the amount of hydrogen gas collected in one test tube is double the amount of oxygen gas collected in the other because water (H₂O) contains two hydrogen atoms for every one oxygen atom. Therefore, during electrolysis, hydrogen is produced in a 2:1 ratio by volume compared to oxygen.
Elaboration:
When electricity is passed through water (H₂O), it decomposes into its constituent elements: hydrogen and oxygen. The chemical equation for this process is:
2H₂O → 2H₂ + O₂
This equation shows that for every two molecules of water that decompose, two molecules of hydrogen gas (2H₂) and one molecule of oxygen gas (O₂) are produced. This explains why the volume of hydrogen gas collected in one of the test tubes is double the volume of oxygen gas collected in the other.