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Compound C
This is a white powder. The flame test produces a persistent orange-yellow flame. On addition of dilute hydrochloric acid rapid effervescence is seen. This gas turns limewater milky. A solution of C has a pH of 10.9.
The orange-yellow flame test suggests a sodium salt. The gas which turns limewater milky is carbon dioxide. The production of carbon dioxide gas on addition of acid is an indication of either a carbonate or hydrogencarbonate. The pH of the solution distinguishes between these two possibilities. A carbonate would have the higher pH in the range 10-12.
Answer: sodium carbonate.
We can see here that some of these chemical tests do not always eliminate all possibilities. A further test may be necessary to confirm our initial suspicions.