Not because they are small, but because they have no color.
Say Hydrogen and Oxygen, yes we can see water, but, how did scientists distinguish that there were one oxygen and two hydrogen atoms in it if these elements are invisible to begin with?
In: Physics
There are many, many more measurable attributes than color that people used to figure these things out.
To use your example of water: if you run electricity through it, it breaks apart. Early scientists wouldn’t know that, but they would know that it makes the water turn into gas.
You place a tube over this to capture it, and you can find by experimentation that the top 2/3 is much lighter, and when released carefully without disturbing the bottom 1/3, is flammable, while the bottom 1/3 is not. Clearly there are 2 different substances here
Put a glowing splint in the bottom gas, and it bursts into flame. This gas must support combustion. Gather it and burn different things, you’ll find that it makes the same substances burn that burn in normal air, but much faster. Perhaps this is the part of air that allows things to burn?
Put a small animal in a container with the gases. With the lighter one, it dies. The heavier one, it lives! This gas supports life while the other does not.
On and on and on until you discover the attributes of this substance and realize you cannot break it down into anything else, and it must be fundamental.
When you run electricity though water, bubbles of gas form – and the gas that forms on the negative electrode behaves *very* differently from the gas that forms on the positive electrode. So they must be different substances.
You can collect these two gases and recombine them into water. But *you can only combine them in a very specific proportion:* it’s always 2-to-1. If you use a different ratio, you will have one of the gases left over when you’re done. So there has to be two of *something* combining with each one *something else*.
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