Na → NaOH → Na2S → NaCl → Na2SO4
1) 2Na + 2H2O → 2NaOH + H2 ↑ (substitution reaction; sodium, water, sodium hydroxide, hydrogen);
2) 2NaOH + H2S → Na2S + 2H2O (exchange reaction; sodium hydroxide, hydrogen sulfide, sodium sulfide, water);
3) Na2S + CaCl2 → 2NaCl + CaS (exchange reaction; sodium sulfide, calcium chloride, sodium chloride, calcium sulfide);
4) 2NaCl + CaSO4 → Na2SO4 + CaCl2 (exchange reaction; sodium chloride, calcium sulfate, sodium sulfate, calcium chloride).
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