physical chemistry - Why is snow white?

I know that this is a rather ambiguous question; but my question is, whenever we take water and freeze it in the freezer, it still tends to stay clear. Since snow is just frozen water, why is it white? Is it due to contents of the air - i.e. dust - that make snow this color?

Or is my freezer just weird?

Answer

The difference between snow and ordinary ice cubes is mainly about the size of the particles. Snow is made from small, irregular crystals with many edges at a very small scale. Light is refracted or scattered by the edges (or the interface between air and the edges). Snow is white because the scattering effect of those edges dominates what happens to light shining on the snow. In a large block of ice like an ice cube there is very little refraction or scattering as, for any ray of light, there are only two edges (one when light enters the cube, one when it leaves). So transmission dominates (and there is little colour as ice only weakly absorbs visible light).

This is common in many other compounds. Titanium dioxide is a transparent mineral but is used as the primary ingredient in many "white" products like paint. The secret is to use TiO₂ particles that are just the right size to maximise the scattering at the particle edges thereby creating "whiteness" as all light is equally scattered.