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 initially a gas (steam), but observation shows it condensed as liquid mist.

Step-by-step solution

Understanding Matter

Matter can exist in three main states: solid, liquid, and gas. Each state has distinct characteristics that differentiate it from the others. Steam represents a specific state of matter, typically the gas state of water.

Identifying Steam

Steam is formed when water transitions from the liquid state to the gas state due to heat. This transition occurs when the water molecules gain sufficient energy to break free from the liquid and disperse into the air as a vapor.

Observing Condensation

The 'steam' seen rising from a cup of coffee is actually not pure steam, but rather condensation. As hot steam (gas) leaves the surface, it cools down quickly in the surrounding air, turning back into tiny droplets of liquid water, which is visible as a mist.

Conclusion

Based on the understanding of steam and observation of the mist, the 'steam' rising from the coffee cup is initially in a gaseous state (steam) but is seen as a liquid in mist form due to rapid cooling and condensation.

Key concepts

Phase Transition

Phase transitions are fascinating processes where a substance changes its state of matter. These transitions occur through the absorption or release of energy. Such changes include a solid melting to become a liquid, or a liquid boiling to form a gas. Each phase transition has unique characteristics and requires specific conditions of temperature and pressure to occur.

For instance, when boiling water to create steam, the liquid water absorbs heat and gains enough energy for molecules to break free into the gaseous state. This illustrates a liquid-to-gas phase transition, also known as vaporization. Conversely, when this gaseous state undergoes cooling, it can change back into a liquid through condensation. Understanding these transitions helps explain everyday phenomena like the condensation of steam above a coffee cup.

Steam

Steam is a term used to describe water in its gaseous state. It's created when liquid water is heated and its molecules gain sufficient energy to enter the gas phase. In practical terms, steam is generated when water is boiled. The heat energy transfers to the water molecules, helping them overcome intermolecular attractions.

This propulsion of water molecules into the air not only forms steam but is critical in driving numerous industrial applications, such as turbines in power plants. Nevertheless, the steam we visually perceive in settings like a steaming cup of coffee isn't pure steam but actually contains tiny water droplets, making it visible.

Condensation

Condensation arises when a gas loses energy and transforms back into a liquid. This occurs when steam in the air cools down and clusters into droplets. It is often noticeable on cold surfaces where the cooler air rapidly drops the vapor temperature.

In the context of a steaming cup of coffee, steam cools as it escapes the cup, causing water vapor in the air to condense into fine droplets, visible as mist. This process beautifully illustrates the principle of condensation as it showcases the gas-to-liquid state change.

Gas State

The gas state of matter is characterized by the lack of a definite shape or volume. In this state, molecules are far apart with very weak forces holding them together, allowing them to move freely. This movement enables gases to expand and fill any available space.

When water assumes the gas state, such as through boiling, it transitions into water vapor. This expansion is key to understanding how steam arises from processes like boiling, where heat energy helps water molecules break free into the gaseous environment.

Liquid State

The liquid state is one in which matter maintains a definite volume but adapts to the shape of its container. Molecules in this state are more closely packed than in gases but can still move relative to each other. This flow ability gives liquids their unique characteristics, such as the ease with which they can pour or spill.

When it comes to water, its liquid state is what we normally encounter. However, with enough added heat, water will transform from this state to the gaseous state, a process exemplified in the formation of steam.

Water Vapor

Water vapor is the gaseous form of water and constitutes a key aspect of the water cycle. It occurs not just as steam but across all conditions where water evaporates into the surrounding air.

In many situations, especially in nature, water vapor is invisible. The steam seen over a boiling pot or a hot beverage is water vapor mixed with air and consists of moisture droplets forming a visible mist.

Understanding water vapor is significant for various scientific fields, including meteorology, as it plays a crucial role in weather patterns and climatic conditions.