(A) The $COD$ (Chemical Oxygen Demand) is a measure of the amount of oxygen required to chemically oxidize organic matter in a water sample. It is det

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(A) The $COD$ (Chemical Oxygen Demand) is a measure of the amount of oxygen required to chemically oxidize organic matter in a water sample. It is determined by refluxing the water sample with a known excess of potassium dichromate $(K_2Cr_2O_7)$ in the presence of concentrated sulfuric acid $(H_2SO_4)$. The excess $K_2Cr_2O_7$ is then back-titrated with ferrous ammonium sulfate. Therefore,the correct reagents used are $50 \% H_2SO_4$ and $K_2Cr_2O_7$.

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The $COD$ (Chemical Oxygen Demand) of a water sample is determined using:

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A
$50 \% H_2SO_4, K_2Cr_2O_7$
B
$50 \% NaOH, K_2Cr_2O_7$
C
$50 \% CH_3COOH, KMnO_4$
D
$50 \% H_2SO_4, FeSO_4$

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Which of the following statements about smog is/are correct?
$(i)$ Smog formed on sunny days contains oxidizing molecules.
$(ii)$ Photochemical smog contains many reactive molecules.
$(iii)$ The main polluting components of smog are oxides of carbon.
$(iv)$ The presence of carbon monoxide in air leads to the formation of ozone in smog.

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Match the particulate pollutants given in List-$I$ with their descriptions given in List-$II$.

List-$I$ List-$II$

$(A)$ Smoke $(1)$ Formed by condensation of vapours during chemical reactions.

$(B)$ Dust $(2)$ Fine solid particles.

$(C)$ Mist $(3)$ Produced by particles of spray liquids and by condensation of vapours.

$(D)$ Fumes $(4)$ Mixture of solid and liquid particles formed by combustion of organic matter.

List-$I$	List-$II$
$(A)$ Smoke	$(1)$ Formed by condensation of vapours during chemical reactions.
$(B)$ Dust	$(2)$ Fine solid particles.
$(C)$ Mist	$(3)$ Produced by particles of spray liquids and by condensation of vapours.
$(D)$ Fumes	$(4)$ Mixture of solid and liquid particles formed by combustion of organic matter.