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Compound D
This is a white powder which gives a brick red flame test. On treatment of solid D with concentrated sulfuric acid a brown vapour is seen. A solution of B gives a pale yellow precipitate when treated with acidified silver nitrate solution.
The brick red flame test suggests a calcium salt. The brown gas is likely to be either bromine or nitrogen dioxide gas. The silver nitrate test confirms the presence of a halide indicating that the brown gas is bromine. This means that the original salt before oxidation by the concentrated sulfuric acid was a bromide.
Answer: calcium bromide.
The group 7 reactions are a popular source of observation practical exercises. The result of the silver nitrate test shows that care needs to be taken here. Pale yellow is the colour I have given for silver iodide. I have given cream as the colour of silver bromide. However, there is not a lot of difference between cream and pale yellow, and these two results are easy to confuse. A confirmatory test, such as the solubility in ammonia solution is helpful here.