Give Scientific Reasons:
a. When the gas formed on heating limstone is passed through freshly prepared lime water, the lime water turns milky.
  1. When limestone is heated, calcium oxide is formed and carbon dioxide gas is liberated.
  2. When this carbon dioxide gas is passes through freshly prepared lime water, white coloured insoluble calcium carbonate and water are formed.
  3. Because of this insoluble, white coloured calcium carbonate, the lime water turns milky.
  4. The reactions involved in this are as follows:
    $\ce{CaCO3 ->[\Delta] CaO + CO2}$
    $\ce {CaOH2 + CO2 -> CaCO3 + H2O}$
2 Marks
b. It takes time for pieces of Shahabad tile to disappear in HCl, but its powder disappears rapidly.
  1. The rate of chemical reaction depends upon the size of the particles of the reactants.
  2. The smaller the size of particles of the reactants, the faster is the rate of chemical reaction.
  3. The size of Shahabad tile particles is much larger than the size of its powder.
  4. Therefore, it takes time for pieces of Shahabad tile to disappear in HCl, but its powder disappears rapidly.
2 Marks
c. While preparing dilute sulphuric acid from concentrated sulphuric acid in the laboratory, the concentrated sulphuric acid is added slowly to water with constant stirring.
  1. Preparation of dilute sulphuric acid is a highly exothermic reaction.
  2. Sulphuric acid is heavier than water. While diluting it, if water is poured in the acid, the water being lighter tends to float on the acid. The generated heat causes water to evaporate and results in splashing. This can cause an accident.
  3. On the other hand, when acid is poured in water, acid being heavier it tends to go down and mix well with water. The constant stirring also enables the generated heat to dissipate faster.
  4. Therefore, while preparing dilute sulphuric acid from concentrated sulphuric acid in the laboratory, the concentrated sulphuric acid is added slowly to water with constant stirring.
2 Marks
d. It is recommended to use airtight container for storing oil for a long time.
  1. If edible oil is allowed to stand exposed to air for a long time, it undergoes slow oxidation. It becomes rancid, its taste deteriorates and oil gets a foul smell.
  2. The process of rancidity can be significantly slowed down by storing it in an airtight container. This prevents its contact with fresh air.
  3. Hence, it is recommended to use airtight container for storing oil for a long time.
2 Marks
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09 August 2019 at 03:21

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