Step 1: In a double displacement reaction, two compounds exchange their ions to form two new compounds. Step 2: 2H₂ + O₂ → 2H₂O: two elements combine into one compound → combination reaction. Step 3: CaCO₃ → CaO + CO₂: one compound breaks into two → decomposition reaction. Step 4: Zn + H₂SO₄ → ZnSO₄ + H₂: zinc (an element) displaces hydrogen from the acid → single displacement. Step 5: Na₂SO₄ + BaCl₂ → BaSO₄ + 2NaCl: Ba²⁺ and Na⁺ exchange partners and white BaSO₄ precipitates → double displacement. Answer: Na₂SO₄ + BaCl₂ → BaSO₄ + 2NaCl.