A change in which the chemical composition of a substance remains the same but physical properties are changed is known as Physical Change.
A change which leads to a change in the molecular composition of a substance along with the possible physical change is called Chemical Change.
Characteristics of a Chemical Change
- It is a permanent change.
- The chemical properties along with the
physical properties change.
- New substances are formed.
- There is always an exchange of energy
i.e. either heat is absorbed or released.
- This change is irreversible.
Condition for Chemical Change
- The reacting species when comes in close contact then the number of collisions between them increases which causes chemical change.
- The chemical change in bond fission of reactant and bond formation to produce new substances causes net energy change.
- In some chemical changes, the released
amount of energy converted into light energy. This causes flames in combustion.
- Electricity provides electrons which get involved in chemical change for various redox reaction.
- In gaseous reactants the reacting
species come close to each other with the help of high pressure which causes
required chemical change.
- Catalyst change the rate of reaction involved in a particular chemical change but, it cannot initiate the chemical change.
Few examples of chemical change:
- Burning of wood due to the formation of new
substances and release of energy.
- Formation of yogurt from milk due to
formation of a new substance by bacterial action.
- Preparation of alcohol from grapes due
to the formation of a new substance with release of gas and energy change.
- Baking of a cake due to the formation of
new substance and change in color.
- Rusting of iron due to the formation of
new substance and change in colour.
- Bursting of crackers due to heat and
light release along with new substances.
- Burning of magnesium ribbon due to the
formation of new products and the release of heat and light energy.
- Ripening of bananas due to the formation of
new substance.
- Burning of sugar due to the formation of
new substance along with a change in color.
- Burning of LPG gas in a stove due to
formation of new substance and release of heat and light
- Digestion of food due to energy change
with the formation of new substance like CO2.
- Photosynthesis in plants due to the formation of new compounds in plants.
Types of Chemical Changes
- In direct combination two or more
compounds combine directly to form new substances.
- In decomposition reaction reactants
break down to form two or more new substances.
- In displacement reaction an element
displaces the anion of a compound to combine with its cation and form a new
compound.
- In double displacement reaction more the reactive anion of a compound displaces the less reactive anion of other
compound to form a new product.
- In redox reaction a compound reduces
the other compound and itself get oxidized at the same time.
- In photochemical reaction two or more
substances react with each other due to the presence of light.
- Energy Change = Total Energy Released in Product Side (Q1) – Total Energy Absorbed in Reactant Side (Q2)
- A chemical change in which there is the release of heat is called exothermic change (Q1 > Q2).
- A chemical change in which there is the absorption of heat is called endothermic change (Q1< Q2).
- Effect of electricity on chemicals (electrolytes) is known as electrolysis.
- Chemical change which takes place due to the passage of electricity which decomposes the electrolyte into its constituents at the electrodes.
Burning is a chemical process in which a substance reacts with oxygen and form oxides along with a large amount of heat and light energy.
Conditions of Burning
- Presence of Combustible Substance
- Presence of supporter of combustion i.e., oxygen
- Heating of combustible substance up to ignition temperature
The temperature at which combustible substance catches fire is defined as its ignition temperature.
Fire can be extinguished if we remove anyone cause of burning out of the following three causes:
- Removal of combustible substance
- Removal of a supporter of combustion
i.e., oxygen
- Decrease the temperature of burning
substance below its ignition temperature.
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