Problem 144 - Chemistry

Question

A student heated a beaker of cold water (on a tripod) with a Bunsen burner. When the gas is ignited, she noticed that there was water condensed on the outside of the beaker. Explain what happened.

Step-by-Step Solution

Answer

The water condensed on the outside of the beaker because of the process of condensation. When the water in the beaker was heated, it evaporated into a gaseous state but when it came into contact with the cooler surface of the beaker, it converted back into its liquid state causing condensation on the outside of the beaker.

Step 1: Understanding the process of heating

First, comprehend the procedure; when the cold water in the beaker was heated, it gained energy which caused the water molecules to move quickly, thereby increasing the temperature.

Step 2: Process of Evaporation

When the temperature rose, the water molecules gained enough energy to change from liquid state to gaseous state. This change of state from liquid to gas is called evaporation.

Step 3: Process of Condensation

The evaporated water, now as water vapour, comes into contact with the cooler surface of the beaker. This causes the water vapour to lose its energy and revert back to its liquid state. This change of state from gas to liquid is called condensation which causes water to gather on the outside of the beaker.

Key Concepts

Evaporation

Evaporation is a fascinating process in which a liquid turns into a gas, typically at temperatures below boiling. As the student heated the water in the beaker, the heat was transferred to the water molecules. These molecules absorbed the energy, moved faster, and some with enough energy broke free from the liquid's surface.
This transition from the liquid phase to a gaseous phase is known as evaporation.

• Evaporation occurs more rapidly at higher temperatures because more water molecules reach the required energy threshold.
• It is different from boiling, which involves the rapid vaporization of a liquid at its boiling point.

Evaporation isn't confined to the water body under direct heat; any open surface can experience evaporation. That is why, even at room temperature, water eventually evaporates from a glass left uncovered. Understanding this process explains why some of the water in the student's beaker became water vapor shortly after heating.

Condensation

Condensation is essentially the reverse of evaporation, where a gas turns back into a liquid. In the scenario with the student, once the water evaporated, the water vapor came into contact with the cooler surface of the beaker.
This contact caused the vapor to lose kinetic energy, and the molecules slowed down, eventually sticking back together into droplets of liquid.

• Condensation is important for various natural phenomena, such as rain formation and dew on grass.
• The temperature at which condensation begins is called the dew point.

When the cool surface interacts with humid air, it triggers condensation, leading to the formation of water droplets on surfaces, as seen on the outside of the beaker.

Heat Transfer

Heat transfer is the process of thermal energy moving from a warmer object to a cooler one. It can occur through conduction, convection, or radiation. In the student's experiment, the heat from the Bunsen burner was transferred to the beaker and then to the water inside.
The transfer of energy caused the temperature of the water to rise, enabling both evaporation and subsequent condensation to occur.

• Conduction refers to energy transfer through direct contact, like the beaker and the water.
• Convection involves energy transfer through fluids; in this scenario, the movement of heated water molecules facilitates thermal mixing.
• Radiation is the transfer of energy through electromagnetic waves, such as when heat from the Bunsen burner's flame reaches the beaker.

The balance of heat transfer is pivotal in understanding phase changes, marking its presence in heating and cooling scenarios, like how evaporated water condenses on a cooler surface when losing heat.