A colourless gas is collected and several tests are carried out on it. Which single observation would confirm that this gas is carbon dioxide rather than on...
Question 1 Report
A colourless gas is collected and several tests are carried out on it. Which single observation would confirm that this gas is carbon dioxide rather than one of the other colourless gases?
Answer Details
Each common colourless gas has its own test, so the confirming observation must be one that no other gas gives. Carbon dioxide reacts with the calcium hydroxide in limewater to form insoluble calcium carbonate, and that fine white solid is what makes the limewater look milky:
\[\text{Ca(OH)}_2 + \text{CO}_2 \rightarrow \text{CaCO}_3 + \text{H}_2\text{O}\]
So turning limewater milky is the observation that confirms carbon dioxide.
A squeaky pop with a lighted splint identifies hydrogen, relighting a glowing splint identifies oxygen, and turning damp red litmus blue identifies ammonia. Carbon dioxide in fact does the opposite of that last test, since its solution is weakly acidic and turns damp blue litmus red.
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