States of matter

Solids liquids and gases

• 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.

• A gasis 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 boundarythat 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.

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

Density 26/1/22

We write d = m/v where d is density, m is mass, and v is volume

The units used to describe density often differ for the phases of matter: solids (g/cm3), liquids (g/mL), and gases (g/L)

Mass and volume, as we learned in the previous unit, are measures of the quantity of a substance, and as such are defined as extensive properties of matter.).

You will recall that the ratio of two extensive properties is always an intensive property – one that characterises a particular kind of matter, independently of its size or mass. It

is this ratio, (mass ÷ volume), that we are concerned with in this lesson.

These plots show how the masses of three liquids vary with their volumes. Notice that

• the plots all have the same origin of (0,0): if the mass is zero, so is the volume;

• the plots are all straight lines, which signify direct proportionality.

The only difference between these plots is their slopes. Denoting mass and volume by m and V respectively, we can write the equation of each line as m = ρV, where the slope ρ (Greek lower-case rho) is the proportionality constant that relates mass to volume. This quantity ρ is known as the density, which is usually defined as the mass per unit volume: ρ = **m/**V

The volume units milliliter (mL) and cubic centimeter (cm3) are almost identical and are commonly used interchangeably.

Density can be expressed in any combination of mass and volume units; the most commonly seen units are grams per mL (g mL–1, g cm–3), or kilograms per liter.

1 kg m–3 = 1 g L–1 = .0624 lb ft–3

Density = mass over volume

SI unit + kg over m3