10.1 An overview of Physical States and Phase Changes

1. States of matter 

All the matter around you occurs in one or more of the three physical states-gas, liquid or solid. Under certain conditions, many pure substances can occur in any one of the states.  

The fundamental difference between states of matter is the strength of the intermolecular forces of attraction. Stronger forces bring molecules closer together. Solids and liquids are referred to as the condensed phases. 

Characteristic Properties of states of Matter: 

Table 10.1 

So, How do we know which one is state of a matter ? The answer to this question largely relies on the balance between the kinetic energies of the particles. Inter-particle energies of attraction. In gas, kinetic energy of the molecules is much greater than energies of attraction whereas, in solid, energies of attraction is very high compared to kinetic energies. In liquid, we see comparable level of kinetic energies and energies of attraction.  

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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 

• 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. 

Macroscopic physical properties of gases Figure 10.3 

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. 

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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! 

Condensed states of matter 

Liquids and solids share most of the properties of having their molecular units in direct contact as discussed in the previous section on condensed states of matter. At they same time, liquids, like gases, are fluids, meaning that their molecular units can move more or less independently of each other. But whereas the volume of a gas depends entirely on the pressure (and thus generally on the volume within which it is confined), the volume of a liquid is largely independent of the pressure. 

We discuss the properties of liquids in some detail in another section. In the next section  we offer just enough to help you see how they these condensed states of matter originate and the factors controlling the strength to bind them together.