Which of water's properties directly causes mechanical weathering?

Answer 1

Two different properties aid mechanical weathering. First, simple dissolution and subsequent precipitation of salts will help destroy rock's surface structure.

And the expansion of water below -4oC causes a 'freeze-thaw' weathering as it breaks up the rock.

[note, in both Roman times, and more recently in Scotland, a method of breaking large boulders was to drill a series of holes across the stone, and on a frosty night allow water in the holes to break the stone apart. A minor variant of this is to hammer dry hardwood pegs into the holes as above, and wet the surface. As the wood swells, it breaks the stone. ]

Answer 2

Water's ability to freeze and expand when it turns into ice is the property that directly causes mechanical weathering. When water seeps into cracks in rocks, expands as it freezes, and then thaws, it exerts pressure on the rock and can break it apart over time.