Chapter 12, Problem 1GST

To determine

The state of matter of steam rising from the accompanying photograph.

Answer to Problem 1GST

The steam is actually in liquid state. Water evaporates out of the coffee cup and as it rises, it cools and condenses into tiny liquid droplets. Hence, the steam is actually composed of tiny liquid droplets and hence, a person can see it. If it was water vapor, one cannot see it.

Explanation of Solution

Evaporation is the conversion of water from a liquid to gas state when subjected to heat. This absorbed heat helps the molecules to escape the liquid surface and become a gas. Condensation is the conversion of water from a gas to liquid state. It is the reverse process of evaporation. The water molecules release energy that was absorbed during evaporation.

The liquid in the coffee cup is below the boiling point of water and the process of evaporation takes place in it, which involves the conversion of liquid water into the gaseous water.

The molecules in the surface of the liquid gain enough energy to escape from the liquid surface and become a gas in the atmosphere.

However, the atmosphere can only hold too much of water in it. It reaches a point where the air above the coffee cup becomes saturated and attains the dew point.

At dew point, atmosphere can no longer hold enough water vapor molecules and must condense back into liquid water. This is when condensation takes place and it is the process that happens above the steaming cup of coffee.

Some of the water molecules that evaporate cause the atmosphere right above it to reach the dew point. A portion of the water vapor condenses back into tiny droplets of liquid water, which one can see as fog or steam.