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Salt Analysis (Qualitative)
Chapter summary, hard words and model exam answers for Class 10 Hindi.
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Chemistry · ICSE Class 10
Summary
Sodium hydroxide is a strong alkali, so it provides plenty of hydroxide (OH minus) ions. When added drop by drop to a salt solution, the metal ion grabs these hydroxide ions and an insoluble metal hydroxide drops out as a coloured precipitate. The colour is the first clue: ferrous (Fe 2 plus) gives a dirty green precipitate, ferric (Fe 3 plus) gives a reddish brown one, copper (Cu 2 plus) gives a pale blue one, and zinc, lead and calcium each give a white one. So three salts can give white precipitates, which is why the next test matters.
To tell the three white precipitates apart, keep adding NaOH in excess. Zinc hydroxide and lead hydroxide are amphoteric, meaning they react with extra alkali and redissolve to give a clear, colourless solution. Calcium hydroxide does not dissolve in excess; it stays as a faint white precipitate (it is only sparingly soluble). The iron and copper hydroxides also stay put in excess NaOH. This single step splits the white group neatly into zinc/lead (dissolve) and calcium (stays).
Ammonium hydroxide is a weak alkali, so it ionises only slightly and gives few hydroxide ions. With most metals it still forms the same coloured hydroxide, but it cannot precipitate calcium at all, because the OH minus concentration is too low to reach calcium hydroxide. With copper there is a striking extra step: copper hydroxide dissolves in excess ammonium hydroxide to give a deep inky blue solution (a complex ion), a famous confirmatory test for copper. Zinc hydroxide also dissolves in excess ammonium hydroxide, giving a colourless solution.
Both reagents supply hydroxide ions, but a strong alkali like NaOH is almost fully ionised in water while a weak alkali like NH4OH is only partly ionised. The difference in OH minus concentration is exactly why calcium is precipitated by NaOH but not by NH4OH, and why the two reagents are used together in analysis. Reading the colour, then the behaviour in excess of each reagent, lets you name the metal ion with confidence, the heart of qualitative salt analysis.
Hard words & meanings
| qualitative analysis | finding out which ions are present in a substance, not how much |
| cation | a positively charged ion, usually the metal part of a salt |
| precipitate | an insoluble solid that forms and separates out when two solutions react |
| hydroxide | a compound containing the OH minus ion; metal hydroxides are often insoluble and coloured |
| amphoteric | able to react with both acids and alkalis; an amphoteric hydroxide dissolves in excess alkali |
| strong alkali | an alkali that is almost completely ionised in water, giving a high OH minus concentration, e.g. NaOH |
| weak alkali | an alkali only partly ionised in water, giving a low OH minus concentration, e.g. NH4OH |
| sparingly soluble | dissolving only to a very small extent, e.g. calcium hydroxide |
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