When the solution of substance X is added to a solution of potassium iodide, then a yellow solid separates out from the solution.

(a) What do you think substance X is likely to be?

(b) Name the substance which the yellow solid consists.

(c) Which characteristic of chemical reactions is illustrated by this example?

(d) Write a balanced chemical equation for the reaction which takes place. Mention the physical states of all the reactants and products involved in the chemical equation.