點解係flame test中..一定要用platinum/nichrome wie??

The reasons are :
1. Platinum and nichrome are unreactive. They do not react with concentrated hydrochloric acid, do not react with oxygen when heated, and do not undergo displacement reaction with the compound to be tested in flame test.
2. Platinum and nichrome do not show characteristic flame colour when heated in Bunsen flame. For example, green flame would be observed when copper wire is used instead.

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Points to be noted.
1. If the mixture of the sample and concentrated hydrochloric acid is heated, the cation will evaporate with chloride ion in the form of metal chloride. In fact, no catalyst is required.
2. Cl2 gas does not react with platinum wire. However, if the reaction occurs at the surface of platinum, chlorine atoms would be formed and then two chlorine atoms combine to give a chlorine molecule. Chlorine atoms is very reactive, they can attack platinum before combining to form molecule.
3. Besides, KMnO4 can oxidized the Cl- to Cl2 only when KMnO4 is dropped into the concentrated hydrochloric acid.

原因有二:

1)
Platinum/nichrome 較為不活躍, 不易受 conc. HCl 侵蝕.

2)
Platinum/nichrome 可作為一種 catalyst, 使正離子的一方在 conc. HCl 的 presence 下形成一種揮發性強的氯化物 (volatile chloride), 而令到正離子在 luminous flame 中能發揮出其顏色.

但要留意, 做 flame test 時要 make sure 附近無 KMnO4 的存在, 因為 MnO4- ion 會 oxidize Cl- to Cl2, 然後 Cl2 gas 會侵蝕 platinum 的.