Gases in Air

In this often repeated experiment, we see how a lit candle acts when the air is limited. We also see how heated, then cooled, air behaves.

Materials Required

• Shallow pan or bowl

• Narrow water glass

• Candle

• Water

Procedure

1. Pour a small amount of water into a shallow pan. One half inch of water will be fine.

2. Pace a candle in the center of the pan and light it.

3. As you prepare for the last step of the procedure, watch every detail of what happens.  Place a water glass over the candle and allow it to "sit" in the water on the pan.

4. Watch carefully. Allow the apparatus to sit for at least a minute,

5. Record your observations. Make sketches of what you see happening in the glass and with the gases in and out of the glass.

Questions

What was the first thing that happened in the experiment after the glass was placed over the flame?

Why did the flame go out?

What was the level of the water inside the glass at first compared to later?

What was the volume of gas inside the glass at first compared to later?

Extensions

Repeat the procedure in various ways:  

A. When you have finished the experiment, carefully remove the glass and place it quickly on the table mouth down. This keeps the gases from mixing with the air around the glass. Repeat the experiment with this "used" gas.  How long did the candle burn compared to the first experiment?  

B. Do the experiment like you did the first time, only after the candle goes out and after 1 minute, place an ice cube on top of the inverted glass. Watch the gas volume and water level. What does that tell you about how temperature affects the volume of gases?

Discussion

Observation 1. There is a fixed amount of oxygen in the glass.  The candle needs oxygen to burn. When the oxygen is used up the candle can no longer stay lit, so it goes out.

Observation 2. When the glass is first placed over the candle one observes bubbles coming out of the glass and up through the water in the pan. As the gas is heated in the glass by the flame, it expands. The expanding gas pushes out and presses the water out and some escapes out of the glass.

Observation 3. After the candle goes out, the water level slowly rises in the glass. The candle will ultimately float and rise inside the glass. After the candle goes out, the heated gas inside the glass starts to cool. When gas is reduced in temperature it has a smaller volume--it takes up less space. As a result, the water takes up that space and rises in the glass.

Observation 4. If you repeat the experiment and place the glass atop the candle without allowing the gas inside the glass to mix (using only the gas left in the glass from the first experiment), the candle will not burn for very long and the experiment will not work as well.  All the oxygen was used up and no new oxygen was allowed into the glass.

Observation 5. When you put an ice cube atop the glass, it causes the gas to become even colder and take up even less space and the water level will rise even further.

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