we can now image individual molecules and atoms as they participate in chemical reactions.
When I was first learning about science, in the '50s and '60s, I'm sure most scientists would have flatly stated that such a thing was impossible. Most likely they'd cite the fact that the atoms and (most) molecules were smaller than the wavelength of any conceivable radiation that might be used to image them (and certainly smaller than the wavelength of visible light). Therefore imaging wasn't possible. Simple!
But wrong. The advent of atomic force microscopes (which don't use any radiation to create their images) completely changed the context – and the impossible became possible. Now you can buy such a microscope, if you've got a few hundred thousand dollars to spare (though I did find a used one for just $32,500).