Explanation When two salts are mixed they can form either double salt or coordination compound . if they form double salt ,they will ionized and give test of each ion but if they form coordination compound then they will not ionized and do not give test of each ion. In mixture of iron sulphate and ammonium sulphate, a double salt is formed so give test of Fe२- but in another case it form coordination compound and don’t give test. Final answer Mixing of ferrous sulphate and ammonium sulphate give result the formation of double salt (Mohr‘s salt) but in case of copper a coordination compound is formed which does not give this test.
