$S ⟶ SO_{2} ⟶ H_{2}SO_{3} ⟶ CaSO_{3}$
$S + O_{2} = SO_{2}$
$SO_{2} + H_{2}O = H_{2}SO_{3}$
$H_{2}SO_{3} + Ca(OH)_{2} = CaSO_{3}↓ + 2H_{2}O$

$Fe ⟶ Fe_{2}O_{3} ⟶ FeCl_{2} ⟶ Fe(OH)_{3}$
$4Fe + 3O_{2} = 2Fe_{2}O_{3}$
$Fe_{2}O_{3} + 6HCl = 2FeCl_{3} + 3H_{2}O$
$FeCl_{3} + 3NaOH = 3NaCl + Fe(OH)_{3}↓$

$Ca ⟶ CaO ⟶ Ca(OH)_{2} ⟶ Ca(NO_{3})_{2}$
$2Ca + O_{2} = 2CaO$
$CaO + H_{2}O = Ca(OH)_{2}$
$Ca(OH)_{2} + 2HNO_{3} = Ca(NO_{3})_{2} + 2H_{2}O$

$SO_{3}$ $\overset{+H_{2}O}{⟶}$ ? $\overset{+Mg}{⟶}$ ? $\overset{+NaOH}{⟶}$ ?
$SO_{3} + H_{2}O = H_{2}SO_{3}$
$H_{2}SO_{3} + Mg = MgSO_{3} + H_{2}↑$
$MgSO_{3} + 2NaOH = Na_{2}SO_{3} + Mg(OH)_{2}↓$

$Mg(OH)_{2}$ $\overset{t}{⟶}$ ? $\overset{+HCl}{⟶}$ ? $\overset{+KOH}{⟶}$ ? + $\overset{H_{2}SO_{4}}{⟶}$ ?
$Mg(OH)_{2} = MgO + H_{2}O$
$MgO + 2HCl = MgCl_{2} + H_{2}O$
$MgCl_{2} + 2KOH = 2KCl + Mg(OH)_{2}$
$Mg(OH)_{2} + H_{2}SO_{4} = MgSO_{4} + 2H_{2}O$