Chemical reaction

• Chemical reaction is a process in which a chemical substance is transformed into a new substance with new properties.
• A chemical change occurs leading to formation of a new substance unlike physical change which involves change in state or colour but no new substances are formed.

CHANGES TAKING PLACE DUE TO CHEMICAL REACTIONS

Status of atoms after/during a chemical reaction:

• During a chemical reaction, atoms of one element do not change into those of another element.
• Only rearrangement of atoms takes place in a chemical reaction.

REACTANTS AND PRODUCTS

• The substances which take part and undergo change during a chemical reaction are called reactants.
• The new substances produced in a chemical reaction are called products.

CHEMICAL REACTIONS IN DAILY LIFE:

Some of the observable chemical reactions in daily life are:

• Souring of milk, when left at room temperature
• Formation of curd from milk
• Fermentation of grapes
• Cooking of food
• Process of Digestion of food
• Process of respiration
• Rusting of iron, when exposed to humid atmosphere
• Burning of fuel

CHARACTERISTICS OF CHEMICAL REACTIONS

The important characteristics of chemical reactions are:

• Evolution of a gas,
• Formation of a precipitate,
• Change in colour,
• Change in temperature, and
• Change in state

(i) Evolution of a Gas:

Some chemical reactions are characterized by the evolution of a gas. For instance, the chemical reaction between zinc and diluted sulphuric acid, results in the evolution of hydrogen gas.

(ii) Formation of a Precipitate

Some chemical reactions are characterized by the evolution of a gas. For instance, the chemical reaction between zinc and diluted sulphuric acid, results in the evolution of hydrogen gas.

(iii) Change in Colour

Some chemical reactions are characterised by a change in colour. E.g. In a reaction of potassium permanganate solution with the citric acid, where the purple colour of potassium permanganate solution disappears to become colourless.

(iv) Change in Temperature

Some chemical reactions are characterised by a change in temperature.

Example :

1. Production of heat energy is observed in quicklime when it reacts with water to form slaked lime. This process of production of heat is known as exothermic reaction.
2. When Barium hydroxide [Ba(OH)₂] is added to ammonium chloride NH₄Cl) taken in a test-tube and mixed with a glass rod. This results in the formation of then ammonia, barium chloride and water.

• A lot of heat energy is absorbed in this reaction, resulting in the drop of temperature, which in turn makes the bottom of test-tube very cold. This heat absorbing reaction is called endothermic reaction.

Change in State Some chemical reactions are characterised by a change in state.
E.g. Burning of wax in the form of wax candle results in the formation of water and carbon dioxide. Here, the wax is in the solid form, water is in the liquid form and carbon dioxide is in the gaseous form. This means that during the combustion reaction of wax, the physical state changes from solid to liquid and then to gas.