Short notes of chapter 1( based on matter )of class 8

*Matter

- matter is something which occupies space, has mass and can be perceived by our senses.

Composition-

•Indian philosophers- 5 tatvas- akash,vayu,Tejas(Agni),jal & prithvi.

•Maharishi kanada - tiny particles-anu

•John Dalton-tiny particles-molecules.

A molecule can exist free in nature and it has all the properties of that substance.

-a molecule can be of:

*one atom-monoatomic molecule.eg helium, neon,argon ,etc.

*two atoms-diatomic molecule.eg hydrogen molecule, oxygen molecule, nitrogen molecule, etc.

*more than two atoms-Polyatomic molecule.eg water, ammonia, carbon dioxide, etc.

*Characteristics of molecules:

1. Molecules are very small in size-

     -nearly 10-¹⁰ m

     -•crystals of potassium permanganate-dark purple colour (to demonstrate that matter is composed of tiny particles)

2. The molecules of matter have spaces between them- inter molecular space.

S<L<G.

•salt+water experiment = the molecules of salt occupy the spaces between the molecules of water.

3. Molecules are in constant motion

   Solid-vibrate about their mean positions without leaving their position.

Liquid-free to move within the boundary of the container.

Gas-can move in random manner everywhere

•Sprayed perfume experiment

•Lycopodium powder (yellow coloured dust powder/jash powder)-zig zag part of a fine particle of lycopodium powder.

4. Molecules attract each other:

   -inter-molecular force-is effective between the molecules only up to a separation of 10-⁹ m.

S>L>G

  •force of cohesion-intermolecular force of attraction between the molecules of the same substance.

•force of adhesion-the force of attraction between the molecules of two different substances.

•the particles of coal are bound by the strong forces of attraction amongst themselves.

*States of matter:

Three physical States of a substance can be changed by changing its physical condition such as temperature and pressure.

-•solids-rigid, definite size (volume) & definite shape.

•liquids-not rigid, definite volume but no definite shape.

•gases-not Rigid, neither a definite volume nor a definite shape.

 The properties which decide the state of a substance:

1. Intermolecular space

2. Intermolecular force

3. Kinetic energy of molecules due to their motion.

*Molecular model of solids:

1. Made up of very tiny particles-molecules.

   Like tiny rigid balls.

2. Inter molecular spacing is very small.

3. Can only vibrate to and fro about their mean positions. They do not leave their positions.

4. Closely packed due to the strong inter-molecular force between them.

*Molecular model of liquids:

1. made up of very tiny particles -molecules.

2. Intermolecular spacing in liquids are more than solids.

3.can move about freely within the boundary of The vessel in which the liquid is kept.

4.  less closely packed because the intermolecular forces in a liquid are weak in comparison to that in solids.

*Molecular model of gases:

1. Made up of very tiny particles-molecules.

   Like non-rigid balls.

2. Intermolecular spacing is very large.

3. can move about freely in the space available to them.

4. Are wide apart because the intermolecular forces in them are very weak.

    -the process of change from one state to another state either by absorption or rejection of heat at a constant temperature is called change of state.

Melting/fusion-the process of change from solid state to  liquid state by absorption of heat at a constant temperature.

Melting point-the constant temperature at which solid changes into liquid by absorbing heat energy.

•The wax melts at 55 degree Celsius during which heat is supplied, but temperature does not rise. After The melting of whole wax, the temperature begins to rise.energy used in increasing the potential energy of molecules.

*Melting point of ice decreases with increase of pressure on it,but the melting point of wax increases with increase of pressure on it.

*melting point of ice decreases when salt is added to it. This mixture of salt added to Ice is called freezing mixture. It is used for preparing kulfis.

•

Freezing-the process of change from liquid state to solid state by rejection of heat at a constant temperature.

*Water expands on freezing whereas wax, lead etc., Contract on freezing.

Freezing point-the constant temperature at which liquid changes into solid state by rejecting heat energy.

*Freezing point and melting point are the same.

*Explanation of melting by molecular model-

-on heating a solid,the kinetic energy of molecules increases due to which they begin to vibrate more violently.the molecules acquire sufficient kinetic energy to become far separated from each other by overcoming the forces of attraction between them & gets converted into liquid state.

AT MELTING POINT-the heat energy absorbed by the substance does not change the temperature of the substance,but is utilised in doing work against the forces of attraction in increasing the separation between the molecules that is their potential energy increases.

Boiling/vaporization-the process of change from liquid state to gaseous state by absorption of heat at a constant temperature.

Condensation-the process of change from gaseous state to liquid state by rejection of heat at a constant temperature.

* boiling point and condensation point are same.

*Explanation of vaporization by molecular model-on heating, the average kinetic energy of molecules of liquid increases.the molecules acquire sufficient kinetic energy to overcome the forces of attraction between themselves and they become free to leave the surface,thus converted into gases state.

AT BOILING POINT-The heat energy absorbed does not change the kinetic energy of the molecules,but is utilised in doing work against the force of attraction in increasing the separation between the molecules that is an increasing their potential energy.

Sublimation-the process of change from solid state to gaseous state directly by absorption of heat at a constant temperature .eg. ammonium chloride, iodine, camphor, naphthalene and solid carbon dioxide (dry ice).

Sometimes sublimation also takes place without heating. example naphthalene balls (moth balls) which is used to protect clothes, decreases in size with time & becomes small.

Deposition/solidification-process of change from gaseous state to solid state directly by rejection of heat at a constant temperature.

Hidden heat/latent heat-the quantity of heat given to or taken from the substance during the the change of state at a constant temperature.

• *Evaporation-the change from liquid state to gasses state on absorbing heat energy at all temperatures.
• *Explanation of evaporation by the molecular model:
• The molecules are in motion within its boundary and collide with each other. Some molecules below the surface of liquid acquire sufficient kinetic energy to overcome the force of attraction of other molecules and their intermolecular spacing increases such molecules have  to move to the surface of the liquid and they got it from surroundings so as to escape out into the atmosphere with atmospheric air molecules.
• *Factors affecting rate of evaporation:
• The temperature of liquid
• The area of the exposed surface cloth dries up faster if it is spread out than if it is folded up.
• The nature of liquid-volatile liquid such as alcohol, spirit, ether etc. evaporates much faster than water. The force of attraction among molecules is negligible in volatile liquids . So, they escape out more easily.
• The flow of air above  the liquid.to cool the milk, we blow air above it. On blowing air,the air molecules take away the liquid molecules from the surface with them and other molecules of the liquid occupy their place. This increases the rate of evaporation.
• The presence of moisture or humidity.wet clothes dry faster in dry summer days than in rainy season.
• *Evaporation produces cooling:
• When a liquid changes into vapour, it requires heat. Heat is supplied by the surroundings of the liquid .this results in fall in temperature in the surroundings.
• *Applications of evaporation:
• 1. Water gets cold  in an earthen pot.the heat required for evaporation is taken from water inside the pot.
• 2. The the strips of wet cloth on the fore head of a patient having high fever. During evaporation water takes heat it from the body of the patient.
• 3.tea in a saucer, the surface area of tea increases and evaporation becomes faster.
• 4.sweat from our body helps to maintain the body temperature at 37°C or 98.6°F. when sweat evaporates, it requires heat which. it takes away from our body.
• The average kinetic energy of molecules of a substance is a measure of its temperature.
• D the average kinetic energy of its molecules increases due to which its temperature rises. But is a substance gives out heat,the average kinetic energy of its molecules decreases due to which its temperature falls.
• 
  
  
  
  
  
• *Explanation of sublimation by molecular model:
• the intermolecular forces of attractions are weak in some solids, on heating the molecules acquire sufficient kinetic energy to overcome the force of of attraction and they become free to move and thus intermolecular separation becomes very large and they change to vapour directly.