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  for

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.

Heat Heat

Solid State 

Cool Cool

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

Solid State Liquid State Gaseous State

Finish Line & Beyond

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.