A balanced chemical equation is in accordance with

  1. Avogadro's law
  2. Law of multiple proportion
  3. Law of conservation of mass
  4. Law of gaseous volumes



  1. Law of conservation of mass


Explanation - In a balanced chemical equation, the number of atoms of each element is balanced on the left and right side of the equation and thus the mass is considered to be balanced in the process. 

Hence it is in accordance with the Law of conservation of mass


Which of the following is not a balanced equation?

  1. Ca(OH) 2 + CO 2 → CaCO 3 + H 2 O


  1. Fe + CuSO 4 → FeSO 4 + Cu


  1. KClO 4 → KCl + 2O 2


  1. Cu+ 2HNO 3 → Cu(NO 3 ) 2 + 2NO 2 + H 2 O



  1. Cu+ 2HNO 3 → Cu(NO 3 ) 2 + 2NO 2 + H 2 O
    Explanation - The number of atoms of each element on the reactants and products side of the equation are equal.


Write the balanced chemical equations for the following reactions and identify the type of reaction in each case.

(a) Thermite reaction, iron (III) oxide reacts with aluminium and gives molten iron and aluminium oxide.

(b) Magnesium ribbon is burnt in an atmosphere of nitrogen gas to form solid magnesium nitride.

(c) Chlorine gas is passed in an aqueous potassium iodide solution to form potassium chloride solution and solid iodine.

(d) Ethanol is burnt in air to form carbon dioxide, water and releases heat 

(e) Nitrogen gas is treated with hydrogen gas in the presence of a catalyst at 773 K to form ammonia gas.

(f) Sodium hydroxide solution is treated with acetic acid to form sodium acetate and water.


  1. Fe 2 O 3 (s) + 2Al(s) → Al 2 O 3 (s) + 2Fe(l) + Heat
    Iron (III) Oxide     Aluminium    Aluminium Oxide    Molten Iron
    This is a displacement reaction ; Al is more reactive than Fe and displaces Fe from Fe2O3



  1. 3Mg(s) + N 2 (g)        →         Mg 3 N 2 (s)
    Magnesium    Nitrogen         BURNING       Magnesium Nitride
    This is a combination reaction ; Mg burns and reacts with N 2 to form a single product  Mg 3 N 2

                                                                      MAGNESIUM         NITROGEN           MAGNESIUM NITRIDE


Mg burns and reacts with N2 to form a single product  Mg3N2 - Teachoo.jpg


  1. Cl 2 (g)     +  2KI(aq)     →   2KCl(aq)     +     I 2 (s)
    Chlorine          Potassium Iodide         Potassium Chloride        Iodine
    This is a displacement reaction ; Chlorine is more reactive than Iodine and displaces Iodine from KI and this form KCl and solid I2


Displacement reaction. - Teachoo.jpg

  1. C 2 H 5 OH(l) + 3O 2 (g) → 2CO 2 (g) + 3H 2 O(l) + Heat
    Ethanol                 Oxygen              Carbon Dioxide       Water
    It is a Combustion reaction , which is a type of Oxidation reaction

Combustion reaction - Teachoo.jpg

  1. N 2 (g) + 3H 2 (g)       →    2NH 3 (g)
    Nitrogen     Hydrogen     773K       Ammonia
    This is a combination reaction . N 2 and H 2 react to form one product NH 3




  1. NaOH + CH 3 COOH → CH 3 COONa + H 2 O
    Sodium Hydroxide      Acetic acid         Sodium acetate        Water
    This is a double displacement reaction as the ions of both the reactants get exchanged to form the products.


Double - replacement Reaction - Teachoo.jpg





Grapes hanging on the plant do not ferment but after being plucked from the plant can be fermented. Under what conditions do these grapes ferment? Is it a chemical or a physical change?



  • Fermentation takes place when microbes utilise the sugars and convert it into alcohol in the absence of air
  • When hanging on the plant , there is an abundance of air around . The immune system of the plant also prevents the action of microbes on the grapes. 
  • After plucking , grapes can be stored in anaerobic (free of air) conditions to undergo fermentation
  • This is a chemical change , that follows the process:




During the reaction of some metals with dilute hydrochloric acid, following observations were made.

  1. Silver metal does not show any change.
  2. The temperature of the reaction mixture rises when aluminium (Al) is added.
  3. The reaction of sodium metal is found to be highly explosive.
  4. Some bubbles of a gas are seen when lead (Pb) is reacted with the acid. Explain these observations giving suitable reasons.




  1. According to the reactivity series, Silver is less reactive than Hydrogen and cannot displace Hydrogen from the dilute Hydrochloric acid
    Ag + dil HCl → No reaction


  1. Reaction of Aluminium and Hydrochloric acid is an exothermic reaction and thus there is an increase in the temperature.

Exothermic - Teachoo.jpg

  1. In the presence of moisture (H 2 O), Sodium forms Hydrogen gas;
    Na + H 2 O → NaOH + H 2
    It thus catches fire as Hydrogen is highly inflammable and the reaction is exothermic.


  1. The bubbles are due to the evolution of gas. Here Hydrogen gas is evolved following the reaction;
    Pb(s) + 2 dil HCl → PbCl 2 (s) + H 2 (g)
    After a while, the surface of Lead (Pb) gets coated with Lead Chloride ( PbCl 2 ) and the reaction becomes slow.

Hydrochloride acide - Teachoo.jpg

A silver article generally turns black when kept in the open for a few days. The article when rubbed with toothpaste again starts shining.

(a) Why do silver articles turn black when kept in the open for a few days? Name the phenomenon involved.

(b) Name the black substance formed and give its chemical formula.



  1. Reaction of Silver with Hydrogen Sulphide in the atmosphere results in Silver articles turning black.
    The phenomenon is called Corrosion .


  1. The black coating on silver articles is Silver Sulphide .



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CA Maninder Singh is a Chartered Accountant for the past 13 years and a teacher from the past 17 years. He teaches Science, Economics, Accounting and English at Teachoo