Carbon dioxide, and how to prove it
Almost any acid poured on a carbonate fizzes. The fizzing alone proves nothing — plenty of reactions give off a gas. This activity makes the gas from marble chips and dilute hydrochloric acid, leads it through a delivery tube into lime water, and uses the one test that identifies it: lime water turns milky in carbon dioxide and in almost nothing else. It is the confirmatory test every Class 10 syllabus asks for, and the reason a chemist never stops at "a gas came off".
5 steps · Acids, Bases & Salts · Chemical Reactions
What you will use
Apparatus
- Reaction flask
- Lime water
Chemicals
- Marble Chips
- Limewater
- Dilute Hydrochloric Acid
The steps
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Drop a few marble chips into the reaction flask.
Nothing happens yet. A carbonate on its own is stable.
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Pour lime water into the test tube.
Clear and colourless, which is what makes any change in it easy to see.
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Right-click the flask and run a delivery tube into the lime water.
Set up before the reaction starts — a gas that escapes into the room cannot be tested.
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Pour dilute hydrochloric acid onto the marble chips.
Brisk effervescence. The chips fizz as gas is driven off and travels down the tube.
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Watch the lime water.
It turns milky — a fine white solid clouding a solution that was clear a moment ago.
What it shows
The gas is carbon dioxide. It turns lime water milky because it reacts with the calcium hydroxide to form calcium carbonate, which will not dissolve — the same compound the marble chips were made of, back again as a white cloud.