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The experts at Team Jagran Josh have framed the Model Test Paper for Chemistry subject which strictly follows the Board pattern of CBSE Class 12 for the year 2016. The Model Test Paper is extremely beneficial for the Class 12 Students appearing for their respective examinations in 2016.

To Get the Complete Solved Model Paper, Click Here |

**A few Questions from this Model Test Paper are given below:**

**Q.** How the conductivity of an intrinsic semiconductor may be increased?

**Solution:** The conductivity of an intrinsic semiconductor can be increased by doping that is adding appropriate amount of suitable impurities in the semiconductor.

**Q.** How is copper extracted from a low grade ore of it?

**Solution:** Copper can be extracted from a low grade ore of it through the process of bacterial leaching.

**Q.** You work in a large university's physical plant department, which oversees the day-to-day operations of the buildings on campus. The university currently maintains 14 days-scale-air-conditioning units that still use CFC as the refrigerant. These units were installed well before the 1996 ban on CFC production. A member of the budget committee comes to you because she is concerned about the potential costs of replacing so many units.

She asks you these questions.

(i) Since a cooling unit is sealed (the refrigerant is kept under pressure in a sealed system) why does the continued use of CFC pose any risk to the environment?

(ii) Can't the university simply buy a different refrigerant to replace the CFCs and use-it in the existing units? What would be the concern associated with such refrigerant replacement?

**Solution:**

(i) The earth's surface has been protected from too much UV light by a layer of ozone. The large scale depletion of O_{3} in the layer (termed as ozone hole) observed over Antarctica has been attributed mainly due to massive buildup, in the atmosphere of CFCs due to their increasing use as refrigerants. The CFCs, which are stable in lower atmosphere, float to the stratosphere and decompose releasing atomic chlorine which attacks O_{3}.

(ii) Urgent research and development programs have resulted in a series of replacement refrigerants that are already being incorporated in new refrigeration systems, but they cannot be used in existing systems (including 1994 and earlier-model automobiles). The most prominent materials are

HFC - 134 a (CF_{3}-CH_{2}F)

HCFC - 141 b (CH_{3}-CCl_{2}F)

To Get the Complete Solved Model Paper, Click Here |

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