Physical changes are reversible changes and only the physical properties are changed in such changes. Chemical changes are irreversible changes in which new products are formed. The product is different from the reactants and has entirely different properties. Chemical changes that occur are also considered chemical reactions. A short way of writing a chemical reaction or the chemical changes are chemical equations. Chemical equations are a symbolic representation of a chemical reaction. When one writes chemical reaction symbols of elements, arrows, etc are used.
Decomposition reaction - In this type of chemical reaction, one reactant breaks to form several products. It is just opposite to the combination reaction.
AB -----------> A + B
Decomposition in chemistry means breaking of a single reactant to form more stable products compared to the reactant.
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A decomposition reaction is classified into three types-
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It is a type of decomposition in which a single reactant is broken down to more than one product with the help of heating. Thermal energy or heat energy is used to break the reactants. This is an endothermic reaction as heat is used to break the bonds of the reactants. Calcium carbonate is decomposed from heat to form calcium oxide and carbon dioxide.
CaCO3 ----------> CaO + CO2
It is a type of decomposition reaction in which light or photons are used to break the reactants to form several products. Light energy is used to break the bonds in the reactant. This reaction is also known as photolysis.
Silver chloride decomposes in presence of sunlight to form silver metal and chlorine gas.
AgCl ---------> Ag + Cl2
A single reactant is broken down into several products using electricity. This type of decomposition reaction is electrolytic decomposition.
decomposition of water is a good example of electrolytic decomposition.
H2O --------> H2 + O2
There are several examples of decomposition. Some of them are given below-
Carbonic acid in soft drinks decomposes to give carbon dioxide gas.
Hydrogen gas and oxygen gas are released from the decomposition of water.
Digestion of food is a decomposition reaction. Through this reaction, nutrients are released from the food eaten.
Iron sulphate which is of green colour is decomposed to form a brown-coloured ferric oxide and gases like sulphur dioxide and sulphur trioxide gases are released.
Lead nitrate on thermal decomposition gives lead monoxide which is yellow. With this brown gas nitrogen dioxide and oxygen gas are released.
1. Mention uses of decomposition reactions.
Decomposition reactions are used for various purposes. Some of them are given below-
A decomposition reaction is used in the extraction of metals in different ways.
Thermal decomposition is used to obtain mercury metal from its oxide (that is mercuric oxide)
Thermal decomposition is used to obtain zinc oxide from its carbonate ore.
ZnCO3 -----------> ZnO + CO2
Electrolytic decomposition is used to obtain metal from its molten ores. Like sodium is obtained from its ore sodium chloride.
NaCl -----------> Na + Cl2
In the manufacture of cement, a decomposition reaction is used.
Thermal decomposition of limestone is used to manufacture quick lime.
The photodecomposition of silver bromide is used in photography.
AgBr ----------> Ag + Br2
2. Explain the characteristics of chemical reactions
Chemical reactions are accompanied by certain features or signs. The important characteristics of chemical reactions are-
Evolution of gas: When metal like zinc reacts with an acid like sulphuric acid, hydrogen gas is evolved.
Formation of precipitate: The chemical reaction between lead nitrate and potassium iodide is accompanied by the formation of a yellow precipitate of lead iodide.
Change in colour: When iron reacts with copper sulphate (blue) a green colour is obtained due to the formation of iron oxide. Here colour change is seen.
Change in temperature: When quicklime reacts with water a lot of heat is released. This increases the temperature change.