CHEMICAL EQUATIONS AND REACTIONS
Chemical reactions are the process in which new substances with new properties are
formed.
During a chemical reaction, atoms of one element do not change into those of another
element. Only a rearrangement of atoms takes place in a chemical reaction.
The substance which takes part in chemical reaction are called reactants.
" The new substances produced as a result of chemical reaction are called
products.
The products thus formed have properties which are entirely different from those of the
reactants.
For an exanmple-When amagnesium ribbon is heated, it burns in air with a dazzling white
flame to give white powder called magnesium oxide.
Magnesium + Oxygen -+Magnesium oxide.
(as ribbon) (air) (White powder)
Characteristics of chemical reactions
The conversion of reactants into products in a chemicai reaction is often accompanied by
some features which can be observed easily. The important characteristics of chemicat
reactions are:
Evolution of a gas
" Formation of a precipitate.
Change in colour
Change in temperature.
" Change in state.
Any one of these general characteristics can tell us whether a chemical reaction has taken
place or not.
Evolution of gas: Some reactions are characterized by the evolution of gas.
For an example
When zinc granules react with dilute sulphuric acid, then bubbles of hydrogen gas are
produced.
When Sodium carbonate reacts with dilute hydrochloric acid, the effervescences of
carbon dioxide gas.
, Both reactions is accompanied with the evolution of gas.
Formation of precipitate- Aprecipitate is a 'solid product' which separates out from the
solution during a chemical reaction.
Some reactions are characterized by the formation of precipitate.
For example- when potassium iodide solution is added to a solution of lead nitrate, then
a yellow precipitate of lead iodide is formed.
When a dilute sulphuric acid is added to barium chloride solution taken in test tube, then
awhite precipitate of barium sulphate.
Thus, both reactions are accompanied bythe formation of yellow and white precipitate
respectively.
Change in colour
Somereactions are characterized by a change in colour.
For example- When citric acid reacts with potassium
permanganate, then the purple
colour of potassium permanganate disappears.
When sulphur dioxide gas is passed through acidified potassium dichromate solution, then
the orange colour of potassium dichromate solution changes to green.
Thus, both the chemical reactions are accompanied by the change in the colour of the
solution.
Change in temperature
Some reactions are characterised by change in temperature.
For example-The chemical reaction between quicklime and water to, form slaked lime is
characterised by a change in temperature.
The chemical reaction between zinc granules and dilute hydrochloric acid to produce is
also characterised by a change in temperature.
Change in state
Some chemical reactions are characterised by a change in state.
Thecombustion reaction of candle wax is characterised by a change in state from solid to
liquid and gas.
Wax is solid, water formed by change in state from solid to liquid at room temperature
whereas carbon dioxide produce by the reaction of wax is a gas.
Chemical reactions are the process in which new substances with new properties are
formed.
During a chemical reaction, atoms of one element do not change into those of another
element. Only a rearrangement of atoms takes place in a chemical reaction.
The substance which takes part in chemical reaction are called reactants.
" The new substances produced as a result of chemical reaction are called
products.
The products thus formed have properties which are entirely different from those of the
reactants.
For an exanmple-When amagnesium ribbon is heated, it burns in air with a dazzling white
flame to give white powder called magnesium oxide.
Magnesium + Oxygen -+Magnesium oxide.
(as ribbon) (air) (White powder)
Characteristics of chemical reactions
The conversion of reactants into products in a chemicai reaction is often accompanied by
some features which can be observed easily. The important characteristics of chemicat
reactions are:
Evolution of a gas
" Formation of a precipitate.
Change in colour
Change in temperature.
" Change in state.
Any one of these general characteristics can tell us whether a chemical reaction has taken
place or not.
Evolution of gas: Some reactions are characterized by the evolution of gas.
For an example
When zinc granules react with dilute sulphuric acid, then bubbles of hydrogen gas are
produced.
When Sodium carbonate reacts with dilute hydrochloric acid, the effervescences of
carbon dioxide gas.
, Both reactions is accompanied with the evolution of gas.
Formation of precipitate- Aprecipitate is a 'solid product' which separates out from the
solution during a chemical reaction.
Some reactions are characterized by the formation of precipitate.
For example- when potassium iodide solution is added to a solution of lead nitrate, then
a yellow precipitate of lead iodide is formed.
When a dilute sulphuric acid is added to barium chloride solution taken in test tube, then
awhite precipitate of barium sulphate.
Thus, both reactions are accompanied bythe formation of yellow and white precipitate
respectively.
Change in colour
Somereactions are characterized by a change in colour.
For example- When citric acid reacts with potassium
permanganate, then the purple
colour of potassium permanganate disappears.
When sulphur dioxide gas is passed through acidified potassium dichromate solution, then
the orange colour of potassium dichromate solution changes to green.
Thus, both the chemical reactions are accompanied by the change in the colour of the
solution.
Change in temperature
Some reactions are characterised by change in temperature.
For example-The chemical reaction between quicklime and water to, form slaked lime is
characterised by a change in temperature.
The chemical reaction between zinc granules and dilute hydrochloric acid to produce is
also characterised by a change in temperature.
Change in state
Some chemical reactions are characterised by a change in state.
Thecombustion reaction of candle wax is characterised by a change in state from solid to
liquid and gas.
Wax is solid, water formed by change in state from solid to liquid at room temperature
whereas carbon dioxide produce by the reaction of wax is a gas.
