Kinetic Molecular Theory and States of Matter | Nature of matter - Class IX
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Kinetic Molecular Theory and States of Matter
What is matter?
Anything that has mass and occupies space is called matter.
Examples: water, air, wood, stone, etc.
Key Remarks
* Matter is not continuous. It has a particulate nature, i.e, it is made of particles (called molecules) with spaces between them and force of attraction among them.
* Molecules are defined as the smallest particles and the basic units of a matter that can exist independently and still retain the matter's chemical properties.
* Molecules are defined as the smallest particles and the basic units of a matter that can exist independently and still retain the matter's chemical properties.
Kinetic Molecular Theory
The kinetic molecular theory explains matter in terms of the motion of its particles.
Note: Kinetic molecular theory is specially used for gases but it also helps to explain the behaviour of solids and liquids.
Postulates of Kinetic Molecular Theory
Postulates of kinetic molecular theory are the basic assumptions used to explain the behaviour of matter, especially gases.
- Matter is made of very small, tiny particles called molecules.
- There are empty spaces between the molecules. The empty spaces existing between the molecules are called intermolecular spaces.
- Molecules have force of attraction among them. The force of attraction existing among the molecules are called intermolecular forces of attraction.
- The molecules posses kinetic energy due to their continuous random motion.
- The average kinetic energy of molecules increases with with temperature and is proportional to Kelvin Temperature.
Key Remarks
* The molecular force of attraction has two types :
(i) Cohesive force of attraction
(ii) Adhesive force of attraction.
* The force of attraction among similar molecules is called cohesive force of attraction. For example, in a glass of water, water-water molecules pull each other, that is cohesive force.
* The force of attraction among dissimilar molecules is called adhesive force of attraction. For example, in a glass of water, water-glass molecules pull each other, that is adhesive force.
(i) Cohesive force of attraction
(ii) Adhesive force of attraction.
* The force of attraction among similar molecules is called cohesive force of attraction. For example, in a glass of water, water-water molecules pull each other, that is cohesive force.
* The force of attraction among dissimilar molecules is called adhesive force of attraction. For example, in a glass of water, water-glass molecules pull each other, that is adhesive force.
States of Matter
Matter basically exists in three states:
(i) Solids,
(ii) Liquids and
(iii) Gases
Key Remarks
* In modern science, matter can be said to have five states.
* Plasma and Bose-Einstein Condensate are included as its other two states.
* Plasma and Bose-Einstein Condensate are included as its other two states.
Different states of matter and their properties
| Properties | Solids | Liquids | Gases |
|---|---|---|---|
| Mass | definite | definite | definite |
| Volume | definite | definite | indefinite |
| Shape | definite | indefinite | indefinite |
| Density | High | lesser | least |
| Compressibility | incompressible | slightly compressible | highly compressible |
| Rigidity | rigid (cannot flow) | fluid (can flow) | fluid |
| Free surfaces | any number of free surfaces | 1 (only the upper surface) | 0 |
| Thermal Expansion | very low | higher | very high |
| Diffusion | generally cannot diffuse | some liquids can diffuse spontaneously into another | diffuses spontaneously and rapidly |
Key Remarks
* The amount of matter in a body is called its mass.
* The space occupied by a matter is called its volume.
* The external form of a matter is called its shape.
* Mass per unit volume of a matter is called its density.
* The ability of a matter to decrease in its volume when pressure is applied is called its compressibility.
* The property of a matter by which it resists change in its shape or size when external force is applied is called rigidity.
* The surface of a matter open to the atmospheric air is called its free surface.
* The increase in length, area, or volume of a matter when it is heated is called thermal expansion.
* The spreading of the molecules of a matter from a region of higher concentration to a region of lower concentration until they are evenly mixed is called diffusion.
* The space occupied by a matter is called its volume.
* The external form of a matter is called its shape.
* Mass per unit volume of a matter is called its density.
* The ability of a matter to decrease in its volume when pressure is applied is called its compressibility.
* The property of a matter by which it resists change in its shape or size when external force is applied is called rigidity.
* The surface of a matter open to the atmospheric air is called its free surface.
* The increase in length, area, or volume of a matter when it is heated is called thermal expansion.
* The spreading of the molecules of a matter from a region of higher concentration to a region of lower concentration until they are evenly mixed is called diffusion.
Comparative Molecular Arrangement in different states
| Parameters | Solids | Liquids | Gases |
|---|---|---|---|
| Packaging of molecules | closely packed | loosely packed | very loosely packed |
| intermolecular space | very low | high | the highest |
| intermolecular force of attraction | very strong | moderate | negligible |
| Molecular movement | posses only vibratory motion | posses translatory, rotatory and vibratory motion in one direction | posses translatory, rotatory and vibratory motion in all directions |
| Kinetic energy | very low | high | the highest |
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