Key Ideas

• A mixture is two or more things simply together, not joined to each other and not changed by each other.
• Burning requires a gas in the air called oxygen. Cut off the supply and the flame stops even with plenty of fuel left.
• Burning also produces carbon dioxide, a gas heavier than air that smothers fire. It is how fire extinguishers work.
• Two gases with opposite behaviors cannot be the same substance, so air must be a mixture.
• Air is about 78% nitrogen, 21% oxygen, and 0.04% carbon dioxide. The nitrogen was found by bookkeeping, not by any single experiment.

• Mixture: Two or more things simply together, not joined and not changed. Remove one kind and the other is unaffected.
• Oxygen: The gas in air that burning requires. About one fifth of the air.
• Carbon dioxide: A gas heavier than the rest of the air. It does not burn and will not let anything else burn, so it smothers fire.
• Nitrogen: The gas making up nearly four fifths of the air. It neither feeds a flame nor puts one out.
• Combustion: The proper word for burning: fuel combining with oxygen and releasing energy as heat and light.
• Atmosphere: The whole envelope of gases surrounding the Earth.
  
  Discussion Questions
  

1. Firefighters sometimes smother a fire with a blanket instead of using water. Using what you proved today, explain why that works.
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?

• 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