(i) (a) Zinc is more reactive than silver. It will displace silver from silver nitrate solution.
(b) Magnesium is more reactive than iron. It will displace iron from iron (II) chloride solution.
© Copper is less reactive than magnesium. It will not displace magnesium from magnesium sulphate solution.
(ii) X is potassium (K).
Potassium being soft can be cut by a knife and being lighter than water hence, it will float at the surface. The heat produced during reaction with water will melt it.