The correct option is **D. Steam at 100°C**. ### Explanation of the Correct Answer To understand why steam at 100°C represents the most disorderly arrangement of water molecules, we need to consider the states of matter and how molecular arrangement and energy levels change between them. 1. **States of Matter**: Water can exist in three primary states: solid (ice), liquid (water), and gas (steam). Each state has distinct characteristics regarding molecular arrangement and energy. 2. **Molecular Arrangement**: - **Ice (Solid State)**: In ice, water molecules are arranged in a fixed, orderly structure due to hydrogen bonding. This structure is crystalline, meaning the molecules are held in place and can only vibrate slightly. At -10°C (Option A) and 0°C (Option B), the arrangement remains orderly, but the energy increases slightly as the temperature rises from -10°C to 0°C, causing some minor vibrations. - **Liquid Water**: At 100°C (Option C), water is in a liquid state. The molecules are more disordered than in ice because they can move past one another, but they are still relatively close together and interact through hydrogen bonds. The arrangement is less orderly than in ice but more so than in steam. - **Steam (Gas State)**: At 100°C, when water is in the form of steam, the molecules are in a gaseous state. In this state, the water molecules have enough energy to overcome intermolecular forces, allowing them to move freely and rapidly. This results in a highly disordered arrangement, as the molecules are far apart and can occupy a much larger volume compared to the liquid or solid states. 3. **Energy Levels**: The energy of the molecules increases as we move from solid to liquid to gas. In steam, the water molecules have the highest kinetic energy, which contributes to their disorderly arrangement. The increased temperature at which steam exists (100°C) means that the molecules are moving rapidly and are not constrained by any fixed structure. ### Why the Other Options Are Incorrect - **Option A: Ice at -10°C**: Ice has a very orderly arrangement of molecules in a crystalline structure. The molecules are fixed in place and can only vibrate slightly, leading to minimal disorder. - **Option B: Ice at 0°C**: Similar to -10°C, ice at 0°C still maintains a crystalline structure. Although it may start to melt, the majority of the molecules remain in a fixed arrangement, resulting in low disorder. - **Option C: Water at 100°C**: While water at 100°C is in a liquid state and has more disorder than ice, it is still more ordered than steam. The molecules are closer together and still interact through hydrogen bonds, which keeps them somewhat organized compared to the gaseous state. ### Summary of Key Points - **Steam at 100°C** has the most disorderly arrangement of water molecules due to their high kinetic energy and freedom of movement. - **Ice (both at -10°C and 0°C)** has a fixed, orderly structure with minimal molecular movement. - **Liquid water at 100°C** is more disordered than ice but still retains some organization due to intermolecular forces. - The transition from solid to liquid to gas represents an increase in molecular disorder and energy. This understanding of molecular arrangements and energy levels is crucial for grasping the behavior of substances in different states of matter.