A compound 'X' is bitter to taste. It is a compound of washing powder and reacts with dilute HCl to produce brisk effervescence due to a colourless and odourless gas 'Y' which turns lime water milky due to the formation of 'Z' when excess of `CO_2` is passed, milkiness disappears due to formation of 'P'. Identify 'X' , 'Y' , 'Z' and 'P' . Write the equations involved in the formation of Y, Z and P .