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標題:

問CHEM 2005 MC Q.3 AND Q.8

發問:

問CHEM 2005 MC Q.3 AND Q.8 可唔可以解釋下個答案問黎?

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3. The answer is B. Fractional distillation of liquefied air is to cool down the air to a very low temperature under -200 ℃. Then the liqueified air is warmed readily. As air is a mixture of different chemicals. Different chemicals have different boiling points. So, using this method, the components in air can be separated. The one which has a lower boiling point is being distilled out first. (because it will vapoize to gas first, then it can be collected first.) Considering the boiling points in increasing order, nitrogen < argon < oxygen. So, this is the sequences of collecting the gas. 8. B is the answer. For a redox reaction to occur, the oxidation no. of the species must change For A, the oxidation no. of N increases from -1 to +1 and decreases from +5 to +1. For B, there is no change in oxidation no. For C, the oxidation no. of N decreases from +4 to +3 and increases from +4 to +5. For D, the oxidation no. of O increases from -2 to 0 and that of N decreases from +5 to +4.

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