Questions from Chemical reactions (1)

Chemical Reaction : 
Sulekha Rani.R , PGT Chemistry , KV NTPC Kayamkulam 



Q.1.   A water insoluble substance ‗X‘ on reacting with dilute H2SO4 released a   colourless and  odourless gas accompanied by brisk effervescence. When the gas was passed through water, the solution obtained turned blue litmus red. On bubbling the gas through lime water, it initially became milky and milkyness disappeared when the gas was passed in excess. Identify the substance ‗X‘. Write its chemical equations of the reactions involved

Ans.    The water insoluble substance ‗X‘ is metal carbonate CaCO3.
                    CaCO3 (S) + H2SO4 (Aq) ............... CaSO4(Aq) + H2O (Aq) + CO2 (G)
                  Ca(OH)2 + CO2 (G).........................CaCO3 (S) + H2O (L)
                   (milky)
                CaCO3 (S) + CO2 (G)+ H2O (Aq)....................CaHCO3
               (milkiness)

Q.2. Ahmad took a magnesium ribbon (cleaned) and burned it on a flame. The 
        white powder formed was taken in a test tube and water was added to it. He 
        then tested the solution formed with red and blue litmus paper. What change 
         was seen? Why?
     Ans. Red litmus paper turned blue.
            Blue litmus paper remained blue.
     This is because the magnesium ribbon on burning in air forms the white 
        magnesium oxide. Which dissolved in water, it forms magnesium hydroxide, 
         which is Basic in nature.

Q.3. Give one example of a combination reaction in which an element combines 
         with a compound to give you a new compound.
              Ans.      O2 + 2SO2........ 2SO3
                             8NH3 + 3Cl2........... 6NH4Cl

Q.4. Marble statues often slowly get corroded when kept in open for a long time.  
        Assign a suitable explanation.

Q.5. Mohan tool pure water for the electrolytic decomposition of water but did not  
          see any bubbles  near the electrodes. Explain why?

Q. 6 Rancidity is a process used for spoiling of cooked food materials like 
        vegetables, etc. When kept for long time in open. How can you prevent such 
        process to proceed? Give an example.

Q. 7 A substance ‗X‘ displaces ‗Y‘ from its solution in water. It is called 
         displacement reaction. What other chemical name can be given to such type
         of reactions? Explain, giving an example?


Q. 8 Why is respiration considered an exothermic reaction? Explain.

Q. 9 Why is respiration considered an exothermic reaction? Explain.

Q. 10 Why are decomposition reactions called opposite of combination reactions? 
          Write equations for these reactions.

Q. 11  A shiny brown colored element ‗X‘ on heating in air becomes a black 
           coloured compound. Name the element ‗X‘& black the coloured compound 
           formed. Also write the equation