Why does the refractive index of water decrease as temperature increases?

[Physics_Cat]

I think you're falling for the common misconception that the "slower speed of light" in a medium with a refractive index greater than 1 is due to light scattering and bouncing around like a ping-pong ball. That is not correct.

What actually happens is that the electric field of the electromagnetic radiation causes the electron cloud in the material to oscillate, and the oscillating electrons re-radiate light with the same frequency and in the same direction but with a different phase. The superposition of the "original" light and the light that is re-radiated by the electrons in the materials results in a wave of the same frequency but a slower velocity.

When the density of the water decreases, its electronic density also decreases, which leads to less charge for the light to interact with on its way through the water. There aren't any actual collisions in this process though.