By the method of sodium fusion extract, if sulphur and/or nitrogen is also present in an organic molecule , the test cannot be carried out on a sodium fusion mixture until it has been acidified and boiled. Why is this so?

I suppose you are reffering to test of halogen in presence of N & or S in sodium extract.

The presence of N & S causes hinderance in the detection of halogen in the organic compound. NaCN & Na2S present in the sodium extract will give a white & black ppt of AgCN & Ag2S respectively on treatment with AgNO3. Hence sodium extract is boiled with small amount of conc. HNO3, before testing for halogens. NaCN & Na2S are both decomposed by HNO3.

NaCN + HNO3 .....> NaNO3 + HCN

Na2S + 2 HNO3 ....> 2 NaNO3 + H2S

No.I'm not meaning that.What I ask is when nitrogen or/and sulphur is present in the organic compound,we acid some amount of dil.sulphuric acid and then add ferric chloride.But while testing for halogens,we directly add silver nirate without neutralizing the basicity caused by sodium hydroxide