1. Introduction
All substances are matter. This includes the air, sea, Earth, all living creatures
and even the Sun and Stars.
Matter is anything that has mass and takes up
space.
There are three forms of matter solid, liquid,
gas. These three forms are called states of
matter.
Particle Theory
Solid
.Particles in solid are held tightly and packed fairly close
together - they are strongly attracted to each other.
.Particles are in fixed positions but they do vibrate.
Liquid
. Particles are fairly close together with some attraction
between them.
.Particles are able to move around in all directions but
movement is limited by attractions between particles.
Gas
.Particles have little attraction between them.
. Particles are free to move in all directions and collide
with each other and with the walls of a container and are
widely spaced out.
2. Properties of matter
Solids
Have a definite shape.
Maintain that shape.
Are difficult to compress as the particles are already packed closely
together.
Are often dense as there are many particles packed closely together.
Liquids
Do not have a definite shape.
Flow and fill the bottom of a container. They maintain the same volume
unless the temperature changes.
Are difficult to compress because there are quite a lot of particles in a small
volume.
Are often dense because there are quite a lot of particles in a small volume.
Gases
Do not have a definite shape.
Expand to fill any container.
Are easily compressed because there are only a few particles in a large
volume.
Are often low density as there are not many particles in a large space.
3. Inter conversion of states of matter
Melting
In a solid the strong attractions between the particles hold them tightly packed
together. Even though they are vibrating this is not enough to disrupt the structure.
When a solid is heated the particles gain energy
and start to vibrate faster and faster. Initially the
structure is gradually weakened which has the
effect of expanding the solid. Further heating
provides more energy until the particles start to
break free of the structure. Although the particles
are still loosely connected they are able to move around. At this point the solid is
melting to form a liquid. The particles in the liquid are the same as in the solid but
they have more energy. To melt a solid energy is required to overcome the
attractions between the particles and allow them to pull them apart. The energy is
provided when the solid is heated up. The temperature at which something melts is
called its "melting point" or melting temperature. At room temperature a material is
a solid, liquid or gas depending on its melting temperature. Anything with a melting
temperature higher than about 20oC is likely to be a solid under normal conditions.
Boiling
If a liquid is heated the particles are given more energy and move faster and faster
expanding the liquid. The most energetic particles at the surface escape from the
surface of the liquid as a vapour as it gets warmer.
Liquids evaporate faster as they heat up and more
particles have enough energy to break away. The
particles need energy to overcome the attractions
between them. As the liquid gets warmer more
particles have sufficient energy to escape from the
liquid. Eventually particles in the middle of the liquid
form bubbles of gas in the liquid. At this point the
liquid is boiling and turning to gas. The particles in the
gas are the same as they were in the liquid they just have more energy. At normal
atmospheric pressure all materials have a specific temperature at which boiling
occurs. This is called the "boiling point" or boiling temperature. As with the melting
point the boiling point of materials vary widely.
4. Evaporating
Within a liquid some particles have more energy than other. These "more
energetic particles" may have sufficient energy to escape from the surface of the
liquid as gas or vapour. This process is called evaporation and the result of
evaporation is commonly observed when puddles or clothes dry. Evaporation takes
place at room temperature which is often well
below the boiling point of the liquid.
Evaporation happens from the surface of the
liquid. As the temperature increases the rate of
evaporation increases. Evaporation is also
assisted by windy conditions which help to
remove the vapour particles from the liquid so that
more escape.
Evaporation is a complex idea for children for a
number of reasons. The process involves the apparent disappearance of a liquid
which makes the process difficult for them to understand. It is not easy to see the
water particles in the air. Also, evaporation occurs in a number of quite differing
situations - such as from a puddle or bowl of water where the amount of liquid
obviously changes, to situations where the liquid is less obvious - such as clothes
drying or even those where there is no obvious liquid at all to start with - such as
bread drying out. A further complication is that evaporation may be of a solvent from
a solution e.g. water evaporating from salt water to leave salt. These situations are
quite different yet all involve evaporation.
5. Diffusion
Diffusion is one of several transport phenomena that occur in nature. A
distinguishing feature of diffusion is that it results in mixing or mass transport
without requiring bulk motion. Thus, diffusion should not be confused
with convection or advection, which is other transport mechanisms that use
bulk motion to move particles from one place to another.
From the atomistic point of view, diffusion is considered as a result of
the random walk of the diffusing particles. In molecular diffusion, the moving
molecules are self propelled by thermal energy. Random walk of small
particles in suspension in a fluid was discovered in 1827 by Robert Brown.
The theory of the Brownian motion and the atomistic backgrounds of diffusion
were developed by Albert Einstein.
Now, the concept of diffusion is widely used in science: in physics (particle
diffusion), chemistry and biology, in sociology, economics and finance
(diffusion of people, ideas and of price values). It appears every time, when
the concept of random walk in ensembles of individuals is applicable.
6. Graham’s Law of Diffusion
Inter mixing of two or more gases to form a homogeneous mixture without any
chemical change is called "DIFFUSION OF GASES". Diffusion is purely a physical
phenomenon. Gases diffuse very quickly due to large empty spaces among
molecules. Different gases diffuse with different rates (velocities).
The law stating that the rate of diffusion of a gas through a liquid (or the
alveolar-capillary membrane) is directly proportional to its solubility coefficient
and inversely proportional to the square root of its density.
Graham's law is a quantitative relation between the density and rate
of diffusion of gases.
Mathematical Representation of the Law
Since density a molecular mass, therefore, we can replace density d by Molecular
mass M.
7. Effects of Pressure and Temperature on gases
If the volume of the container is not fixed, increasing the temperature will cause
a gas to expand (increase the volume), and contract when cooled (decreasing
the volume). This would be the case for a gas
inside a piston, or inside a rubber balloon.
If the volume is fixed, then increasing the
temperature will increase the pressure, and
decreasing the temperature will decrease the
pressure. This would be the case for a gas in a
closed solid container, like a canister or sealed
metal box.
Increasing pressure will cause the gas to contract
(reducing the volume), and decreasing
the pressure will cause the gas to
expand (increasing the volume).
Again, this is if the volume is not
fixed. If the volume is fixed, then
increasing the pressure will increase
the temperature, and decreasing the
pressure will decrease the
temperature.
Gases can also be changed to a liquid
or solid if the temperature is too low
or the pressure is too high. As an
example steam changes to a liquid
when it touches a cold object, and
nitrogen gas can be converted to liquid nitrogen by compressing it to very high
pressures.
