• # question_answer Which of the following pair of ions makes the water hard [AMU 2002] A) $N{{a}^{+}},\ SO_{4}^{2-}$ B) ${{K}^{+}},\ HCO_{3}^{-}$ C) $C{{a}^{2+}},\ NO_{3}^{-}$ D) $NH_{4}^{+},\ C{{l}^{-}}$

$HCO_{3}^{-}$ is main reason of temporary hardness of water.