Gases, liquids, and solids

Why does matter exist in three separate states, and how does it change from one state to another? How can a substance like water be sometimes hard and solid, sometimes soft and liquid, and sometimes an invisible vapor? The movement of the particles—the atoms or molecules—that make up a substance holds the key, and the particles move according to temperature and pressure.


Temperatures and pressures at the earth’s surface fall within a relatively small range. Under the conditions that are considered normal, oxygen (02) is always a gas, so it is usually considered as such. On the coldest day as well as the warmest, 02 molecules are in constant motion in the air around us. The space between these molecules is so great, compared with the molecules of a liquid or solid, that they move almost independently.

The atoms or molecules of a gas are spaced very far apart. They can, however, be disturbed, such as by the impact of an explosion. When a firecracker explodes, the energy of the explosion, in the form of heat and pressure, is immediately passed on to all the molecules in the surrounding air. For an instant, the molecules are driven against one another by the shock—those nearest the explosion absorb heat, those farther away absorb only pressure. The noise heard is the collision of gas molecules. But in the vacuum of outer space, not a thing would be heard—an American Fourth of July fireworks show would be like a silent movie.

Heat is another thing that disturbs air. Whether it comes from the sun or from a raging fire, heat sets all the air molecules moving in the same general direction—flowing along.

If a person is in their way, he or she feels their impact on the skin and calls it “wind.”

When gases expand—when the molecules get farther and farther apart—they lose heat, they cool; when they’re compressed—when the molecules are squeezed together—they get hot This is how a refrigerator works. It uses electricity to compress a gas like Freon outside the refrigerated enclosure, then lets it expand through a series of coils, getting colder and colder as it flows through the coils, and thus it cools the interior of the refrigerator.

Air inside an elastic container, such as a balloon, also demonstrates how gases respond to temperature. If an air-filled balloon is heated, its contents expand—and so does the balloon itself. Expanded, the air inside becomes lighter than the air outside, and the heated balloon rises. The higher the balloon climbs, the cooler the outside air gets. So unless the air inside the balloon is continually heated, it starts to cool, and the balloon shrinks, stops rising, and begins to fall.

Robert Boyle applied math to chemistry and described how the volume of a gas is related to its pressure. He stated that when the pressure (P) increases, the volume (V) decreases, and vice versa, if the temperature remains the same:

V = k/P

One of Boyle’s followers was a Frenchman named Jacques Alexandre Cesar Charles (1746-1823) who was interested in lighter-than-air balloons. Balloon rides were beginning to be attempted in France when the Revolutionary War in America was winding down, and Charles went up every chance he got. But he wasn’t just having fun—he was taking notes. And like Boyle, he applied math to his observations. Today, thanks in part to those balloon rides, there is Charles’ Law equating the temperature (T) and volume (V) of a gas [where (k) is a constant):

V/T = k or


These formulas are ways of saying that if the temperature of a gas increases, so does the volume—and vice versa, provided the pressure stays the same.

The three states of matter reflect basic differences in the ways that molecules can be arranged (A). Matter can be compressed in the gaseous state (B), but not in the liquid or solid states.

This works well for balloons, which expand and contract under the influence of atmospheric pressure, but what happens when gas is kept in a rigid container, like a steel tank, so that the volume can’t change? That’s what Joseph Gay-Lussac wanted to know. He discovered that when a gas is kept in a rigid container, the pressure (P) goes up when the tem-erature (T) goes up (the volume in this case eing kept constant by the container). Following in the footsteps of Boyle and Charles, he expressed this law mathematically, as follows:

P/T = k or


Once these three equations had been formulated, it took just simple algebra to come up with what is now known as the Combined Gas Law, an equation that allows for the calculation of change in any one of the three variables—V, T, or P—when there is a change in both of the other two variables:

P,V,/T, = P2V2/T2

So it is clear that gases all behave similarly despite their chemical identity, with molecules moving in all directions, and enormous space between them compared to their size.


As might be expected, the molecules in a liquid are much closer together and there is very little space between them. They are packed almost as closely as the molecules in a solid, but still have enough room between them to slip, slide, and roll over one another like tiny ball bearings. These molecules are constantly moving, but held together loosely by the forces of intermolecular attraction. In liquid water, this force is called the hydrogen bond.

The way molecules of a liquid are packed, compared with molecules of a gas, explains why liquids behave differently from gases. For instance, gas can be compressed because the space between molecules is so great, but liquid can’t be compressed. If a liquid is pushed, it pushes back—a capacity that provides the basis of the science of hydraulics and hydraulically powered machines. A gas will expand to fill the volume of the container that holds it, but a liquid poured into a rigid container fills only part of the container and takes on the shape of the container—a shape as distinct as that of a solid.

If enough heat energy is put into a liquid, raising its temperature to a certain point, large numbers of its molecules fly off in every direction. The process by which a substance passes from a liquid state to a gas is called boiling.

Atoms of many different elements combine in groups of 2 or more to form a huge array of molec-. ar compounds. This dia-sram illustrates 13 major elements and some of the : i mpounds they form that tre familiar as gases, liquids, or solids. The + and – signs are oxidation numbers.

When this process occurs at room temperature so slowly it can hardly be seen it’s called evaporation. But draw off the heat energy of a liquid or cool it and the movement of the molecules slows down. The liquid becomes more viscous (thicker), until the molecules stop moving altogether and become locked in place, or frozen. This is how a substance passes from the liquid state to the solid state.

arbon has three crystalline forms. In diamond, each atom is bonded to four others in a tetrahedral arrangement. Graphite is formed of six-membered rings joined together to form flat sheets. Fullerenes are composed of 5- and 6-membered rings joined together in nearly spherical molecules.


The solid state is the state that is most familiar.

The key to this state the chief thing that sets a solid apart from a liquid and a gas is structure.

Molecules and atoms in a solid are immobile, and nearly always arranged in a definite, orderly structure. As a solid, every substance has its characteristic atomic structure, and differences in structure express themselves in differences in physical properties such as color, hardness, and luster.

For example, as a pure element in nature, carbon has long been known to exist in two allotropes, or forms: graphite soft, gray, and greasy; and diamond hard, brilliant, and transparent. Within the rocks of the earth’s

crust, at certain temperatures and pressures, carbon takes the form of graphite. In this form, its atoms are tightly bound in two dimensions but loosely bound in the third, so that they slide over one another like a deck of cards, giving graphite its slippery, lubricant quality. But at much higher temperatures and pressures, far below the earth’s surface, carbon atoms rearrange themselves into a tight interlocking framework and form diamond, the hardest of all substances.

In 1985, another allotrope of carbon, called fullerene, was discovered. As a fullerene, carbon exists in nearly spherical clusters of 60 to 70 atoms. Small amounts of fullerene are found in the soot deposited by flames.

The geometric orderliness of the structures of the solid state is expressed in the property known as crystallinity, a characteristic of substances as dissimilar as diamond and ice. Organic solids such as plastics are composed of carbon/hydrogen molecules arranged in polymers, endless intertwining chains that resemble braided hair. In contrast, the atoms that comprise inorganic solids are arranged in ordered, three-dimensional frameworks, which is what is meant when a substance is said to be crystalline.

Although individual crystals of a substance are usually too small to see without a microscope, under certain natural conditions some crystals grow very slowly to huge sizes, their lengths measured in meters. These conditions occur deep within the earth where, as a giant body of molten granitic magma cools, water-rich offshoots solidify more slowly than the main mass. This allows time for crystals of common minerals such as mica, quartz, and feldspar to grow to huge size, their weight measured in tons.

Sodium chloride crystals (A) with ions at each corner (table salt) form a lattice of and at the centers of each sodium ions (Na+) and chlo- face (B). The ion at the cen-ride ions (CI-). These ions ter (C)—sodium or chloare held together by the ride—is closest to six ions electrostatic attraction be- of the other type arranged tween positive and negative octahedrally around it. This forces. This is known as octahedron sits in the cenionic bonding. The struc- ter of the lattice, ture is a face-centered cube