1. Chemical changes are expressed in the form of chemical equations.

2. Chemical equation is a short hand representation of a chemical change or reaction in terms of symbols and formulae.

3. The substances which undergo chemical change in the reaction are called reactants and the new substances formed are called products.

4. Reactants are written on the left - hand side of the equation, connected by a plus (+) sign.

5. Products are written on the right - hand side of the equation, connected by a plus (+) sign.

6. Reactants and products are separated by an arrow (→). The arrow head point towards the product shows the direction of the reaction.

7. Calcium oxide reacts with water to form calcium hydroxide.
CaO + H₂O  Ca(OH)₂

8. Sodium sulphate reacts with barium chloride to give white precipitate of barium sulphate.
Na₂SO₄ + BaCl₂   BaSO₄ + 2 NaCl

9. Zinc metal reacts with dilute hydrochloric acid to form zinc chloride and hydrogen gas.
Zn + 2 HCl  ZnCl₂ + H_2↑ 

10. The total mass of the products formed in chemical reaction must be equal to the mass of reactants consumed. It is called law of conservation of mass.

11. The number of atoms of each element before and after reaction must be same.

12. Atoms are neither created nor destroyed in chemical reactions.

13. A chemical equation in which the numbers of atoms of different elements on the reactions side are same as those on product side is called a balanced reaction.
e.g.: 2 H₂ + O₂  2 H₂O
C₃H₈ + 5 O₂  3 CO₂ + 4 H₂O
Fe₂O₃ + 2 Al  2 Fe + Al₂O₃

14. Chemical equations can be made more informative by expressing following characteristics of the reactants and products.
(i) Physical state       (ii) Heat changes        (iii) Gas evolved        (iv) Precipitate formed

15. To make the chemical equation more informative, the Physical states of the substances may be mentioned along with their chemical formula. The different states are i.e., Gaseous, liquid and solid states are represented by the notations (g), (l) and (s) respectively, If the substance is present as a solution in water, the word aqueous is written.
The balanced equation is written along with the physical states as
Fe₂O₃ + 2 Al  2 Fe + Al₂O₃
  (s)          (s)               (s)         (s)

16. Heat is liberated in exothermic reactions and heat is obsorbed in endothermic reactions.
e.g.: C + O₂ 

 CO₂ + Q (exothermic reaction)
       (s)   (g)              (g)
N₂ + O₂  2 NO − Q (endothermic reaction)
(g)    (g)               (g)
Here 'Q' is heat energy which is shown with plus (+) sign on product side for exothermic reactions and minus (−) sign on product side for endothermic reactions.

17. If a gas is evolved in a reaction, it is denoted by an upward arrow _↑ or (g)
e.g.: Zn + H₂SO₄  ZnSO₄ + H_2↑
        (s)       (g)                  (aq)       (aq)

18. If a precipitate is formed in the reaction it is denoted by a downward arrow (or) the physical state (S) is written subscript for precipitate substance.
e.g.: AgNO₃ + NaCl  AgCl_↓ + NaNO₃
           (aq)        (aq)               (s)            (aq)
19. Sometimes the reaction conditions such as temperature, pressure, catalyst, etc. are indicated above and or below the arrow in chemical equations.


20. Interpreting a balanced chemical equation
(i) A chemical equation gives information about the reactants and products through their symbols and formulae.
(ii) It gives the ratio of molecules of reactants and products.
(iii) As molecular masses are expressed in 'Unified masses' (U), the relative masses of reactants and products are known from the equation.
(iv) If the masses are expressed in grams then the equation also give the molar ratios of reactants and products.
(v) If gases are involved, we can equate their masses to their volumes and calculate the volumes or those gases liberated at given condition of temperature and pressure using molar mass and molar volume relationship.
(vi) Using molar mass and Avagadro's number we can calculate the number of molecules and atoms of different substances from the equation.
(vii) It gives information about relative masses of reactants and products from the equation we get
a) mass - mass relationship
b) mass - volume relationship
c) volume - volume relationship
d) mass - volume - number of molecules relationship etc.

Chemical Equations
Conceptual Flowchart