Chemical Reactions and Equations
The processes like crystallisation, boiling, vaporisation, burning of colour, melting point, hardness etc, are called physical changes. These are reversible in nature. The processes like photosynthesis, digestion, ripening of fruits, burning of paper which involve the change in composition as well as chemical properties and result in the formation of a new substance are called chemical changes. These are irreversible in nature. Hence we can say, chemical reaction is the process of combination of different atoms to form different products which involve a change in the physical and chemical properties like:
- Change in the colour of the substance
- Change in the state of the substance
- Change in heat energy - Absorption of energy or release of energy
- Release of gas
- Evolution of light and sound
The symbolic representation of the reactants and the products using their chemical formulae is known as Chemical Equation. A chemical equation consists of:
- An arrow separates the reactants and products
Reactants are those substances which take parts in the chemical reaction.
Products are those substances which produced during chemical reaction.
C + O2 → CO2
Representation of physical states of reactants and products:
- For solids it is "(s)".
- For liquids it is "(l)".
- For gases it is "(g)".
- For aqueous solutions it is “(aq)".
- For gas produced in the reaction it is "(↑)".
- For precipitate formed in the reaction it is “(↓)".
- Direction of reaction is indicated by "(→)".
For Example: Zn (s) + dil.H2SO4 (aq) → ZnSO4 (aq) + H2 (g) (↑)
A chemical equation is helpful to understand a chemical reaction in an easy way. In a chemical equation the masses of reactants and products may or may not be equal. But according to law of conservation of mass "the total mass of the reactants and the products should be equal".
So in order maintain the law true it is necessary to balance a chemical equation.
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