Meet Carbon Dioxide
The gas that puts out fires and fizzes your drinks.
What this lesson covers
Why it matters
Drop a chip of chalk into some acid and it starts to bubble like a fizzy drink. Those bubbles are carbon dioxide, the gas that can smother a flame.
Predict first
When marble chips (calcium carbonate) meet dilute hydrochloric acid, they fizz. Which gas is escaping in the bubbles?
CaCO₃ + 2HCl → CaCl₂ + H₂O + CO₂. The acid attacks the carbonate and carbon dioxide gas fizzes out.
- Oxygen
- Carbon dioxide — correct
- Hydrogen
- Water vapour
What you do
Add dilute hydrochloric acid to the marble chips and watch the carbon dioxide bubble off. Then bubble the gas through lime water.
Check yourself
How do you test that a gas really is carbon dioxide?
Carbon dioxide turns colourless lime water milky (chalky white). This is the standard test for carbon dioxide.
Why can carbon dioxide be used to put out a small fire?
Carbon dioxide is denser than air, so it sinks and forms a blanket over the fire. Because it does not support burning, it keeps oxygen away and the flame goes out.
Which everyday use depends on carbon dioxide?
Carbon dioxide is dissolved under pressure to make fizzy drinks, and solid carbon dioxide (dry ice) is used to keep things very cold.
- Bubble it through lime water and it turns milky — correct
- It relights a glowing splint
- It burns with a squeaky pop
- It turns damp blue litmus red instantly and only that
- It is lighter than air and floats away the flames
- It is heavier than air and does not support burning, so it smothers the flame — correct
- It cools the fire because it is always ice-cold
- It reacts with the fuel and makes it wet
- Making the fizz in soft drinks and freezing things as dry ice — correct
- Filling balloons so they rise high in the sky
- Helping patients breathe in hospitals
- Burning as a cooking fuel