ANY medium in which you pass a light beam, more or less, take away some power from the beam (can be reflection, refraction, absorption .....) , so, any beam passing through a medium, lost power (also just the air, if you think it in a scientifical way, take away power from your beam ..... when you see the beam, it's all light reflected and dispersed from air mist, dust and particles, so it's all power taken away from the beam, after all)
Ofcourse, more density and opacity your medium have, more power it take away.
The experiment, in that form, is not so useful, cause you need also to take in consideration reflections and refractions and absorption from the container that hold the liquid, so as reference you have to use an identical container, if you want to present it in the right way.
Anyway, i have a suggestion, if you want ..... as you know, there are some solutions and liquids that looks transparents for visible light, and totally block infrareds, as the same as there are some liquid solutions that are totally dark for visible light, and left pass infrareds .....
You can then use a green module, specifying that it emits both infrareds and 532nm green, then do tests with some different liquids, and a pair of filters, one blocking IR and one blocking visible, and demonstrate not just the drop of power through the various liquids, but also the differences with visible and IR drops ..... it may require a bit more work and research, and something reliable as light detector (a luxmeter without IR filtering ?), if you don't have any LPM, but can be an idea
