Identify process A.

Process A shows a change from Gas to Solid. This process is called deposition or desublimation.
25. (i) A: $\boxed{\text{Deposition}}$

Process C shows a change from Gas to Liquid. This process is called condensation.
25. (i) C: $\boxed{\text{Condensation}}$

Naphthalene and Iodine are substances that change directly from solid to gas without passing through a liquid state. This process is called sublimation.
25. (ii) Process: $\boxed{\text{Sublimation}}$

The diagram shows sodium peroxide reacting to produce oxygen gas, which is collected over water. Liquid Y is added to the sodium peroxide to initiate the reaction. Sodium peroxide reacts with water to produce oxygen. Therefore, Y is water.
26. (a) Y: $\boxed{\text{Water}}$

The reaction between sodium peroxide and water is highly exothermic and can be vigorous, producing a lot of heat and gas quickly. To control the reaction and prevent it from becoming too violent or overflowing, the water should be added slowly.
26. (c) Precaution: Add water slowly to control the rate of reaction and prevent overheating or frothing.