Solids, Liquids, and Gases

The three phases of matter are solid, liquid, and gas. Note the differences between these three phases of matter on the microscopic and macroscopic levels.

  • Solids, like wooden blocks, have definite shape and definite volume. The particles are ordered and close together.
  • Liquids have definite volume and indefinite shape, meaning they take on the shape of the container. A liquid's particles are less ordered, but still relatively close together.
  • Gases, such as the air inside balloons, take the shape and volume of their container. Their particles are highly disordered.

Read this text. Pay attention to the first image which shows the microscopic differences between the phases of matter.

What distinguishes solids, liquids, and gases – the three major states of matter – from each other? Let us begin at the microscopic level, by reviewing what we know about gases, the simplest state in which matter can exist.

At ordinary pressures, the molecules of a gas are so far apart that intermolecular forces have an insignificant effect on the random thermal motions of the individual particles.

As the temperature decreases and the pressure increases, intermolecular attractions become more important, and there will be an increasing tendency for molecules to form temporary clusters. These are so short-lived, however, that even under extreme conditions, gases cannot be said to possess structure in the usual sense.

solids, liquids and gases

The contrast at the microscopic level between solids, liquids and gases is most clearly seen in the simplified schematic views above. The molecular units of crystalline solids tend to be highly ordered, with each unit occupying a fixed position with respect to the others.

In liquids, the molecules are able to slip around each other, introducing an element of disorder and creating some void spaces that decrease the density.

Gases present a picture of almost total disorder, with practically no restrictions on where any one molecule can be.

Solids, Liquids and Gases: How to Tell Them Apart

Having lived our lives in a world composed of solids, liquids, and gases, few of us ever have any difficulty deciding into which of these categories a given sample of matter falls. Our decision is most commonly based on purely visual cues:

  • A gas is transparent and has no definite boundaries other than those that might be imposed by the walls of a confining vessel.

  • Liquids and solids possess a clearly delineated phase boundary that gives solids their definite shapes and whose light-reflecting properties enable us to distinguish one phase from another.

  • Solids can have any conceivable shape, and their surfaces are usually too irregular to show specular (mirror-like) reflection of light. Liquids, on the other hand, are mobile; except when in the form of tiny droplets, liquids have no inherent shape of their own, but assume the shape of their container and show an approximately flat upper surface.

Our experience also tells us that these categories are quite distinct; a phase, which you will recall is a region of matter having uniform intensive properties, is either a gas, a liquid, or a solid. Thus the three states of matter are not simply three points on a continuum; when an ordinary solid melts, it usually does so at a definite temperature, without apparently passing through any states that are intermediate between a solid and a liquid.

Some Solids Can Flow – Slowly!

glacial flow

Kluane Glacier in Canada's Yukon Territory. Typical glacial flow rates are 10–200 meters per year.

Although these common-sense perceptions are usually correct, they are not infallible, and in fact there are gases that are not transparent, there are solids such as glasses and many plastics that do not have sharp melting points,

Macroscopic Physical Properties of Gases

Photo of a child with a balloon, with a photo of a drop of water.

A more scientific approach would be to compare the macroscopic physical properties of the three states of matter, but even here we run into difficulty. It is true, for example, that the density of a gas is usually about a thousandth of that of the liquid or solid at the same temperature and pressure; thus one gram of water vapor at 100°C and 1 atm pressure occupies a volume of 1671 mL; when it condenses to liquid water at the same temperature, it occupies only 1.043 mL.

Comparison of the molar volumes of neon in its three states. For the gaseous state, P = 1 atm and T = 0°C. The excluded volume is the volume actually taken up by the neon atoms according to the van der Waals model.

Gas 22,400 cm3/mol total volume (42 cm3/mol excluded volume)
Liquid 16.8 cm3/mol
Solid 13.9 cm3/mol

Here we compare the molar volumes of neon in its three states. For the gaseous state, P = 1 atm and T = 0°C. The excluded volume is the volume actually taken up by the neon atoms according to the Van der Waals model.

It is this extreme contrast with the gaseous states that leads to the appellation
condensed states of matter for liquids and solids. However, gases at very high pressures can have densities that exceed those of other solid and liquid substances, so density alone is not a sufficiently comprehensive criterion for distinguishing between the gaseous and condensed states of matter.

Similarly, the density of a solid is usually greater than that of the corresponding liquid at the same temperature and pressure, but not always: you have certainly seen ice floating on water!

Problem Example: Density of Xenon Gas

Compare the density of gaseous xenon (molar mass 131 g) at 100 atm and 0°C with that of a hydrocarbon liquid for which ρ = 0.104 g/mL at the same temperature.

Solution: For simplicity, we will pretend that xenon approximates an ideal gas under these conditions, which it really does not.

The ideal molar volume at 0° C and 1 atm is 22.4 L; at 100 atm, this would be reduced to .22 L or 220 mL, giving a density ρ = (131 g) / (224 mL) = 0.58 g/mL.

In his autobiographical Uncle Tungsten, the late physician/author Oliver Sacks describes his experience with xenon-filled balloons of "astonishing density — as near to 'lead balloons' as could be [imagined]. If one twirled these xenon balloons in one's hand, then stopped, the heavy gas, by its own momentum, would continue rotating for a minute, almost as if it were a liquid."

Other physical properties, such as the compressibility, surface tension, and viscosity, are somewhat more useful for distinguishing between the different states of matter. Even these, however, provide no well-defined dividing lines between the various states.

Rather than try to develop a strict scheme for classifying the three states of matter, it will be more useful to simply present a few generalizations.

Relative Magnitudes of Some Properties of the Three States of Matter

Property Gas Liquid Solid
Density very small large large
Thermal expansion coefficient large (= R/P) small small
Cohesiveness nil small large
Surface tension nil medium very large
Viscosity small medium very large
Kinetic energy per molecule large small smaller
Disorder random medium small

Definitions for the Terms Used in this Chart

Density is the mass per unit volume of a substance; it expresses how closely matter is packed together.

Thermal expansion coefficient usually refers to the volume thermal expansion coefficient that expresses the fractional change in volume per degree of temperature change.

Cohesiveness is a general term that refers to how difficult it is to pull a material apart, that is to overcome the attractive forces between its molecules.

Surface tension is a measure of the work required to move a molecule from the interior of a substance to its surface; it is a rough measure of the strength of the attractive forces between the molecular units. 

Viscosity is a measure of the resistance to flow; the term is normally applied to fluids.

Kinetic energy per molecule expresses the intensity of thermal motions in the molecular units of a substance; these motions are opposed by the intermolecular attractive forces.

Disorder in this context expresses the likelihood that the locations of the molecular units will change with time.


Source: Stephen Lower,
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Last modified: Monday, November 29, 2021, 5:59 PM