When hydrogen gas is burnt and the products are cooled, a colorless liquid is formed. What is one chemical test that could be carried out to identify the liquid?
It's a pretty safe bet that the "colorless liquid" is water, since hydrogen burned in air combines with the oxygen in air to produce water. From the context of the question, it's clear we aren't permitted to "know" this beforehand, and we can't avoid the point of the questions simply by going around it and asserting our certainty that it's water. This can be frustrating because there's no single chemical test that will conclusively demonstrate this is water, but the point is to demonstrate a means of narrowing down the possibilities so we could apply the same line of thinking to a truly unknown substance in the future.
First, consider the nature of water and which of its properties we could empirically test;
- Water freezes and boils at known temperatures
- It is a highly polar molecule
- Many ionic compounds, like sodium chloride, dissolve in it
- It is not soluble with oil or other nonpolar compounds
- It has a neutral pH
- It has no smell or taste
So, some of the experiments we could conduct include
- cooling and heating it, and evaluating the temperatures at which it boils and freezes
- exposing it to a magnetic field to see if it bends (you can do this easily with a strong magnet and a moving column of liquid, e.g. from a faucet)
- attempting to dissolve table salt in it
- attempting to dissolve olive oil in it
- using pH strips or universal indicator
- smelling it
Generally, I advise against tasting compounds under any circumstances due to the possibility of contamination.