Lesson 47: Air as a Mixture
Air is a mixture of gases with different properties. Candle and gas demonstrations introduce oxygen and carbon dioxide, while the composition of air shows the larger roles of nitrogen and other gases.
Key Ideas
- Air is a mixture of gases. The gases are together without becoming one new substance.
- A candle needs enough oxygen to burn. Under a glass it eventually goes out, but this does not mean every bit of oxygen was used.
- Burning candle wax uses oxygen and makes carbon dioxide and water. Carbon dioxide can put out a candle by reducing the oxygen around its flame.
- Gases can behave differently even when we cannot see them. Scientists use careful tests to identify gases and measure how much of each is present.
- Dry air is roughly four-fifths nitrogen and one-fifth oxygen, with small amounts of other gases. The amount of water vapor in air can change.
Vocabulary
- Mixture: Two or more substances together without forming one new substance.
- Oxygen: A gas in air that our bodies use and that a candle needs to burn.
- Carbon Dioxide: A gas in air that plants use to make sugars. Breathing and burning add carbon dioxide to the air.
- Nitrogen: The gas that makes up most of Earth’s air. It does not help a candle burn.
- Combustion: Burning: a chemical change that releases energy.
- Atmosphere: The layer of gases surrounding Earth.
Discussion Questions
1. Firefighters sometimes smother suitable small fires with a fire blanket. How can reducing the oxygen supply affect combustion?
2. A neat but wrong explanation for a science observation can survive for a long time even when it's wrong. Why do you think that happens?
3. You breathe in air and breathe out air. If you could test both, what do you predict would be different about them, and what does that suggest is happening inside you?
Supplies
- A candle stub, a soup bowl, water, a clear drinking glass, matches, and an adult to handle the flame
- A candle, a jar, a tablespoon of baking soda, a quarter cup of vinegar, and an adult for the flame