Why does water 'reflect' a standing sound wave in a tube I'm reading about (standing) sound waves and one example is that of an vertical pipe that has a standing sound wave in it. It is then filled halfway with water and since the length of the pipe decreases, the required fundamental frequency increases.
Now what I don't understand is how the water just acts like the bottom of the tube. The standing wave should be caused by the reflection of the sound waves that are sent in, but don't the waves continue into the water?
I found a picture that illustrates a standing sound wave on top of the water, where the water surface works like a displacement node.

 A: When a wave meets a boundary between 2 media in which the wave speed and/or density is different, there is both transmission and reflection. The bigger the difference in wave speed or density, the higher the % of the wave energy which is reflected. 
For normal incidence, the ratio of reflected to incident amplitudes for sound waves in fluids is 
$R=\frac{\rho_1 c_2 - \rho_2 c_1}{\rho_1 c_2+\rho_2 c_1}$
where $\rho, c$ are the densities and speeds in the two media. See section 2 of these notes from MIT.
The speed of sound in air is about $c_1=330m/s$, while in water it is about $c_2=1484m/s$. The densities are $\rho_1=1.225kg/m^3$ and $\rho_2=1000kg/m^3$. So the reflection coefficient is $R=0.989$ and the % reflection of energy is $R^2=97.8$%. Almost all of the sound energy is reflected by the water surface. 
A: Actually most of the sound is reflected from the water surface but not all. Some sound indeed transmits into the water. But for purposes of analyzing, measuring a standing wave it has to do with impedance matching at the air-water boundary. Water has a higher transmissive impedance than air for the sound. Most of the energy is therefore reflected back into and contained within the air space.
And as one of the comments mentioned some is transmitted through the rigid walls.
