical thermometers that change color with temperature. As body heat changes, the molecules in the liquid crystal change, the light they transmit changes, and different colors appear.

Liquid crystals are also used for the display in electronic calculators. When you press a button on the calculator, you send an electric current through the display. The electric current causes molecules in the liquid crystal to change their positions. More or less light passes through the crystal, and numbers either light up or go dark.

The motion of the particles of matter is a function of the energy they contain. Suppose that you add heat, a form of energy, to a solid. That heat is used to increase the speed with which the solid particles are moving. If enough heat is added, the particles eventually move rapidly enough that the substance turns into a liquid: it melts.

How states of matter differ from each other

One can distinguish among solids, liquids, and gases on two levels: the macroscopic and submicroscopic. The term macroscopic refers to properties that can be observed by the five human senses, aided or unaided. The term submicroscopic refers to properties that are too small to be seen even with the very best of microscopes.

On the macroscopic level, solids, liquids, and gases can be distinguished from each other on the basis of shape and volume. That is, solids have both constant shape and constant volume. A cube of sugar always looks exactly the same as long as it is not melted, dissolved, or changed in some other way.

Liquids have constant volume but indefinite shape. Take 100 milliliters of water in a wide pan and pour it into a tall, thin container. The total volume of the water remains the same, 100 milliliters, but the shape it takes changes.

Finally, gases have neither constant volume nor constant shape. They take the size and shape of whatever container they are placed into. Suppose you have a small container of compressed oxygen in a one-liter tank. The volume of the gas is one liter, and its shape is cylindrical (the shape of the tank). If you open the valve of the tank inside a closed room, the gas escapes to fill the room. Its volume is now much greater than 1 liter, and its shape is the shape of the room.

These macroscopic differences among solids, liquids, and gases reflect properties of the particles of which they are made. In solids, those particles are moving very slowly and tend to exert strong forces of attraction on each other. Since they have little tendency to pull away from each other, they remain in the same shape and volume.

The particles of a liquid are moving more rapidly, but they still exert a significant force on each other. These particles have the ability to flow past each other but not to escape from the attraction they feel for each other.

The particles of a gas are moving very rapidly and feel very little attraction for each other. They fly off in every direction, preventing the gas from taking on either definite shape or volume.

Plasma

Plasma is considered to be the fourth state of matter. Plasmas have been well studied in only the last few decades. They rarely exist on Earth, although they occur commonly in stars and other parts of the universe.

A plasma is a gaslike mixture with a very high temperature. The temperature of the plasma is so high that the atoms of which it is made are completely ionized. That means that the electrons that normally occur in an atom have been stripped away by the high temperature and exist independently of the atoms from which they came. A plasma is, therefore, a very hot mixture of electrons and positive ions, the atoms that are left after their electrons have been removed.

[See also Atom; Crystal; Element, chemical; Gases, properties of; Ionization; Mass; Molecule ]

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states of matter

states of matter, forms of matter differing in several properties because of differences in the motions and forces of the molecules (or atoms, ions, or elementary particles) of which they are composed. The states of matter are also known as phases of matter or states of aggregation. There are three commonly recognized states of matter: solid, liquid, and gas. The molecules of a solid are limited to vibration about a fixed position. This restriction gives a solid both a definite volume and a definite shape. As energy in the form of heat is added to a solid, its molecules begin to vibrate more rapidly until they break out of their fixed positions and the solid becomes a liquid. The change from solid to liquid is called melting and occurs at a definite temperature, the melting point. The molecules of a liquid are free to move throughout the liquid but are held from escaping from the liquid by intermolecular forces (see adhesion and cohesion). This gives a liquid a definite volume but no definite shape. As more heat is added to the liquid, some molecules gain enough energy to break away completely from the liquid and escape into the surrounding space (see evaporation). Finally a temperature is reached at which molecules throughout the liquid are becoming energetic enough to escape and bubbles of vapor form and rise to the surface. The change of the liquid to a vapor, or gas, in this manner is called boiling and occurs at the boiling point. The molecules of a gas are free to move in every possible way; a gas has neither a definite shape nor a definite volume but expands to fill any container in which it is placed. In addition to these three states of matter, scientists also distinguish three additional states—plasma and the Bose-Einstein and the fermionic condensates. A plasma is formed by adding still more heat to the molecules of a gas. Eventually a point is reached where the molecules are moving so rapidly that the molecules become torn apart into their component atoms and individual electrons are pulled away from the atoms. This very hot mixture of negatively charged electrons and positively charged ions has properties distinct from those of the other states of matter. Bose-Einstein condensate and fermionic condensates are formed by chilling the molecules of a gas. As temperatures approach absolute zero (-273.15°C), the motion of the individual atoms slows to the point where they combine to form a single "super atom" with properties distinct from those of other states of matter.

W. H. Bragg, Concerning the Nature of Things (2004).

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matter, states of

matter, states of Classification of matter according to its structural characteristics. Four states (or phases) of matter are generally recognized: solid, liquid, gas, and plasma. Any one element or compound may exist sequentially or simultaneously in two or more of these states. Solids may be crystalline, as in salt and metals; or amorphous, as in tar or glass. Liquids have molecules that can flow past one another but that remain almost as close as in a solid. In a gas, molecules are so far from one another that they travel in relatively straight lines until they collide. In a plasma, atoms are torn apart into electrons and nuclei by the high temperatures, such as those in stars.

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States of Matter

States of Matter

Nature of matter

Solids

Liquids

Boiling

Gases

Plasma

Resources

Matter is defined, generally, in science as anything within the universe that has mass (is influenced by gravity and inertia), occupies space, and can be converted to energy. The last characteristic of matter (that is, can be converted to energy) has only been around since GermanAmerican physicist Albert Einstein (18791955) declared that energy and matter are the same. Einstein said that energy and matter are equated by the equation E = mc2, where the energy (E ) of a body is equal to the mass (m ) of that body times the speed of light (c ) squared.

As easy to identify examples, matter includes: the food eaten, the water drunk, the air breathed, the ores deep within the Earth, as well as the atmosphere above it, the substances that make up the moon, and the stars as well as the dust in the tail of a comet. It is fairly easy to observe that matter exists in different forms or states: solids, liquids, gases, and the less familiar plasma state.

Nature of matter

All matter is composed of very small, discrete particles, either atoms, ions, or molecules. The nature of a particular substance depends on the type and arrangement of the atoms within the molecule.

It is possible for these particles to assume different arrangements in space. For example, they can be arranged close together or far apart. They can be neat and orderly or random and disordered. Since two particles cannot occupy the same place at the same time, they can be pushed closer together only if there is empty space between particles. Sometimes they slip and slide past each other and sometimes they are locked rigidly into a specific position. The state in which any particular piece of matter exists depends on these properties. Under the right conditions, any

substance can exist in all of the states of matter: solid, liquid, gas, or plasma.

The atoms, molecules, and ions that make up all matter are in constant motion, which can range from vibrating within a fairly rigid position to moving randomly at very high speeds. Movement is always in a straight line until some other force interferes with the motion. Like billiard balls, the moving particles can hit other particles or objects such as the walls of their container. These collisions cause the particles to change direction and, although no energy is lost, it can be transferred to other particles.

Various forces exist between the particles of matter. The degree of attraction or repulsion depends on such factors as whether the particles are electrically neutral or carry a charge, whether the charges are localized or balanced out, how big or small the particles are, and how far apart the particles are from each other.

Solids

Matter is said to be in the solid state when it is rigid; that is, when it retains a definite shape and volume against the pull of gravity. Strong attractive forces exist among the particles that make up solids, causing them to position themselves close together in an orderly and definite arrangement in space. Their motion consists mainly of vibrating in a fixed position so the shape and the volume (amount of space they

occupy) are maintained. The atoms, ions, or molecules cannot be pushed closer together; therefore, solids cannot generally, in normal circumstances, be compressed any closer together.

Many solids exist in the form of crystals, which have simple geometric shapes, reflecting the regular spatial arrangement forms and shapes depending on the arrangement of the atoms, ions, or molecules of which they are made. This arrangement is called a lattice. Other solids, such as lumps of clay, seem to have no preferred shapes at all. They are said to be amorphous (without form). This is true because the individual crystals may be very tiny or because the substance consists of several kinds of crystals, randomly mixed together. Other solids, such as glass, contain no crystals at all.

When solids are cooled to lower temperatures, the orderly arrangement of their particles stays basically the same. The vibrations become slightly slower and weaker, however, causing the particles to move closer together and the solid contracts slightly. However, when solids are heated, the vibrations become faster and broader. This causes the particles to move slightly farther apart, and the solid expands a little. If heated enough, the particles will vibrate so vigorously that the rigid structure can no longer be maintained. The lattice begins to fall apart, first into clumps, and eventually into individual particles that can slip and slide past each other as they move about freely. At this point, the solid has become a liquid.

The temperature at which a solids loses its rigid form and turns into a liquid is called the melting point. Different substances have different melting points that are dependent on the sizes of the particles and the strength of the attractions between the particles. In general, heavier particles require more energy (higher temperatures) in order to vibrate vigorously enough to come apart. In addition, the stronger the attractions between the particles, the more energy is required to break them apart and change the solid into a liquid. In both casesheavier particles and stronger attractionsthe melting point will be higher.

Water serves as a good example. Liquid water freezes at the same temperature that ice melts, and the melting and freezing points are therefore identical. This is true for all substances. Ice melts at 32°F(0°C), which is uncharacteristically high for particles the weight of water molecules. This unusually high melt-ing/freezing point is caused by the very strong attractive forces that exist between the molecules, making it very difficult for particles to move away from their neighbors and for the crystalline structure to collapse. Metals melt at much higher temperatures than ice. For example, copper is made into various shapes by melting it at 1,985°F (1,085°C), pouring it into molds, and cooling it. It is then usually purified further by electrolysis before it is commercially useful. Since pure substances have unique melting points that are usually quite easy to determine, chemists often use them as the first step in identifying unknown substances.

The amount of energy required to change a solid to a liquid varies from substance to substance. This energy is called the heat of fusion. Ice, for example, must absorb 80 calories per gram in order to melt into liquid water. Similarly, water releases 80 calories per gram of water to freeze into ice. Each of these changes occurs at the melting/freezing point of water, 32°F (0°C). In melting, since all the heat energy is used up in breaking the crystalline lattice, there is no change in temperature. However, once all the ice has melted, the absorbed energy causes the temperature of the liquid water to rise. This is generally true of the melting of all solids.

Liquids

The change from solid to liquid is a physical rather than chemical change because no chemical bonds have been broken. The individual particlesatoms, ions, or moleculesthat made up the solid are the same individual particles that make up the liquid. What does change is the arrangement of the particles. In the liquid, the particles are at a higher temperature, having more energy than in the solid, and this allows them to move away from their nearest neighbors. Attractions between liquid particlesthough less than those of solidsis still fairly strong. This keeps the particles close to each other and touching, even though they can move around past one another. They cannot be pushed closer together, and so, like solids, liquids maintain their volume and cannot be compressed. Because their particles move freely around, liquids can flow, and they will assume the shape of any container.

Like solids, the particles of liquids are close to each other; therefore, the amount of space occupied by liquids is quite close to that of their corresponding solids. However, because of the disorderly arrangement, the empty space between the liquid particles is usually slightly greater than that between the particles of the solid. Therefore, liquids usually have a slightly larger volumethat is, they are less densethan solids. A very unusual exception to this is the case of ice melting to form water, when the volume actually decreases. The crystalline lattice of ice has a cage-like structure of H2O molecules with big, open spaces in the middle of the cages. When the ice melts and the crystal breaks down, the cages collapse and the molecules move closer together, taking up less space. Consequently, a given weight of water occupies more volume as ice than as liquid. In other words, ice is less dense than water. Therefore, ice floats on liquid water. In addition, a full, closed container of water will break as it freezes because the ice must expand. A water pipe may break if it freezes in winter because of this unusual property of water.

Boiling

As the temperature of a liquid is increased, the particles gain more energy and move faster and faster. Jostling about and colliding increases until eventually the particles at the surface gain enough energy to overcome the attractive forces from their neighbors and break away into the surrounding space. At this point, the liquid is becoming a gas (also called a vapor). The temperature at which this happens depends on what the substance is. This temperature, known as the boiling point, remains constant during the entire process of boiling because the added heat is being used up to break the attraction between the particles. The reverse process, condensation, occurs at the same temperature as boiling. Like the melting point, the boiling point is unique for each pure substance, and can be used as an analytical tool for determining the identities of unknown substances.

The amount of energy required for a given amount of a liquid to vaporize or become a gas is called the heat of vaporization (or condensation). It varies from substance to substance because the particles of different substances may be heavier or lighter and may exert different attractive forces. The amount of energy absorbed when 1 gram of water completely changes to a vapor is 540 calories. Conversely, 540 calories are released when 1 gram of water vapor changes back to liquid.

When a liquid reaches the boiling point, particles on the surface actually gain enough energy to break away from the surface. However, as heating continues, particles throughout the liquid are also increasing in energy and moving faster. In a body of the liquid, however, the particles cannot escape into the air, as those on the surface can. This action happens because they happen to be buried deep down below the surface.

It also happens because the atmosphere is pushing down on the entire liquid and all the particles within it, and, in order to break away, these particles deep within the liquid must acquire enough energy to overcome this additional pressure. (The surface particles can just fly off into the spaces between the air molecules.) When a group of interior particles finally do get enough energyget hot enoughto overcome the atmospheric pressure, they can push each other away, leaving a hollow space within the liquid. This is a bubble. It is not entirely empty, however, because it contains many trapped particles, flying around inside. The lightweight bubble then rises through the liquid and breaks at the surface, releasing its trapped particles as vapor. Scientists then say the liquid is boiling.

Since the pressure inside the bubbles must overcome atmospheric pressure in order for the bubbles to form, the boiling point of a substance depends on atmospheric pressure. Liquids will boil at lower temperatures if the atmospheric pressure is lower, as it is on a mountain. At the top of Mount Everest (the highest point on the surface of the Earth and within the Himalaya range in Asia), 29,000 ft (8,839 m) above sea level, where the pressure is only about one-third that at sea level, water boils at 158°F (70°C). At 10,000 ft (3,048 m) above sea level, water boils at 192°F (89°C). It would take longer to cook an egg where the boiling point is 192°F (89°C) than at sea level where the boiling point is 212°F (100°C). The normal boiling point of a liquid is defined as its boiling point when the atmospheric pressure is exactly 760 mm Hg, or 1 atmosphere.

With the diminishing supplies of fresh water today, it is increasingly important to find ways of desalinatingremoving the salt fromseawater in order to make it useful for human consumption, agriculture, and industry. Changes in state, both boiling and freezing, are useful for this purpose. When salt water is heated to boiling and the vapors cooled, they condense to form water again, but the salt stays behind in a very salty residue called brine. By this process, called distillation, freshwater has been recovered from salt water. Similarly, when salt water freezes, much of the salt stays behind as a very salty slush. The ice is removed from the brine and melted to produce relatively fresh water.

Gases

When a substance has reached the gaseous state, the particles are moving at relatively high speeds, and in straight lines, until they encounter other particles or some other barrier. The spaces between the particles are many times the size of the particles themselves. Generally, gas particles travel large distances through space before colliding with another particle or object. When colliding, although energy can be lost by one particle, it is gained by another and there is no net gain or loss of energy.

Because the particles are flying freely in the gaseous state, gases will fill whatever space is available to them. Thus, 100, 1,000, or ten million particles of gas in a container spread out and fill the entire container.

The characteristics of gases vary widely. While some gases dissolve in water, smell strong, react with most substances, and appear colorless, others have properties exactly opposite to these characteristics. Thus, while most gases are transparent, some show colors such as the yellowish-green colored chlorine. Most gases have no smell, but some gases have a strong smell that is very noticeable, such as ammonia. Some of the gases that react strongly are oxygen and fluorine. In fact, fluorine reacts with most substances. On the other hand, the noble gases are mostly unreactive. In fact, neon has never been shown to react with any other substance.

Plasma

Plasmas are considered by some to be the fourth phase of matter. They are closely related to gases. In a plasma, the particles are neither atoms nor molecules, but electrons and positive ions. Plasmas can be formed at very high temperatureshigh enough to ionize (remove electrons from) the atoms. The resulting electrons and positive ions can then move freely, like the particles in a gas. Although not found naturally on the Earth except in the outermost atmosphere, plasmas are probably more prevalent in the universe than the other three states of matter. The stars, comets tails, and the aurora borealis are all plasmas. Because their particles are electrically charged, plasmas are greatly influenced by electric and magnetic fields.

The name plasma was given to this state by American chemist Irving Langmuir (18811957) in 1920. Langmuir was one of the first modern scientists to study ionized gases. He called them plasma because they looked similar to blood plasma. Much research today involves the study of plasmas and the ability to control them. One possible method for producing enormous amounts of energy through nuclear fusion involves the production and control of plasmas.

See also Density; Desalination; Evaporation; Gases, properties of.

Resources

BOOKS

De Podesta, Michael. Understanding the Properties of Matter. London, UK, and New York: Taylor & Francis, 2002.

Dove, Martin T. Structure and Dynamics: An Atomic View of Materials. Oxford, UK, and New York: Oxford University Press, 2003.

Lief, Elliott H. The Stability of Matter: From Atoms to Stars. Berlin, Germany, and New York: Springer, 2005.

Souza, Mario Everaldo de. The General Structure of Matter. Sergipe, Brazil: Universidade Federal de Sergipe, 2001.

Leona B. Bronstein

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States of Matter

States of matter

Matter includes all the material that makes up the universe. It has mass and it takes up space. It includes everything around us: the food we eat, the water we drink, the air we breathe, the ores deep within the earth , as well as the atmosphere above it, the substances that make up the moon , and the stars as well as the dust in the tail of a comet. It is fairly easy to observe that matter exists in different forms or states: solids, liquids, gases, and the less familiar plasma state.


Nature of matter

All matter is composed of very small, discrete particles, either atoms , ions, or molecules. The nature of a particular substance depends on the type and arrangement of the atoms within the molecule .

It is possible for these particles to assume different arrangements in space. For example, they can be arranged close together or far apart. They can be neat and orderly or random and disordered. Since two particles cannot occupy the same place at the same time , they can be pushed closer together only if there is empty space between particles. Sometimes they slip and slide past each other and sometimes they are locked rigidly into a specific position. The state in which any particular piece of matter exists depends on these properties. Under the right conditions, any substance can exist in all of the states of matter: solid, liquid, gas, or plasma.

The atoms, molecules, and ions that make up all matter are in constant motion , which can range from vibrating within a fairly rigid position to moving randomly at very high speeds. Movement is always in a straight line until some other force interferes with the motion. Like billiard balls, the moving particles can hit other particles or objects such as the walls of their container. These collisions cause the particles to change direction and, although no energy is lost, it can be transferred to other particles.

Various forces exist between the particles of matter. The degree of attraction or repulsion depends on such factors as whether the particles are electrically neutral or carry a charge, whether the charges are localized or balanced out, how big or small the particles are, and how far apart the particles are from each other.


Solids

Matter is said to be in the solid state when it is rigid, that is, when it retains a definite shape and volume against the pull of gravity. Strong attractive forces exist among the particles that make up solids, causing them to position themselves close together in an orderly and definite arrangement in space. Their motion consists mainly of vibrating in a fixed position so the shape and the volume (amount of space they occupy) are maintained. The
atoms, ions, or molecules cannot be pushed closer together; therefore, solids cannot be compressed.

Many solids exist in the form of crystals, which have simple geometric shapes, reflecting the regular spatial arrangement forms and shapes depending on the arrangement of the atoms, ions, or molecules of which they are made. This arrangement is called a lattice. Other solids, such as lumps of clay, seem to have no preferred shapes at all. They are said to be amorphous (without form). This is true because the individual crystals may be very tiny or because the substance consists of several kinds of crystals, randomly mixed together. Other solids, such as glass , contain no crystals at all.

When solids are cooled to lower temperatures, the orderly arrangement of their particles stays basically the same. The vibrations become slightly slower and weaker, however, causing the particles to move closer together, and the solid contracts slightly. But when solids are heated, the vibrations become faster and broader. This causes the particles to move slightly farther apart, and the solid expands a little. If heated enough, the particles will vibrate so vigorously that the rigid structure can no longer be maintained. The lattice begins to fall apart, first into clumps, and eventually into individual particles which can slip and slide past each other as they move about freely. At this point, the solid has become a liquid.

The temperature at which a solids loses its rigid form and turns into a liquid is called the melting point. Different substances have different melting points that are dependent on the sizes of the particles and the strength of the attractions between the particles. In general, heavier particles require more energy (higher temperatures) in order to vibrate vigorously enough to come apart. Also, the stronger the attractions between the particles, the more energy is required to break them apart and change the solid into a liquid. In both cases—heavier particles and stronger attractions—the melting point will be higher. Water serves as a good example. Liquid water freezes at the same temperature that ice melts, and the melting and freezing points are therefore identical. This is true for all substances. Ice melts at 32°F (0°C) which is uncharacteristically high for particles the weight of water molecules. This unusually high melting/freezing point is caused by the very strong attractive forces that exist between the molecules, making it very difficult for particles to move away from their neighbors and for the crystalline structure to collapse. Metals melt at much higher temperatures than ice. For example, copper is made into various shapes by melting it at 1,985°F (1,085°C), pouring it into molds, and cooling it. It is then usually purified further by electrolysis before it is commercially useful. Since pure substances have unique melting points which are usually quite easy to determine, chemists often use them as the first step in identifying unknown substances.

The amount of energy required to change a solid to a liquid varies from substance to substance. This energy is called the heat of fusion. Ice, for example, must absorb 80 calories per gram in order to melt into liquid water. Similarly, water releases 80 calories per gram of water to freeze into ice. Each of these changes occurs at the melting/freezing point of water, 32°F (0°C). In melting, since all the heat energy is used up in breaking the crystalline lattice, there is no change in temperature. However, once all the ice has melted, the absorbed energy causes the temperature of the liquid water to rise. This is generally true of the melting of all solids.


Liquids

The change from solid to liquid is a physical rather than chemical change because no chemical bonds have been broken. The individual particles—atoms, ions, or molecules—that made up the solid are the same individual particles that make up the liquid. What does change is the arrangement of the particles. In the liquid, the particles are at a higher temperature, having more energy than in the solid, and this allows them to move away from their nearest neighbors. The attractions between liquid particles, though less than those of solids, is still fairly strong. This keeps the particles close to each other and touching, even though they can around past one another. They cannot be pushed closer together, and so, like solids, liquids maintain their volume and cannot be compressed. Because their particles move freely around, liquids can flow, and they will assume the shape of any container.

Like solids, the particles of liquids are close to each other; therefore, the amount of space occupied by liquids is quite close to that of their corresponding solids. However, because of the disorderly arrangement, the empty space between the liquid particles is usually slightly greater than that between the particles of the solid. Therefore, liquids usually have a slightly larger volume-that is, they are less dense-than solids. A very unusual exception to this is the case of ice melting to form water, when the volume actually decreases. The crystalline lattice of ice has a cage-like structure of H2O molecules with big, open spaces in the middle of the cages. When the ice melts and the crystal breaks down, the cages collapse and the molecules move closer together, taking up less space. Consequently, a given weight of water occupies more volume as ice than as liquid. In other words, ice is less dense than water. Therefore, ice floats on liquid water. Also, a full, closed container of water will break as it freezes because the ice must expand. A water pipe may break if it freezes in winter because of this unusual property of water.


Boiling

As the temperature of a liquid is increased, the particles gain more energy and move faster and faster. Jostling about and colliding increases until eventually the particles at the surface gain enough energy to overcome the attractive forces from their neighbors and break away into the surrounding space. At this point, the liquid is becoming a gas (also called a vapor). The temperature at which this happens depends on what the substance is. This temperature, known as the boiling point , remains constant during the entire process of boiling because the added heat is being used up to break the attraction between the particles. The reverse process, condensation, occurs at the same temperature as boiling. Like the melting point, the boiling point is unique for each pure substance, and can be used as an analytical tool for determining the identities of unknown substances.

The amount of energy required for a given amount of a liquid to vaporize or become a gas is called the heat of vaporization (or condensation). It varies from substance to substance because the particle of different substances may be heavier or lighter and may exert different attractive forces. The amount of energy absorbed when 1 gram of water completely changes to a vapor is 540 calories. Conversely, 540 calories are released when 1 gram of water vapor changes back to liquid.

When a liquid reaches the boiling point, particles on the surface actually gain enough energy to break away from the surface. But as heating continues, particles
throughout the liquid are also increasing in energy and moving faster. In a body of the liquid, however, the particles cannot escape into the air, as those on the surface can. That is not only because they happen to be buried deep down below the surface. It is also because the atmosphere is pushing down on the entire liquid and all the particles within it, and, in order to break away, these particles deep within the liquid must acquire enough energy to overcome this additional pressure . (The surface particles can just fly off into the spaces between the air molecules.) When a group of interior particles finally do get enough energy-get hot enough-to overcome the atmospheric pressure , they can push each other away, leaving a hollow space within the liquid. This is a bubble. It is not entirely empty, however, because it contains many trapped particles, flying around inside. The light-weight bubble then rises through the liquid and breaks at the surface, releasing its trapped particles as vapor. We then say the liquid is boiling.

Since the pressure inside the bubbles must overcome atmospheric pressure in order for the bubbles to form, the boiling point of a substance depends on atmospheric pressure. Liquids will boil at lower temperatures if the atmospheric pressure is lower, as it is on a mountain. At the top of Mount Everest, 29,000 ft (8,839 m) above sea level , where the pressure is only about one-third that at sea level, water boils at 158°F (70°C). At 10,000 ft (3,048 m) above sea level, water boils at 192°F (89°C). It would take longer to cook an egg where the boiling point is 192°F (89°C) than at sea level where the boiling point is 212°F (100°C). The normal boiling point of a liquid is defined as its boiling point when the atmospheric pressure is exactly 760 mm Hg, or 1 atmosphere.

With the diminishing supplies of fresh water today, it is increasingly important to find ways of desalinating-removing the salt from sea water in order to make it useful for human consumption, agriculture, and industry. Changes in state, both boiling and freezing, are useful for this purpose. When salt water is heated to boiling and the vapors cooled, they condense to form water again, but the salt stays behind in a very salty residue called brine. By this process, called distillation , freshwater has been recovered from salt water. Similarly, when salt water freezes, much of the salt stays behind as a very salty slush. The ice is removed from the brine and melted to produce relatively fresh water.


Gases

When a substance has reached the gaseous state, the particles are moving at relatively high speeds, and in straight lines, until they encounter other particles or some other barrier. The spaces between the particles are many times the size of the particles themselves. Generally, gas particles travel large distances through space before colliding with another particle or object. When colliding, although energy can be lost by one particle, it is gained by another and there is no net gain or loss of energy.

Because the particles are flying freely in the gaseous state, gases will fill whatever space is available to them. Thus, 100, 1000, or ten million particles of gas in a container spread out and fill the entire container.


Plasma

Plasmas are considered by some to be the fourth phase of matter. They are closely related to gases. In a plasma, the particles are neither atoms nor molecules, but electrons and positive ions. Plasmas can be formed at very high temperatures-high enough to ionize (remove electrons from) the atoms. The resulting electrons and positive ions can then move freely, like the particles in a gas. Although not found on Earth except in the outermost atmosphere, plasmas are probably more prevalent in the universe than all of the other three states of matter. The stars, comets' tails, and the aurora borealis are all plasmas. Because their particles are electrically charged, plasmas are greatly influenced by electric and magnetic fields.

Much research today involves the study of plasmas and the ability to control them. One possible method for producing enormous amounts of energy through nuclear fusion involves the production and control of plasmas.

See also Density; Desalination; Evaporation; Gases, properties of.


Resources

books

Caro, Paul. Water. New York: McGraw Hill, 1993.

Close, Frank. Too Hot to Handle: The Race for Cold Fusion. Princeton: Princeton University Press, 1991.

periodicals

Burgess, David. "Stronger than Atoms." New Scientist 140, (November 1993): 28-33.

Fortman, John J. "States of Matter." Journal of Chemical Education 70, (January 1993): 56-57.


Leona B. Bronstein

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