1. If Ca is burnt in air, Ca will give brick-red flame. However, if CaSO4, Ca(NO3)2, etc, i.e. Ca2+ is burnt in air, will it also give brick-red flame? If yes, does other metal ion, such as K, Na, etc also give the same result?
2. To distinguish between table salt and sugar, we can add conc. H2SO4. Sugar of course will char. But table salt will also give colourless gas or white fume (marking scheme states). What is the gas or fume? Since I expect that there is no reaction between conc. H2SO4 and table salt.
3. From (2), can I disinguish between table salt and sugar by flame test? (Table salt is NaCl. So table salt will give golden-yellow flame. While sugar is C12H22O11. So sugar will only char but not coloured flame will be shown) I expect flame test works. However, marking scheme does not state this method. Is there any mistake here? [The question is asking using chemical test to distinguish between table salt and sugar]
Please answer me as much as possible. I demand detailed but not quick answer. Thanks a lot!!!!!!!!!!!!!!
F.4 Chem (20 points)
2009-05-22 5:45 am
回答 (1)
2009-05-23 12:27 am
✔ 最佳答案
1.When a metal ion (no matter what the cation is) is heated (NOT burned) in a Bunsen flame, it will give a characteristic flame colour. This is known as "flame test".
For example, calcium ion (e.g. in CaSO4 and Ca(NO3)2) give a brick red flame colour. When calcium is burned in air, calcium oxide is formed and the calcium ion formed also gives a brick red flame colour.
Some metal ions give characteristic flame colours. For example, sodium ion (e.g. in NaCl, NaNO3) gives a bright (golden) yellow flame, potassium ion give a lilac flame, copper ion gives a bluish green flame, etc.
However, most metal ions (e.g. magnesium ion, zinc ion, etc.) do not give characteristic flame colours.
2.
The colourless gas is hydrogen chloride gas (HCl), which gives a while fume when it absorbs moisture in air and then forms small droplets of hydrochloric acid.
H2SO4 is an acid with a high boiling point. Volatile acids can be formed by heating its salt with concentrated sulphuric acid. For example, when the mixture of sodium chloride (table salt) and concentrated sulphuric acid is heated, hydrogen chloride gas is formed.
H2SO4 + NaCl → NaHSO4 + HCl
3.
It is an I.S. experiment on food test that sugar can burn with a yellow flame when sugar is heated in air. Besides, table salt gives a bright (golden) yellow flame in flame test. Therefore, it cannot distinguish between table salt and sugar by their flame colours.
To distinguish between them, I would like to add silver nitrate solution separately to each of their aqueous solution. The solution of table salt give a white precipitate of silver chloride, and that of sugar gives no observable change.
收錄日期: 2021-04-13 16:37:45
原文連結 [永久失效]:
https://hk.answers.yahoo.com/question/index?qid=20090521000051KK01750