chapter no.1 

Chemical Reactions And Equations

Very short Answer type Questions
Q.1. Why should a magnesium ribbon be cleaned before burning in air ?
Ans. Magnesium ribbon should be cleaned before burning in air so that that the unwanted impurities deposited on it can be removed and only pure magnesium can burn.

Q.2. Why do we apply paint on iron articles ?
Ans. We apply paint on iron articles because paint act as antioxidant and saves iron from corrosion.

Q.3. Oil and fat containing food items are flushed with nitrogen. Why ? 
Ans. When the oil and fat containing food items comes in contact with air they get oxidized. To keep these items save from oxidising effects, they are flushed with nitrogen.

Q.4. What is the unit of rate of reaction ? 
Ans. The unit of rate of reaction" mole per litre per second"

Q.5. What is pH value of pure water ?
Ans. pH value of pure water is 7.

Q.6.why does the colour of cooper sulphate solution change when an iron nail 
Ans.When an iron nail is dipped in copper sulphate solution, its colour becomes brownish because the following reaction take place 

     CuSO4(aq)  +Fe ➡FeSO4+Cu

Short Answer type Questions
Q.1. Give differences between strong and weak electrolytes.
Ans. Differences between strong and weak electrolytes.

Q.2. Why is respiration considered as exothermic reaction ? Explain.
Ans. The complex molecules of food are decomposed by the oxygen inhaled during respiration. Food like : rice, potato etc. are made up of carbohydrates. These are broken down to glucose. Glucose during respiration is oxidised and in this process energy is liberated.
Reaction is given below: 
         C6H12O6+6O2→6CO2+6H2O+Energy


Long Answer Type Questions
Q.1. What is a balanced chemical equation ? Why should chemical equations be balanced ?
Ans. The balanced chemical equation is that equation in which number of each element remains the same on both side of equation i.e., the number of each element before and after reaction should be the same. A balanced equation should follow the conservation of mass as well as that of charge.
     A chemical equation must be balanced with respect to mass because mass can neither be created nor be destroyed. The chemical equation must be balanced with respect to charge, because the charge can neither be created nor be destroyed.
  Example:  Zn+H2SO4→ZnSO4+H2

Q.2. Why are decomposition reactions called the opposite of combination reaction ? Write equations for these reactions.
Ans. In a decomposition reaction, a single reactant breaks down to give simpler products .
Examples:  CaCO3➡CaO+CO2
But in a combination reaction, two more reactants combine together to from a single product.


  • Example: CaO+CO2➡CaCO3
  • Thus, it is clear that decomposition reaction is opposite to the combination reaction.

  • Q.3. State the factors that affect the rate of a reaction. Explain in detail. ( MP 2011 ) 
  • Ans. The following are the factors which affect the rate of a chemical reaction:
  • 1.Concentration: Rate of reaction increases with increase in concentration of the reactants.
  • 2.Temperature: Normally at high temperature, rate of chemical reaction is high.
  • 3.Surface area: On increasing surface area of a solid reactant, the rate of reactions increases.
  • 4.Effect of radiations: Certain reaction take place faster in presence of light.
  • 5.Pressure: With increase in pressure, the rate of reactions increases, when all the reactants and product are in gaseous state.