Question

The accompanying photo shows a cup of hot coffee. What state of matter is the "steam" rising from the liquid? Explain your answer.

Short answer

The steam is in the gaseous state as it's composed of water vapor from evaporating coffee.

Step-by-step solution

Understand the Composition of Steam

Steam rising from a cup of coffee is formed by water molecules that have transitioned from a liquid to a gaseous state due to heat from the hot coffee. This process is known as evaporation, where liquid water molecules gain enough energy to overcome intermolecular forces holding them in the liquid phase and move freely as a gas.

Identify the State of Matter

The steam visible above the cup of hot coffee is in the gaseous state. This is because the water molecules have gained sufficient energy to break free from the liquid state and disperse in the air as vapor, which is a gas.

Evidence From Observation

The fact that the steam rises and disperses quickly into the air supports the conclusion that it is in the gaseous state. Gases tend to expand and fill the space available to them, unlike liquids and solids, which have more fixed volumes and shapes.

Key concepts

Evaporation

Evaporation is a fascinating and important concept when discussing the states of matter. It is the process through which water, in liquid form, changes to a gas. This occurs when water molecules at the surface gain enough energy from heat to escape the liquid state.
This energy helps them overcome the forces that bind them together in the liquid phase. As these molecules leave the liquid, they become vapor, or steam, which is essentially a gaseous form of water. Several factors can affect evaporation:

• Temperature: Higher temperatures provide more energy for molecules to escape into the gaseous state.
• Surface Area: More surface area allows more molecules to escape at a time.
• Air Movement: Wind or moving air can carry away evaporated molecules, allowing more molecules to escape.
• Humidity: Lower humidity means less water vapor in the air, which can speed up evaporation.

Understanding these factors helps explain why a hot cup of coffee readily turns liquid water into steam.

Gaseous State

The gaseous state is one of the three primary states of matter, alongside solid and liquid. In this state, substances have unique characteristics. They do not have a fixed shape or volume, instead, they expand to fill the container or space they occupy. This is because the molecules move rapidly and are spaced far apart compared to liquids and solids. Key features of gases include:

• High Compressibility: Gases can be compressed easily because the molecules are spread out.
• Low Density: Gases are less dense than liquids and solids, making the spread easier.
• Indefinite Shape: Unlike solids, gases do not hold their own shape and take the shape of their container.
• Expansive: They expand to fill any available space, and this expansion is observable with substances like steam rising from coffee.

Considering the gaseous state helps explain why steam from coffee disperses quickly into the air.

Water Vapor

Water vapor is the name given to water in its gaseous state. It is produced by the process of evaporation when water molecules gain enough energy to leave the surface of a liquid and enter the air. Once in the air, these water molecules function like other gas molecules, spreading out and occupying space. In everyday life, we can observe water vapor:

• Weather Phenomena: Clouds, fog, and precipitation are all natural examples of water vapor condensing.
• Invisible Gas: Though often visible as steam, most water vapor in the air is invisible.
• Humidity: Humidity levels in the air are directly related to the presence of water vapor.
• Breath Exhalation: When we breathe out, we release water vapor into the air.

Understanding water vapor’s properties helps clarify why we see steam when hot coffee evaporates, and how this invisible gas plays a role in various atmospheric processes.