States of Matter

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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.