Question

Describe, on both a microscopic and a macroscopic basis, what happens to a sample of water as it is heated from room temperature to \(50^{\circ} \mathrm{C}\) above its normal boiling point.

Short answer

As a sample of water is heated from room temperature to 50°C above its boiling point, on a microscopic level, the water molecules gain kinetic energy, move faster, collide more frequently, and overcome intermolecular forces, transitioning from liquid to gas and eventually superheated steam with weakened intermolecular forces. On a macroscopic level, the water undergoes phase changes from liquid to gas, with visible bubbling during boiling and steam expansion as density decreases.

Step-by-step solution

Identify the phase changes that water undergoes

We need to identify the phase changes that the water will experience as it is heated from room temperature to 50 degrees above its normal boiling point. There are three relevant phases of water: solid (ice), liquid (water), and gas (steam). During this exercise, the water undergoes two main phase transitions: from liquid to gas (boiling) and possible superheated steam.

Describe the water molecules' behavior when heated to boiling point

As a sample of water is heated, the average kinetic energy of its molecules increases as a result of the added thermal energy. On a microscopic level, this means that the water molecules are moving faster, colliding more frequently, and with greater force. On a macroscopic level, the water's temperature increases until it reaches its boiling point (100°C or 212°F), at which point the liquid water begins to transition into a gas (steam).

Explain the boiling events in detail

During boiling, water molecules have enough energy to overcome the attractive forces between them, so they transition from the liquid phase to the gas phase. On a microscopic level, water molecules at the surface break free from the liquid's intermolecular forces and become gaseous molecules, forming bubbles. As the temperature continues to rise, more and more molecules gain the necessary energy to transition into the gas phase. On a macroscopic level, this forms visible bubbling and steam production as liquid water is converted into gas.

Describe the behavior of water molecules in superheated steam

As the water keeps getting heated beyond its boiling point, it will eventually reach a state of superheated steam, which means the steam's temperature is higher than its saturation temperature (100°C) at the same pressure. On a microscopic level, the water molecules have even more kinetic energy in this state, and they move even faster and collide more violently. The intermolecular forces between the gaseous water molecules are weaker in comparison to those in the liquid phase. On a macroscopic level, the steam expands and becomes less dense as a result of the increased energy and weakening intermolecular forces.

Recap the entire process from room temperature to 50°C above boiling point

To summarize, as we heat a sample of water from room temperature to 50°C above its normal boiling point, we observe the following changes on both microscopic and macroscopic bases: 1. The water molecules gain kinetic energy as they are heated, causing them to move faster and collide with increased frequency and force. 2. The water reaches its boiling point, and molecules with sufficient energy overcome the intermolecular forces, transitioning from the liquid phase to the gas phase. 3. As the water continues to be heated beyond its boiling point, it becomes superheated steam, with water molecules moving even faster and colliding more violently. 4. On a macroscopic level, the water undergoes phase changes from liquid to gas, including the visible bubbling during boiling, and expansion and decrease in density of the steam when heated beyond its saturation temperature.

Key concepts

Microscopic Level

On a microscopic level, heating water leads to increased molecular motion. When you start heating water from room temperature, its molecules which are initially moving at moderate speeds, start to gain kinetic energy. This increased energy causes them to move faster and collide with each other more frequently. As the temperature approaches the boiling point, the molecules have enough kinetic energy to overcome the forces that hold them together in the liquid state. At this point, molecules at the surface start to escape into the air as steam. If heating continues, moving to superheated steam, the water molecules possess such high kinetic energy that they exhibit increased velocity and forceful collisions, making them very active within their gaseous state.

Macroscopic Level

On a macroscopic level, heating water results in observable changes. Initially, as you heat a water sample, its temperature rises steadily until it reaches boiling point at 100°C (212°F). At boiling, you can observe the transition from liquid to gas, which appears as bubbling due to steam production. This bubbling happens because gas bubbles form within the liquid and rise to the surface. As the temperature continues to increase beyond the boiling point, you see the production of superheated steam. This steam expands in volume and becomes less dense, signifying the transformation from simple steam to superheated steam that can carry more energy and heat.

Superheated Steam

Superheated steam occurs when steam is heated beyond its boiling point temperature, without an increase in pressure. On a microscopic level, superheated steam is characterized by water molecules that possess extremely high kinetic energy and are moving much faster than they did in the liquid phase. These molecules have weak interactions between them, as the energy they contain overcomes the existing intermolecular forces. Macroscopically, superheated steam appears less dense and occupies a larger volume compared to regular steam. This state is particularly useful in industrial applications, such as turbines in power plants, because it can do more work due to its high energy content. It's important to handle superheated steam cautiously, as it can be far hotter than boiling water and cause severe burns.