Chemistry REVISION OF PHYSICAL AND CHEMICAL CHANGES & ELEMENTS, MIXTURES AND COMPOUNDS FOR NDA Part-1
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Physical Changes :

A physical change is a change in which the composition of a substance is not affected, i.e. no new substance is formed. The physical changes are reversible.

Examples : (i) Interconversion of states is a physical change because these changes occur without a change in composition and no change in the chemical nature of the substance.

(ii) When an ordinary piece of iron is magnetised in the presence of a strong magnetic field. Since colour, density, mass and the composition remains the same. Therefore, magnetisation is a physical change. Magnetisation can be reversed by heating the piece of iron.

(iii) Evaporation, distillation, sublimation, condensation and crystallisation are examples of physical changes.

(iv) The blue colour of copper sulphate crystal disappears when it is heated strongly because on heating water of crystallisation of crystal is lost. It regains its blue colour when moistened with a drop of water.

`undersettext(Blue) (CuSO_4*5H_2O) oversettext(Heat)→ undersettext(white) (CuSO_4) +5H_2O`

It is an example of a physical change.

Some facts about the physical changes are

(i) Physical changes are temporary.
(ii) Composition of the constituent of matter remains same during physical change i.e. constituent particles of matter does not change.
(iii) Physical changes are due to the change in physical properties like density, volume, state temporarily.

`text(Note)`
► Amount of space that a substance or object occupies is called volume of that substance.
► Density is the degree of compactness of a substance. It is defined as mass per unit volume.
i.e. `text(Density) = text(mass)/text(volume)`

Chemical Changes :

These changes result in the formation of new substances. In a chemical change, the composition and molecular structure of the substance undergoing changes are considered.

Examples : (i) When a piece of iron or nail is left in moist air for a long time, it gives a reddish brown look due to rust. During rusting process iron combines with oxygen to give a new product. So, rusting is a chemical change which cannot be reversed back by any physical process.

(ii) The food we eat starts digesting from the mouth and during this process many chemical changes take place in the stomach. The complex food material gets converted into simple molecules of glucose which is oxidised in the body cells to release energy.

(iii) Burning of any substance is a chemical change. Burning is always accompanied by the production of heat. Explosion of a fire work is also a chemical change (heat, light, sound and unpleasant gases are produced in explosion).

(iv) Blue copper sulphate crystals on heating loses its all water molecules giving white anhydrous salt which decomposes into black cupric oxide and sulphur trioxide on strong heating.

`undersettext(Blue)(CuSO_4*5H_2O) underset(-5H_2O) overset(Heat)→ undersettext(White) (CuSO_4) undersettext(heat) oversettext(strongly)→ undersettext(Black)(CuO)+SO_3`

`text(Note)`

• Burning of a candle is an example of both physical and chemical changes.
• Ozone is decomposed to oxygen in the presence of ultraviolet rays. It is an example of chemical change.

Some facts about the chemical changes are
(i) During chemical changes, the composition of the substance changes due to change in constituent particles of substance.
(ii) Identity of the substance is lost during a chemical change.
(iii) Energy is absorbed or released during a chemical change.
(iv) Chemical change is permanent and irreversible.

Matter :

Anything that occupies space and possesses mass is termed as matter. It is made up of small particles. There are intermolecular spaces between the constituent particles of matter. The matter may be classified as follows:

(I) Physical classification : It may be classified into five states :

(a) Solid (b) Liquid (c) Gas (d) Plasma (e) Bose-Einstein Condensate.

Plasma and Bose-Einstein condensate states can be seen in specific conditions.

(II) Chemical classification : It may be classified into two category :
(a) pure substances (elements and compounds).
(b) mixtures.

States of Matter :

Almost all substances exist in three states under appropriate conditions of temperature and pressure.

1. Solids : Characteristics of solids are as follows :

(i) Have a definite shape and a definite volume.

(ii) Inter molecular forces between the molecules are very strong due to which their positions are fixed. Molecules can only vibrate to and fro about their mean position.

(iii) Molecules are closely packed and associated with minimum energy.

`text(Note) :`
• The melting point of a solid is an indication of strength of inter molecular forces of attraction.
• Melting point of a solid `prop` strength of intermolecular forces.

Solids can be of the following two types :
(i) Amorphous solids : There is no orderly arrangement of constituent particles over the long range. e.g. glass, rubber and plastic etc. Glass is a supercooled liquid.

(ii) Crystalline solids : There is orderly arrangement of constituent particles over the long range. e.g. graphite, diamond, `NaCl` etc.

Crystalline solids can be further classified as :

(a) Ionic solids : Constituent particles are ions and forces of attraction are strong electrostatic forces of attraction e. g. `NaCl , BaCl_2 , ZnS`.

(b) Metallic solids : Constituent particles are positively charged kernels and free electrons. Forces of attraction are metallic bonds. e.g. `Fe, Cu, AI` etc.

(c) Covalent solids : Constituent particles are atoms and forces of attraction are strong covalent bonds. e.g., diamond, quartz, `SiO_2`.

(d) Molecular solids : Constituent particles are molecules and forces of attraction are hydrogen bonds or van der Waals' forces. e.g. solid `CO_2 , I_2` etc.

2. Liquids : Characteristics of liquids are as follows :

(i) No definite shape but have a definite volume.
(ii) Inter molecular forces operating between the molecules are weak due to which they can slide over one another but cannot escape the boundary in normal conditions.
(iii) Molecules are closely packed and associated with intermediate energy due to motion.

3. Gases : Characteristics of gases are as follows :

(i) Neither definite shape nor volume.
(ii) Inter molecular forces operating between the molecules are very weak due to which their positions are not fixed and thus, they can move freely.
(iii) Molecules are wide apart and are associated with maximum energy.
(iv) Gases exert pressure on the walls of the container due to collision of molecules with the walls of the container.

`text(Note)` : Compressibility depends upon the intermolecular spaces between the particles of matter. Order of compressibility or intermolecular spaces is as follows :

Solid < liquid < gas

Change in State and Latent Heat :

The conversion of a substance from one state to another state at a definite temperature is known as change in state. The state of a substance can be changed by changing temperature or pressure.

e. g. `text(Solid state) undersettext(cool) oversettext(heat) ⇌ text(Liquid state) undersettext(cool) oversettext(heat) ⇌ text(Gaseous state)`

During the change in state the temperature of the substance does not change but internal potential energy of molecules change. (kinetic energy of molecules remain same).

Latent Heat : The amount of heat energy required by a substance from one state to another state at atmospheric pressure is called latent heat of that substance.

Terms Related to Change of State

Melting point : The temperature at which a solid melts to become a liquid at the atmospheric pressure is called its melting point. When a solid melts, its temperature remains the same.

`=>` Melting point of those substances which contract upon melting reduces on increasing pressure.
`=>` Melting point of those substances which expand upon melting increases with increasing pressure.

Freezing point : The temperature at which a liquid becomes solid at the atmospheric pressure is called its freezing point.

Boiling point : The temperature at which a liquid starts boiling at the atmospheric pressure is called its boiling point. It is a bulk phenomenon.

`=>` Boiling point of water increases in presence of impurities in the water.
`=>` Boiling point increases on increasing pressure upon liquid that's why food cooks comparatively faster in pressure cooker.

 
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