Dropping dilute hydrochloric acid is to verify whether sodium carbonate exceeds the standard, and dropping sodium carbonate solution is to verify whether calcium hydroxide exceeds the standard.

Ⅲ There is no obvious phenomenon, indicating that calcium hydroxide is either insufficient or sufficient.

By measuring the mass of calcium carbonate generated by the reaction between calcium hydroxide and sodium carbonate, the mass of sodium carbonate is calculated and the mass fraction is obtained.

Insufficient calcium hydroxide, low calcium carbonate, low sodium carbonate and low mass fraction. If it is only enough, it will have no effect.