Motion of gas molecules Español: Animación mostrando la agitación térmica de un gas. Cinco partículas han sido coloreadas de rojo para facilitar el seguimiento de sus movimientos. Русский: Хаотическое тепловое движение на плоскости частиц газа таких как атомы и молекулы (Photo credit: Wikipedia)
Deutsch: Teilchemodellbild eines flüssigen Stoffes Kategorie:Datei:Chemie (Photo credit: Wikipedia)
Space-filling model of the part of the crystal structure of solid carbon dioxide (dry ice), CO 2 . Crystal structure data from AMCSD. (Photo credit: Wikipedia)
Diagram of particles in solid, liquid, and gas states (Photo credit: Wikipedia)
Matter in Our Surroundings
Characteristics of Particles of Matter
States of Matter
Effect of Change of Temperature
Effect of Change of Pressure
Introduction
Everything in this universe is made up of material which scientists have named “matter”.
The air we breathe, the food we eat, stones, clouds, stars, plants and animals, even a
small drop of water or a particle of sand– each thing is matter.
Early Indian philosophers classified matter in the form of five basic elements –air, earth,
fire, sky and water.
Modern day scientists have evolved two types of classification of matter based on their
physical properties and chemical nature.
Characteristics of Particles of Matter
1. PARTICLES OF MATTER HAVE SPACE BETWEEN THEM- when we make tea,
coffee or lemonade (nimbu paani ), particles of one type of matter get into the spaces
between particles of the other. This shows that there is enough space between particles
of matter. Similarly particles of sugar, salt, Dettol, or potassium permanganate get
evenly distributed in water.
2. PARTICLES OF MATTER ARE CONTINUOUSLY MOVING- Particles of matter are
continuously moving, that is, they possess what we call the kinetic energy. As the
temperature rises, particles move faster. So, we can say that with increase in
temperature the kinetic energy of the particles also increases.
3. PARTICLES OF MATTER ATTRACT EACH OTHER- Particles of matter have force
acting between them. This force keeps the particles together. The strength of this force
of attraction varies from one kind of matter to another.
States of Matter
1. THE SOLID STATE- all solids have a definite shape, distinct boundaries and fixed
volumes, that is, have negligible compressibility. Solids have a tendency to maintain
their shape when subjected to outside force. Solids may break under force but it is
difficult to change their shape, so they are rigid.
2. THE LIQUID STATE- Liquids have no fixed shape but have a fixed volume. They
take up the shape of the container in which they are kept. Liquids flow and change
shape, so they are not rigid but can be called fluid. The rate of diffusion of liquids is
higher than that of solids. This is due to the fact that in the liquid state, particles move
freely and have greater space between each other as compared to particles in the solid
state.
3. THE GASEOUS STATE- Gases are highly compressible as compared to solids and
liquids. The liquefied petroleum gas (LPG) cylinder that we get in our home f or cooking
or the oxygen supplied to hospitals in cylinders is compressed gas. In the gaseous state,
the particles move about randomly at high speed. Due to this random movement, the
particles hit each other and also the walls of the container.
Matter Can Change its State
Water can exist in three states of matter–
• Solid, as ice,
• Liquid, as the familiar water, and
• Gas, as water vapour.
EFFECT OF CHANGE OF TEMPERATURE
On increasing the temperature of solids, the kinetic energy of the particles increases.
Due to the increase in kinetic energy, the particles start vibrating with greater speed.
The energy supplied by heat overcomes the forces of attraction between the particles.
The particles leave their fixed positions and start moving more freely. A stage is reached
when the solid melts and is converted to a liquid. The temperature at which a solid
melts to become a liquid at the atmospheric pressure is called its melting point.
The melting point of ice is 273.16 K*. The process of melting, that is, change of solid state
into liquid state is also known as fusion.
So, we infer that the state of matter can be changed into another state by changing the
temperature.
EFFECT OF CHANGE OF PRESSURE
Increasing or decreasing the pressure can change the state of matter. Applying pressure
and reducing temperature can liquefy gases.
For an example, Solid carbon dioxide (CO2) is stored under high pressure. Solid CO2
gets converted directly to gaseous state on decrease of pressure to 1 atmosphere*
without coming into liquid state. This is the reason that solid carbon dioxide is also
known as dry ice.
Thus, we can say that pressure and temperature determine the state of a substance,
whether it will be solid, liquid or gas.
Evaporation
Particles of matter are always moving and are never at rest. At a given temperature in
any gas, liquid or solid, there are particles with different amounts of kinetic energy. In
the case of liquids, a small fraction of particles at the surface, having higher kinetic
energy, is able to break away from the forces of attraction of other particles and gets
converted into vapour. This phenomenon of change of a liquid into vapours at any
temperature below its boiling point is called evaporation.
FACTORS AFFECTING EVAPORATION
The rate of evaporation increases with–
• An increase of surface area:
We know that evaporation is a surface phenomenon. If the surface area is increased,
the rate of evaporation increases. For example, while putting clothes for drying up we
spread them out.
• An increase of temperature:
With the increase of temperature, more number of particles get enough kinetic energy to
go into the vapour state.
• A decrease in humidity:
Humidity is the amount of water vapour present in air. The air around us cannot hold
more than a definite amount of water vapour at a given temperature. If the amount of
water in air is already high, the rate of evaporation decreases.
• An increase in wind speed:
It is a common observation that clothes dry faster on a windy day. With the increase in
wind speed, the particles of water vapour move away with the wind, decreasing the
amount of water vapour in the surrounding.
HOW DOES EVAPORATION CAUSE COOLING?
In an open vessel, the liquid keeps on evaporating. The particles of liquid absorb energy
from the surrounding to regain the energy lost during evaporation. This absorption of
energy from the surroundings make the surroundings cold.
After a hot sunny day, people sprinkle water on the roof or open ground because the large
latent heat of vaporisation of water helps to cool the hot surface.
Why should we wear cotton clothes in summer?
During summer, we perspire more because of the mechanism of our body which keeps
us cool. We know that during evaporation, the particles at the surface of the liquid gain
energy from the surroundings or body surface and change into vapour. The heat energy
equal to the latent heat of vaporisation is absorbed from the body leaving the body
cool. Cotton, being a good absorber of water helps in absorbing the sweat and exposing
it to the atmosphere for easy evaporation.
Why do we see water droplets on the outer surface of a glass containing icecold
water?
Let us take some ice-cold water in a tumbler. Very soon we see water droplets on the
outer surface of the tumbler. The water vapour present in air, on coming in contact with
the cold glass of water, loses energy and gets converted to liquid state, which we see
as water droplets.
Summary
• Matter is made up of small particles.
• The matter around us exists in three states— solid, liquid and gas.
• The forces of attraction between the particles are maximum in solids, intermediate in
liquids and minimum in gases.
• The spaces in between the constituent particles and kinetic energy of the particles are
minimum in the case of solids, intermediate in liquids and maximum in gases.
• The arrangement of particles is most ordered in the case of solids, in the case of liquids
layers of particles can slip and slide over each other while for gases, there is no order,
particles just move about randomly.
• The states of matter are inter-convertible. The state of matter can be changed by
changing temperature or pressure.
• Sublimation is the change of gaseous state directly to solid state without going through
liquid state, and vice versa.
• Boiling is a bulk phenomenon. Particles from the bulk (whole) of the liquid change into
vapour state.
• Evaporation is a surface phenomenon. Particles from the surface gain enough energy
to overcome the forces of attraction present in the liquid and change into the vapour
state.
• The rate of evaporation depends upon the surface area exposed to the atmosphere, the
temperature, the humidity and the wind speed.
• Evaporation causes cooling.
• Latent heat of vaporisation is the heat energy required to change 1 kg of a liquid to gas
at atmospheric pressure at its boiling point.
• Latent heat of fusion is the amount of heat energy required to change 1 kg of solid into
liquid at its melting point.
• Some measurable quantities and their units to remember:
To change a temperature on the Kelvin scale to the Celsius scale we have to subtract
273 from the given temperature, and to convert a temperature on the Celsius scale to
the Kelvin scale we have to add 273 to the given temperature.
* Atmosphere (atm) is a unit of measuring pressure exerted by a gas. The unit of
pressure is Pascal (Pa):
1 atmosphere = 1.01 × (10 to the power 5) Pa. The pressure of air in atmosphere is
called atmospheric pressure. The atmospheric pressure at sea level is 1 atmosphere, and
is taken as the normal atmospheric pressure.
Note Blast 