A) Tests on C (organic compound):

Here are the observations and conclusions for the exercises on compounds C and D:

a) Tests on C (organic compound):

i)

• Observation: Blue litmus paper turns red, and red litmus paper remains red.
• Conclusion: The solution is acidic.

ii)

• Observation: To the second portion, adding Fehling's solution A and B and boiling, a brick-red precipitate forms.
• Conclusion: A reducing sugar (or aldehyde functional group) is present.

b) Tests on D (inorganic compound):

i)

• Observation: When heated, a colorless gas is evolved which turns limewater milky. The solid residue is yellow when hot and white when cold.
• Conclusion: Carbonate ions ($\text{CO}_3^{2-}$) are present. The solid is likely zinc oxide ($\text{ZnO}$).

ii)

• Observation: To the second portion, adding dilute $\text{HCl}$ causes effervescence, and a colorless gas is evolved which turns limewater milky. The solid dissolves to form a colorless solution.
• Conclusion: Carbonate ions ($\text{CO}_3^{2-}$) are present.

iii)

• Observation: To the first portion from 2(b)(ii), adding $\text{NaOH}_{\text{(aq)}}$ dropwise produces a white precipitate. The precipitate dissolves when excess $\text{NaOH}_{\text{(aq)}}$ is added, forming a colorless solution.
• Conclusion: $\text{Zn}^{2+}$, $\text{Al}^{3+}$, or $\text{Pb}^{2+}$ ions are present.

iv)

• Observation: To the second portion from 2(b)(ii), adding $\text{NH}_{3\text{(aq)}}$ dropwise produces a white precipitate. The precipitate dissolves when excess $\text{NH}_{3\text{(aq)}}$ is added, forming a colorless solution.
• Conclusion: $\text{Zn}^{2+}$ ions are present.