a typical synapse in the cns consists of a presynaptic neuron and a postsynaptic neuron, separated by a narrow space called the

when atoms of different types of elements join together, they make molecules called compounds. water consists of compound molecules made up of 2 hydrogen atoms and 1 oxygen atom. this is why it's called h2o. water will always have 2 times the number of hydrogen atoms as oxygen atoms.

it's diluted or concentrated

in a diluted solution, you can add a lot more solute but in concentrated only a little more can be added