In phase changes and basic thermodynamics, is the process of deposition (when a gas changes directly into a solid) endothermic or exothermic?

Introduction / Context:
Deposition is a phase change in which a substance changes directly from the gaseous state to the solid state without passing through the liquid phase. A familiar example is the formation of frost from water vapour on a cold surface. In basic thermodynamics and chemistry, every phase change is classified as either endothermic (absorbing heat from the surroundings) or exothermic (releasing heat to the surroundings). This question checks whether you correctly understand that deposition is the reverse of sublimation and therefore must be an exothermic process in which heat is released as the gas becomes a solid.

Given Data / Assumptions:
- The phase change being discussed is deposition, gas directly to solid.
- No intermediate liquid phase is involved in the process.
- The classification is between endothermic and exothermic behaviour.
- Standard thermodynamic conventions are used: releasing heat is exothermic, absorbing heat is endothermic.

Concept / Approach:
The key concept is that phase changes in one direction have opposite energy changes to the reverse process. Sublimation (solid directly to gas) is known to be endothermic because energy must be supplied to separate particles and overcome intermolecular forces. Deposition is simply the reverse: gaseous particles lose energy and settle into a solid lattice. When particles move to a lower energy, more ordered state, they release excess energy as heat to the surroundings. Therefore, deposition is exothermic. The approach is to relate deposition to sublimation and apply the idea that forward and reverse phase changes have opposite heat flow directions.

Step-by-Step Solution:
Step 1: Recognise that deposition is the direct change of state from gas to solid. Step 2: Recall that sublimation (solid to gas) is an endothermic process because it requires heat input to break intermolecular forces. Step 3: Understand that deposition is the reverse of sublimation, so the direction of heat flow must also reverse. Step 4: When a gas becomes a solid, particles lose energy and move into a more ordered, lower energy arrangement, releasing energy to the surroundings. Step 5: A process that releases heat to its surroundings is defined as exothermic, so deposition is exothermic.

Verification / Alternative check:
One way to verify this is to think about what happens to the surroundings during frost formation. When water vapour in the air deposits as ice crystals on a cold window, the window and the nearby air receive a small amount of heat as the vapour becomes solid. This is why latent heat of sublimation can be broken into latent heat of fusion plus latent heat of vaporisation, and the reverse process (deposition) releases that combined amount. Heating curve and cooling curve diagrams used in school chemistry also show that moving from gas down to solid steps involves releasing energy at each phase boundary, confirming that deposition is exothermic rather than endothermic.

Why Other Options Are Wrong:
Endothermic is incorrect because endothermic processes absorb heat, which applies to sublimation rather than deposition. Both A and B cannot be right because a specific phase change under given conditions cannot be both endothermic and exothermic at the same time. Cannot be determined is wrong because the thermodynamic nature of deposition is well defined in standard phase change theory and is not ambiguous. Only exothermic correctly describes the energy change for deposition.

Common Pitfalls:
A common mistake is to confuse deposition with sublimation and assume that all gas related changes must absorb heat. Another error is to memorise that melting and vaporisation are endothermic and freezing and condensation are exothermic, but forget to include sublimation and deposition in that pattern. Students may also be misled by the idea that forming a solid must always require energy input, which is not true for phase changes. To avoid these pitfalls, remember that moving to more ordered states (gas to liquid, gas to solid, liquid to solid) releases heat, while moving to less ordered states (solid to liquid, liquid to gas, solid to gas) absorbs heat.

Final Answer:
The process of deposition is exothermic because heat is released when a gas changes directly into a solid.