Determine if the following statements are True or False.
Select all that are True:
Water in the gaseous phase does not exhibit any hydrogen bonds between water molecules.
Heating steam is an exothermic process which decreases the internal energy of the system.
Condensation is an exothermic process because the system gives off energy.
The water in the system is undergoing a reversible process at 273K and 1 atm pressure. Under these conditions, both melting and freezing may occur.
When water freezes (from C to B), hydrogen bonds are formed between water molecules.
The melting of ice is an endothermic process because the system absorbs energy.
Hydrogen bonds between water molecules are broken when evaporation occurs. (From D to E)
The freezing of water is an exothermic process because the system gives off energy.
The Correct Answer and Explanation is:
Here are the correct evaluations of each statement:
✅ True Statements:
- Condensation is an exothermic process because the system gives off energy.
- The water in the system is undergoing a reversible process at 273K and 1 atm pressure. Under these conditions, both melting and freezing may occur.
- When water freezes (from C to B), hydrogen bonds are formed between water molecules.
- The melting of ice is an endothermic process because the system absorbs energy.
- Hydrogen bonds between water molecules are broken when evaporation occurs. (From D to E)
- The freezing of water is an exothermic process because the system gives off energy.
❌ False Statements:
- Water in the gaseous phase does not exhibit any hydrogen bonds between water molecules.
→ False. While hydrogen bonding is significantly reduced in the gas phase due to greater molecular distance, some transient hydrogen bonding may still occur, especially near condensation or in water vapor clusters. - Heating steam is an exothermic process which decreases the internal energy of the system.
→ False. Heating steam is an endothermic process—it requires energy input, which increases the internal energy of the system.
Explanation
Understanding phase changes in water is crucial in thermodynamics. During condensation, gas turns to liquid, releasing energy to the surroundings, making it an exothermic process. Similarly, freezing (liquid to solid) also releases energy as water molecules form stable hydrogen bonds in a crystalline structure.
At 273 K and 1 atm, water exists at its melting/freezing point. These conditions allow a dynamic equilibrium between solid and liquid phases, meaning melting and freezing can occur reversibly, depending on energy flow.
Melting is endothermic—heat is absorbed to overcome hydrogen bonding in the ice lattice, allowing the molecules to move freely in the liquid phase. On the flip side, during freezing, energy is released and hydrogen bonds form, locking molecules into a structured lattice.
Evaporation (or vaporization) involves breaking hydrogen bonds to allow liquid water to become gas, which is also endothermic. Therefore, hydrogen bonds must be broken during this phase change.
Regarding hydrogen bonds in the gas phase, while they’re largely absent due to high molecular kinetic energy and separation, short-lived interactions can still occur, so it’s inaccurate to say they don’t exist at all.
Lastly, heating steam increases the kinetic energy of the vapor molecules and thus raises the system’s internal energy. It is not exothermic, but endothermic.
Understanding these energy changes is essential for interpreting water’s behavior in physical and biological systems.
