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.

Short answer

The water on the beaker formed due to condensation, when the warm air, which was capable of holding more water vapor, cooled down upon contacting the cooler surface of the beaker, resulting in the excess water vapor condensing into liquid water.

Step-by-step solution

Understanding Condensation

Condensation is the process by which water vapor in the air is changed into liquid water. It's the opposite of evaporation. In simple terms, condensation occurs when a vapor gas cools down and becomes a liquid.

Observing the Beaker

When the beaker is heated, the air around it also heats up. Warm air can hold more moisture (as water vapor) than cold air. So when the air close to the beaker is heated, its ability to hold water vapor increases.

Explaining the Dew Point

The 'dew point' is the temperature at which the air cannot hold all the moisture in it and some of the water vapor must condense into a liquid form. So, when the warm air comes into contact with the colder surface of the beaker, it cools down. If this cooled air reaches the dew point, the excess water vapor will condense into liquid water.

Conclusion

In this case, when the gas is ignited, it heats the air around the beaker. But the beaker is also in contact with cooler air. As this air gets warmed up and then cools down again (while being in contact with the beaker), it reaches its dew point and the water vapor condenses, resulting in water droplets on the outside of the beaker.