The lime water test
Four gas jars look the same. Which one holds carbon dioxide?
Four jars, one test
Four jars hold four colourless gases: nitrogen, oxygen, carbon dioxide and hydrogen. You cannot tell them apart by looking. A little clear lime water is added to each jar and shaken.
What will happen to the lime water?
What this lesson covers
The idea
Calcium hydroxide (lime water) + Carbon dioxide → Calcium carbonate + Water; the white, insoluble calcium carbonate turns lime water milky, which is used as a test for carbon dioxide.
Four jars, one test
Four jars hold four colourless gases: nitrogen, oxygen, carbon dioxide and hydrogen. You cannot tell them apart by looking. A little clear lime water is added to each jar and shaken.
What will happen to the lime water?
- It turns milky in all four jars
- It turns milky in only one jar
- It stays clear in all four jars
Test each jar
Tap a jar. A dropper adds a little lime water and the jar is shaken. Test all four.
The test for carbon dioxide
Only the carbon dioxide jar changed. Inside it, a chemical reaction made new substances:
Calcium hydroxide (lime water) + Carbon dioxide → Calcium carbonate (insoluble substance) + Water
The white, insoluble calcium carbonate turns lime water milky. So the turning of lime water milky is used as a test for carbon dioxide.
- Gas in the jar | Lime water
- Nitrogen | stays clear
- Oxygen | stays clear
- Carbon dioxide | turns milky
- Hydrogen | stays clear
Notes
Calcium hydroxide (lime water) + Carbon dioxide → Calcium carbonate + Water. The white, insoluble calcium carbonate turns lime water milky. This is used as a test for carbon dioxide.
Check yourself
Lime water in a jar turns milky after shaking. Which gas does the jar hold?
Lime water stays clear when shaken with the gas in a jar. What do we know?
Calcium hydroxide + Carbon dioxide → Calcium carbonate + Water. How many substances are formed, on the right of the arrow?
Answer: 2
Two: calcium carbonate and a little water. Calcium hydroxide and carbon dioxide are on the left of the arrow. They react.
Why does the lime water look milky?
- Oxygen, the gas we breathe in. Oxygen leaves lime water clear.
- Hydrogen, a very light gas. Hydrogen leaves lime water clear.
- Carbon dioxide — correct. Yes! Only carbon dioxide turned it milky.
- It is not carbon dioxide — correct. Yes! Carbon dioxide would have turned it milky.
- The gas must be oxygen. Nitrogen and hydrogen also leave lime water clear.
- The jar must be empty of gas. Many gases leave lime water clear. The jar can still be full.
- The carbon dioxide dissolves in it. A new white substance forms. That makes it milky.
- A white solid forms in it — correct. Yes! Calcium carbonate is white and does not dissolve.
- The lime water gets hot. Heat is not what makes it milky. The new white substance does.