Phosphourus ppts dissolves in acids forming biphosphates. If we add NaOH it precipitates again as the biphosphate is still in the solution. Carbonate ppts also get dissolved when acids are added but don't precipitate when NaOH is added as CO2 has got out from the system
Let's take MgCO
3. When we add HCl to the ppt , it get dissolved forming MgCl
2 and CO
2 . Yes carbon dioxide goes out but what about magnesium chloride . Won't we get magnesium hydroxide ppt when we are adding NaOH ?