A) iii) To another portion of the filtrate, add 2 drops of barium nitrate followed by 1 cm³ of 2M nitric acid.

Here are the observations and inferences for the tests on solid M:

a) iii) To another portion of the filtrate, add 2 drops of barium nitrate followed by $\text{1 cm}^{3}$ of 2M nitric acid.

Observations	Inferences
A white precipitate forms.	$\boxed{\text{Sulfate ions } (\text{SO}_{4}^{2-}) \text{ are present.}}$

b) Dry the residue using a filter paper.

i) Using a clean spatula, transfer about half of the residue in a dry test tube, heat the residue strongly and test the gas produced using moist blue and red litmus papers.

Observations	Inferences
A colorless gas is produced. The gas turns moist blue litmus paper red. Moist red litmus paper remains red.	$\boxed{\text{An acidic gas is produced, likely carbon dioxide } (\text{CO}_{2}). \text{ Carbonate is present.}}$

ii) Place the remaining residue in a dry test tube. Add 2M nitric acid until all the solid dissolves. Retain the mixture for test (iii) below.

Observations	Inferences
The white residue dissolves with effervescence to form a colorless solution.	$\boxed{\text{The residue is a carbonate or sulfite.}}$