Calcium oxide+water = = = calcium hydroxide 2

2

Firstly, barium chloride is added to generate barium sulfate precipitate, and sulfate radical is removed;

Adding sodium hydroxide to generate magnesium hydroxide precipitate, and removing magnesium ions;

Finally, sodium carbonate is added to generate calcium carbonate precipitation and remove calcium ions.

However, in order to clean it, you need to add too much.

Sodium carbonate can remove excess barium chloride.

In step 6, excess sodium hydroxide and sodium carbonate are removed with hydrochloric acid.

So, you can't change it. If the exchange is carried out, the excess barium chloride will not be removed.

three

Oh, this has been explained in the last sentence.