Question

Heat is applied to an ice cube until only steam is present. Draw a sketch of this process, assuming you can see it at an extremely high level of magnification. What happens to the size of the molecules? What happens to the total mass of the sample?

Short answer

Throughout the heating process of an ice cube turning into steam, the size of the individual water molecules (H2O) remains the same, but their arrangement and movement change. In the ice phase, they are tightly packed in a crystalline structure; in the liquid phase, they are more spread out; and in the steam phase, they are dispersed even further with random movement. The total mass of the sample remains constant, as the number of water molecules does not change during the process.

Step-by-step solution

Sketch the process

Begin by drawing a diagram of the process. From left to right, draw an ice cube, followed by an illustration of melting ice transitioning into liquid water, and finally, water evaporating into steam. To represent the molecules, you can use dots or small circles that would give an idea of their relative size and arrangement at each stage.

Size of the molecules

Throughout the process, the individual water molecules (H2O) will remain the same size. The size of the molecules doesn't change; their position concerning each other and the level of movement they exhibit are what change in different phases of water. Try to draw the molecules closer together in the ice cube, with a more uniform arrangement. In the liquid water illustration, depict the molecules a little further apart, with less uniform arrangement. Finally, in the steam representation, show the molecules furthest apart and randomly dispersed.

Total mass of the sample

As heat is applied to the ice cube, it transitions from solid to liquid to gas; however, the total mass of the sample remains constant throughout the process. The process simply changes the phase of the water, but not its molecular composition, meaning the number of water molecules in the system remains unchanged before and after the transition. The total mass of the water depends on the number of its constituent molecules, so if their number remains constant, so will the total mass of the sample.

High-level view

When visualizing the process at an extremely high level of magnification, you will see the same water molecules (H2O) throughout the process, simply rearranged into different structures and with varying movement patterns. In the ice phase, molecules are tightly packed in a crystalline structure. In the liquid phase, the molecules will be more spread out and moving freely among each other due to the increased kinetic energy. Finally, in the steam phase, the molecules would disperse even further and have even more kinetic energy, resulting in random movement and arrangement. In conclusion, when we heat an ice cube to become steam, the individual molecules (H2O) do not change their size, but rather their arrangement and movement patterns. Furthermore, the total mass of the sample remains constant throughout the process, as the number of water molecules does not change.

Key concepts

States of Matter

The states of matter represent different phases that substances can transition between—solid, liquid, and gas. Each state is characterized by the degree of organization and spacing between molecules. In a solid state, like an ice cube, the molecules are arranged in a definite and rigid pattern, maintaining a fixed shape and volume. The molecules are close together and have the least amount of movement.

When heat is applied, the ice cube begins to melt and turns into liquid water. This state allows molecules more freedom to move, which is why water can flow and take the shape of its container while still maintaining a consistent volume. Although the molecules are still in close contact, they're not held in a rigid structure.

Upon further heating, the liquid water then changes into steam, which is a gaseous state. Here, the molecules are much further apart than in solids or liquids and move at high speeds in all directions. This is why steam expands to fill any available space, taking both the shape and volume of its container. During this entire process, even though the appearance and behavior of water changes significantly, the substance itself remains water throughout all its phases.

Conservation of Mass

The concept of conservation of mass is a fundamental principle in science that states that mass cannot be created or destroyed in a closed system through any physical or chemical changes. Applying this principle to the phase change of water from ice to steam, we understand that no matter is lost or gained. Instead, the molecules simply rearrange their positions and states.

During the melting and boiling of water, the same number of water molecules (\(H_2O\text{'s}\) ) that existed in the ice cube are still present in the steam. Thus, the weight of the ice cube would be the same as the weight of the resultant amount of steam, assuming no water molecules have escaped from the system. Any careful measurement would confirm that the mass before heating (the ice cube) equals the mass after heating (the steam), beautifully demonstrating the conservation of mass in action.

Molecular Kinetics

The behavior of molecules in different states of matter is governed by molecular kinetics, which refers to the movement and energy of molecules. In solids, like an ice cube, molecular movement is minimal due to low kinetic energy. The molecules vibrate in place but do not have enough energy to break free from their fixed positions in the crystal lattice.

As heat is applied, the kinetic energy of the molecules increases. This energy allows the water molecules to break their fixed bonds, turning ice into water. The increase in movement is more obvious in a liquid state, where molecules can slide past each other with relative ease.

Continuing to heat the liquid causes the water molecules to gain even more kinetic energy, enough to overcome the attractive forces holding them together in a liquid state. Consequently, the molecules escape into the air as gas, or steam, moving rapidly and randomly. This heightened state of molecular motion in gases results in molecules spreading out to fill the entirety of any container or space they occupy.