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How to write ionic equations  (IGCSE and O-Level)
Supplement for IGCSE Syllabus,Compulsory for O Level 

Step 1: Write the balanced chemical equation with the state symbols first.

             MgCO3 (s) + 2HCl (aq) → MgCl2 (aq) + H2O (I) + CO2 (g)
Step 2: Break all those substance in aqueous state into their respective ions according to their coefficient in the balanced chemical equation. Each ion must have a charge and the state symbol of the ion remains in aqueous state. Rewrite the rest of the substances in solid, liquid and gaseous state as they appeared in the balanced chemical equation previously.

            MgCO3 (s) + 2H+ (aq) + 2Cl- (aq) → Mg2+ (aq) + 2Cl- (aq) + H2O (I) + CO2 (g)

Step 3: Cancel out the spectator ions. Spectator ions are ions that appear the same on each side of the ionic equation.

           MgCO3 (s) + 2H+ (aq) + 2Cl- (aq) → Mg2+ (aq) + 2Cl- (aq) + H2O (I) + CO2 (g)

Step 4: Write the final equation which is the answer. Ensure the coefficient is reduced to the lowest term.

           MgCO3 (s) + 2H+ (aq) → Mg2+ (aq) + H2O (I) + CO2 (g)

Step 1: (NH4)2SO4(aq) + 2NaOH(aq) → Na2SO4(aq) + 2H2O(l) + 2NH3(g)
Step 2: 2NH4+(aq)+ SO42-(aq)+ 2Na+(aq)+ 2OH-(aq)→ 2Na+(aq)+ SO42-(aq)+ 2H2O(l)+ 2NH3(g)
Step 3: 2NH4+(aq)+ SO42-(aq)+2Na+(aq)+2OH-(aq)→ 2Na+(aq)SO42-(aq)+ 2H2O(l)+ 2NH3(g)
Step 4: the coefficients are divisible by 2. Divide the coefficients by 2 throughout.
             2NH4+(aq)+2OH-(aq)→2H2O(l)+ 2NH3(g)  
Final answer: NH4+(aq)+OH-(aq)→H2O(l)+ NH3(g)